
B. Vermersch, A. Elben, L. Sieberer, N. Y. Yao, P. Zoller Probing scrambling using statistical correlations between randomized measurements,
Phys. Rev. X 9 21061 (20190627),
http://dx.doi.org/10.1103/PhysRevX.9.021061 doi:10.1103/PhysRevX.9.021061 (ID: 720042)
Toggle Abstract
We present a protocol to study scrambling using statistical correlations between measurements, performed after evolving a quantum system from random initial states. We show that the resulting statistical correlations are directly related to OTOCs and can be used to probe scrambling in manybody systems. Our protocol, which does not require reversing time evolution or auxiliary degrees of freedom, can be realized in stateoftheart quantum simulation experiments.

M. Schütz, B. Vermersch, G. Kirchmair, L. Vandersypen, J. I. Cirac, M. Lukin, P. Zoller Quantum Simulation and Optimization in Hot Quantum Networks,
Phys. Rev. B 99 241302 (20190627),
http://dx.doi.org/10.1103/PhysRevB.99.241302 doi:10.1103/PhysRevB.99.241302 (ID: 720058)
Toggle Abstract
We propose and analyze a setup based on (solidstate) qubits coupled to a common multimode transmission line, which allows for coherent spinspin interactions over macroscopic onchip distances, without any groundstate cooling requirements for the data bus. Our approach allows for the realization of fast deterministic quantum gates between distant qubits, the simulation of quantum spin models with engineered (longrange) interactions, and provides a flexible architecture for the implementation of quantum approximate optimization algorithms.

C. Kokail, C. Maier, R. van Bijnen, T. Brydges, M. K. Joshi, P. Jurcevic, C. A. Muschik, P. Silvi, R. Blatt, C. F. Roos, P. Zoller Selfverifying variational quantum simulation of lattice models,
Nature 569 360 (20190515),
http://dx.doi.org/10.1038/s4158601911774 doi:10.1038/s4158601911774 (ID: 720076)
Toggle Abstract
Hybrid classicalquantum algorithms aim at variationally solving optimisation problems, using a feedback loop between a classical computer and a quantum coprocessor, while benefitting from quantum resources. Here we present experiments demonstrating selfverifying, hybrid, variational quantum simulation of lattice models in condensed matter and highenergy physics. Contrary to analog quantum simulation, this approach forgoes the requirement of realising the targeted Hamiltonian directly in the laboratory, thus allowing the study of a wide variety of previously intractable target models. Here, we focus on the Lattice Schwinger model, a gauge theory of 1D quantum electrodynamics. Our quantum coprocessor is a programmable, trappedion analog quantum simulator with up to 20 qubits, capable of generating families of entangled trial states respecting symmetries of the target Hamiltonian. We determine ground states, energy gaps and, by measuring variances of the Schwinger Hamiltonian, we provide algorithmic error bars for energies, thus addressing the longstanding challenge of verifying quantum simulation.

A. Elben, B. Vermersch, C. F. Roos, P. Zoller Statistical correlations between locally randomized measurements: a toolbox for probing entanglement in manybody quantum states,
Phys. Rev. A 99 52323 (20190515),
http://dx.doi.org/10.1103/PhysRevA.99.052323 doi:10.1103/PhysRevA.99.052323 (ID: 720100)
Toggle Abstract
We develop a general theoretical framework for measurement protocols employing statistical correlations of randomized measurements. We focus on locally randomized measurements implemented with local random unitaries in quantum lattice models. In particular, we discuss the theoretical details underlying the recent measurement of the second Rényi entropy of highly mixed quantum states consisting of up to 10 qubits in a trappedion quantum simulator [Brydges et al., arXiv:1806.05747]. We generalize the protocol to access the overlap of quantum states, prepared sequentially in an experiment. Furthermore, we discuss proposals for quantum state tomography based on randomized measurements within our framework and the respective scaling of statistical errors with system size.

T. Brydges, A. Elben, P. Jurcevic, B. Vermersch, C. Maier, B. P. Lanyon, P. Zoller, R. Blatt, C. F. Roos Probing Renyi entanglement entropy via randomized measurements,
Science 364 260 (20190419),
http://dx.doi.org/10.1126/science.aau4963 doi:10.1126/science.aau4963 (ID: 720034)
Toggle Abstract
Entanglement is the key feature of manybody quantum systems, and the development of new tools to probe it in the laboratory is an outstanding challenge. Measuring the entropy of different partitions of a quantum system provides a way to probe its entanglement structure. Here, we present and experimentally demonstrate a new protocol for measuring entropy, based on statistical correlations between randomized measurements. Our experiments, carried out with a trappedion quantum simulator, prove the overall coherent character of the system dynamics and reveal the growth of entanglement between its parts  both in the absence and presence of disorder. Our protocol represents a universal tool for probing and characterizing engineered quantum systems in the laboratory, applicable to arbitrary quantum states of up to several tens of qubits.

M. Heyl, P. Hauke, P. Zoller Quantum localization bounds Trotter errors in digital quantum simulation,
Sci. Adv. 5 (20190412),
http://dx.doi.org/10.1126/sciadv.aau8342 doi:10.1126/sciadv.aau8342 (ID: 720038)
Toggle Abstract
A fundamental challenge in digital quantum simulation (DQS) is the control of inherent errors. These appear when discretizing the time evolution generated by the Hamiltonian of a quantum manybody system as a sequence of quantum gates, called Trotterization. Here, we show that quantum localizationby constraining the time evolution through quantum interferencestrongly bounds these errors for local observables. Consequently, for generic quantum manybody Hamiltonians, Trotter errors can become independent of system size and total simulation time. For local observables, DQS is thus intrinsically much more robust than what one might expect from known error bounds on the global manybody wave function. This robustness is characterized by a sharp threshold as a function of the Trotter step size. The threshold separates a regular region with controllable Trotter errors, where the system exhibits localization in the space of eigenstates of the timeevolution operator, from a quantum chaotic regime where the trajectory is quickly scrambled throughout the entire Hilbert space. Our findings show that DQS with comparatively large Trotter steps can retain controlled Trotter errors for local observables. It is thus possible to reduce the number of quantum gate operations required to represent the desired time evolution faithfully, thereby mitigating the effects of imperfect individual gate operations

A. Keesling, A. Omran, H. Levine, H. Bernien, H. Pichler, S. Choi, R. Samajdar, S. Schwartz, P. Silvi, S. Sachdev, P. Zoller, M. Endres, M. Greiner, V. Vuletic, M. Lukin Quantum Kibble–Zurek mechanism and critical dynamics on a programmable Rydberg simulator,
Nature 568 211 (20190401),
http://dx.doi.org/10.1038/s4158601910701 doi:10.1038/s4158601910701 (ID: 720062)
Toggle Abstract
Quantum phase transitions (QPTs) involve transformations between different states of matter that are driven by quantum fluctuations. These fluctuations play a dominant role in the quantum critical region surrounding the transition point, where the dynamics are governed by the universal properties associated with the QPT. The resulting quantum criticality has been explored by probing linear response for systems near thermal equilibrium. While time dependent phenomena associated with classical phase transitions have been studied in various scientific fields, understanding critical realtime dynamics in isolated, nonequilibrium quantum systems is of fundamental importance both for exploring novel approaches to quantum information processing and realizing exotic new phases of matter. Here, we use a Rydberg atom quantum simulator with programmable interactions to study the quantum critical dynamics associated with several distinct QPTs. By studying the growth of spatial correlations while crossing the QPT at variable speeds, we experimentally verify the quantum KibbleZurek mechanism (QKZM) for an Isingtype QPT, explore scaling universality, and observe corrections beyond simple QKZM predictions. This approach is subsequently used to investigate novel QPTs associated with chiral clock model providing new insights into exotic systems, and opening the door for precision studies of critical phenomena and applications to quantum optimization.

P. Guimond, A. Grankin, D. Vasilyev, B. Vermersch, P. Zoller Subradiant Bell States in Distant Atomic Arrays,
Phys. Rev. Lett. 122 93601 (20190305),
http://dx.doi.org/10.1103/PhysRevLett.122.093601 doi:10.1103/PhysRevLett.122.093601 (ID: 720126)
Toggle Abstract
We study collective “freespace” radiation properties of two distant singlelayer arrays of quantum emitters as twolevel atoms. We show that this system can support a longlived Bell superposition state of atomic excitations exhibiting strong subradiance, which corresponds to a nonlocal excitation of the two arrays. We describe the preparation of these states and their application in quantum information as a resource of nonlocal entanglement, including deterministic quantum state transfer with high fidelity between the arrays representing quantum memories. We discuss experimental realizations using cold atoms in optical trap arrays with subwavelength spacing, and analyze the role of imperfections.

M. Riedel, M. Kovacs, P. Zoller, J. Mlynek, T. Calarco Europe's Quantum Flagship initiative,
Quantum Sci. Technol. 4 (20190222),
http://dx.doi.org/10.1088/20589565/ab042d doi:10.1088/20589565/ab042d (ID: 720172)
Toggle Abstract
As the first applications leap out of research laboratories toward commercialization, the global race for dominance in the maturing field of quantum technologies is becoming ever fiercer. To retain its historical lead and kickstart a continentwide quantumdriven industry and accelerate market takeup, Europe has launched the Quantum Flagship, an ambitious €1 billion, 10 year endeavor. This article provides an overview of the underlying considerations and the current state of the initiative. Furthermore, it briefly presents the 20 projects selected to be at the core of the rampup phase of the initiative, which will address core applications of quantum technologies such as communications, computing, simulation, as well as sensing and metrology, all of which are supported by basic science. Finally, we present the broader ecosystem of European funding instruments and institutions which aim to create the next generation of disruptive technologies within quantum sciences, placing Europe as a worldwide knowledgebased industrial and technological leader in this innovative field.

P. Zoller Quantum Simulation with Atoms and Ions,
QSEC2019 (Heidelberg, 20190923) URL (20190923),
(ID: 720343)
Toggle Abstract
The talk starts with a brief historical review of proposals and concepts for quantum computing, quantum simulation and quantum communication with atoms, ions and photons, and in particular synergy of theoretical and experimental developments. The main part of the talk will focus on our recent research on programmable quantum simulators on various platforms. This includes a theoryexperiment collaboration demonstrating variational quantum simulation of lattice models as hybrid classicalquantum algorithms on trapped ion platforms. In addition, we discuss quantum algorithms for quantum metrology with Rydberg tweezers arrays. Furthermore, we will summarize the recently developed toolbox which employs statistical correlations between randomized measurements to measure entanglement (Renyi) entropies, OutOfTime Ordered Correlators, and we discuss protocols for crossplatform verification of quantum devices. We conclude with an outlook and perspective of future developments in quantum information processing with quantum optical systems.

P. Zoller CrossPlatform Verification of Intermediate Scale Quantum Devices,
Conference on Quantum Information and Quantum ControlVIII (Fields Institute, Toronto, 20190826) URL (20190827),
(ID: 720342)

P. Zoller Hybrid ClassicalQuantum Quantum Simulations of ManyBody Systems: Theory and Experiment,
GRC on Quantum Control of Light and Matter (Salve Regina University, New Port RI, 20190811) URL (20190812),
(ID: 720341)

P. Zoller Programmable Quantum Simulators & Sensors with Atoms and Ions,
ICQT 2019 (Moskow, 20190715) URL (20190716),
URL (ID: 720327)

P. Zoller Hybrid ClassicalQuantum Simulations of the Lattice Schwinger Model in the `Innsbruck Quantum Cloud’,
Humboldt Kolleg on Discoveries and Open Puzzles in Particle Physics and Gravitation (Kitzbühel, 20190623) URL (20190625),
(ID: 720310)

P. Zoller Programming Quantum Simulators,
GRC on Cold Controlled Atoms and Molecules, Ultrafast Spectroscopy and Precision Measurements (Salve Regina University, New Port RI, 20190609) URL (20190610),
(ID: 720311)

P. Zoller Hybrid ClassicalQuantum Algorithms on Programmable Quantum Simulatorswith Atoms and Ions,
NISQ Workshop (Maryland, 20190606) URL (20190607),
(ID: 720312)

P. Zoller Verifying Quantum Machnines,
‘Quantum for ever’ – A symposium to honor H. Rauch’s 80th birthday (Atominstitut TU Wien, 20190521) (20190521),
URL (ID: 720281)

P. Zoller Blackboard lecture: QND Measurement of the manybody Hamiltonian,
Open Quantum System Dynamics: Quantum Simulators and Simulations Far From Equilibrium (KITP Santa Barbara, 20190401) URL (20190425),
(ID: 720267)

P. Zoller, P. Zoller Programmable Quantum Simulators with Atoms and Ions,
International Symposium of Quantum Science & Technology (Purdue University, 20190422) URL (20190422),
(ID: 720265)

P. Zoller Variational Quantum Simulation in the Innsbruck Quantum Cloud,
Google Tech Talk (Google Quantum Goleta, 20190418) (20190418),
(ID: 720266)

P. Zoller Programmable Quantum Simulators with Atoms,
ECAMP13 (Florence, 20190408) URL (20190410),
(ID: 720258)
Toggle Abstract
Atomic physics provides us with the realization of engineered quantum manybody lattice models, as illustrated by spin models with chains of trapped ions [1] and Rydberg tweezer arrays [2], and Hubbard models with bosonic and fermionic cold atoms in optical lattices. Among the most noticeable recent experimental achievements are quantum control and single shot measurements in lattice systems of atoms and ions with single site resolution, as illustrated for atoms in optical lattices by the quantum gas microscope. This ability of single site control provides us with the new paradigm of a `programmable' quantum simulator as a novel quantum architecture; i.e., something between the traditional analog quantum simulator which realizes specific manybody Hamiltonians and quantum dynamics, and the universal, fully programmable quantum computer. We will illustrate implementation, and application of `programmable' quantum simulators by several examples. This includes, first of all, realization of hybrid classicalquantum algorithms, using a feedback loop between a classical computer and a programmable quantum simulator, which acts as a quantum coprocessor [1]. We demonstrate variational quantum simulation of a Lattice Schwinger Model, as 1D quantum electrodynamics. Our quantum coprocessor is a programmable, trappedion analog quantum simulator with up to 20 qubits, capable of generating families of entangled trial states. We determine ground states, energy gaps and, by measuring variances of the Schwinger Hamiltonian, we provide algorithmic error bars for energies, thus addressing the longstanding challenge of verifying quantum simulation. As a second example, we present a new protocol for measuring entanglement entropies, as a quantifier of entanglement in the quantum manybody system, based on statistical correlations between randomized measurements [3]. An experiment is carried out with a trappedion quantum simulator, and we prove the overall coherent character of the system dynamics and reveal the growth of entanglement between its parts. We conclude the talk by discussing, from a theory perspective, generation of entangled states for quantum sensing and metrology with `programmable' quantum simulators, which we illustrate with spinsqueezed states for Ramsey spectroscopy in a Rydberg tweezer array [4].
[1] [1] C. Kokail, C. Maier, R. van Bijnen, T. Brydges, M. K. Joshi, P. Jurcevic, C. A. Muschik, P. Silvi, R. Blatt, C.F. Roos and P. Zoller, SelfVerifying Variational Quantum Simulation of the Lattice Schwinger Model, arXiv:1810.03421
[2] A. Keesling, A. Omran, H. Levine, H. Bernien, H. Pichler, S. Choi, R. Samajdar, S. Schwartz, P. Silvi, S. Sachdev, P. Zoller, M. Endres, M. Greiner, V. Vuletic, M.D. Lukin,
Probing quantum critical dynamics on a programmable Rydberg simulator, arXiv:1809.05540
[3] T. Brydges, A. Elben, P. Jurcevic, B. Vermersch, C. Maier, B.P. Lanyon, P. Zoller, R. Blatt, C.F. Roos, Probing entanglement entropy via randomized measurements, arXiv:1806.05747
[4] R. Kaubrügger, P.S. Silvi, A. Kaufman, and P. Zoller, unpublished

P. Zoller Programmable Quantum Simulators with Atoms and Ions,
PASQuanS Meeting (Institut d’Optique, CNRS, Palaiseau, 20190404) (20190405),
(ID: 720259)
Toggle Abstract
Programmable analog quantum simulators have recently emerged as a new paradigm in quantum information processing. In contrast to the universal quantum computer, programmable quantum simulators are nonuniversal quantum devices with restricted sets of quantum operations, which however can be naturally scaled to a large number of qubits. In this talk we will focus on programmable analog quantum simulators with trapped ions and Rydberg tweezer arrays, and discuss various scenarios and applications of programming these quantum machines. First, we show results from a theoryexperiment collaboration at Innsbruck demonstrating hybrid classicalquantum algorithms where a 20 quit ion analog simulator computes the energy of the ground state of a lattice Schwinger model representing 1D QED. Remarkably, in this experiment we can not only compute the energy but also the algorithmic error (error bar of the energy) on the quantum machine by measuring the energy variance. Further examples include theoretical studies where variational algorithms are applied to generate optimal spin squeezed states for given (restricted) quantum resources provided by Rydberg tweezer arrays, which has potential applications in quantum sensing. We conclude with a discussion of the generic question of cross platform verification of quantum computers and quantum simulators, where the goal is to compare quantum devices on the level of manyquit wavefunctions with protocols, which can be implemented in present day experiments.

P. Zoller Programming QuantumSimulators,
SFBFoQuS International Conference (Innsbruck, 20190204) URL (20190204),
(ID: 720146)
Toggle Abstract
Our SFB FOQUS aims at a collaborative `quantum advantage’, and I will focus on two examples taken from the last year, where new theoretical ideas and concepts found immediate realization in Innsbruck ion trap experiments. The first example is around the paradigm of randomized measurements and its application to atomic quantum manybody physics. I will discuss measurement of entanglement entropy in quench dynamics in a protocol employing single qubit (spin) rotations, and provide a theory outlook. The second example is variational quantum simulation of lattice models. Here families of entangled variational wavefunctions are generated on a programmable 20 ion analog quantum simulator as a quantum resource, and a classicalquantum feedback loop finds the `best' ground state of  in our case  the Schwinger lattice model for the given resources. We provide a first demonstration of selfverification of quantum simulation by measuring the variance of the target Hamiltonian on our quantum device.

A. Elben, B. Vermersch, R. van Bijnen, C. Kokail, T. Brydges, C. Maier, M. K. Joshi, R. Blatt, C. F. Roos, P. Zoller CrossPlatform Verification of Intermediate Scale Quantum Devices,
(20190903),
arXiv:1909.01282v1 arXiv:1909.01282v1 (ID: 720357)
Toggle Abstract
We describe a protocol for crossplatform verification of quantum simulators and quantum computers. We show how to measure directly the overlap Tr[ρ1ρ2] and the purities Tr[ρ21,2], and thus a (mixedstate) fidelity, of two quantum states ρ1 and ρ2 prepared in separate experimental platforms. We require only local measurements in randomized product bases, which are communicated classically. As a proofofprinciple, we present the measurement of experimenttheory fidelities for entangled 10qubit quantum states in a trapped ion quantum simulator.

C. R. Kaubrügger, P. Silvi, C. Kokail, R. van Bijnen, A. M. Rey, J. Ye, A. Kaufman, P. Zoller Variational spinsqueezing algorithms on programmable quantum sensors,
(20190822),
arXiv:1908.08343v1 arXiv:1908.08343v1 (ID: 720356)
Toggle Abstract
Arrays of atoms trapped in optical tweezers combine features of programmable analog quantum simulators with atomic quantum sensors. Here we propose variational quantum algorithms, tailored for tweezer arrays as programmable quantum sensors, capable of generating entangled states ondemand for precision metrology. The scheme is designed to generate metrological enhancement by optimizing it in a feedback loop on the quantum device itself, thus preparing the best entangled states given the available quantum resources. We apply our ideas to generate spinsqueezed states on Sr atom tweezer arrays, where finiterange interactions are generated through Rydberg dressing. The complexity of experimental variational optimization of our quantum circuits is expected to scale favorably with system size. We numerically show our approach to be robust to noise, and surpassing known protocols.

R. Belyansky, J. Young, P. Bienias, Z. Eldredge, A. Kaufman, P. Zoller, A. V. Gorshkov Nondestructive cooling of an atomic quantum register via stateinsensitive Rydberg interactions,
(20190725),
arXiv:1907.11156 arXiv:1907.11156 (ID: 720326)
Toggle Abstract
We propose a protocol for sympathetically cooling neutral atoms without destroying the quantum information stored in their internal states. This is achieved by designing stateinsensitive Rydberg interactions between the datacarrying atoms and cold auxiliary atoms. This can be used to extend the lifetime of quantum storage based on neutral atoms and can have applications for long quantum computations. The protocol can also be modified to realize stateinsensitive interactions between the data and the auxiliary atoms but tunable and nontrivial interactions among the data atoms, allowing one to simultaneously cool and simulate a quantum spinmodel.

A. Celi, B. Vermersch, O. Viyuela, H. Pichler, M. Lukin, P. Zoller Emerging 2D Gauge theories in Rydberg configurable arrays,
(20190707),
arXiv:1907.03311v2 arXiv:1907.03311v2 (ID: 720355)
Toggle Abstract
Solving strongly coupled gauge theories in two or three spatial dimensions is of fundamental importance in several areas of physics ranging from highenergy physics to condensed matter. On a lattice, gauge invariance and gauge invariant (plaquette) interactions involve (at least) fourbody interactions that are challenging to realize. Here we show that Rydberg atoms in configurable arrays realized in current tweezer experiments are the natural platform to realize scalable simulators of the RokhsarKivelson Hamiltonian a 2D U(1) lattice gauge theory that describes quantum dimer and spinice dynamics. Using an electromagnetic duality, we implement the plaquette interactions as Rabi oscillations subject to Rydberg blockade. Remarkably, we show that by controlling the atom arrangement in the array we can engineer anisotropic interactions and generalized blockade conditions for spins built of atom pairs.
We describe how to prepare the resonating valence bond and the crystal phases of the RokhsarKivelson Hamiltonian adiabatically, and probe them and their quench dynamics by onsite measurements of their quantum correlations.
We discuss the potential applications of our Rydberg simulator to lattice gauge theory and exotic spin models.

M. Lacki, P. Zoller, M. Baranov Stroboscopic painting of optical potentials for atoms with subwavelength resolution,
(20190618),
arXiv:1906.07649 arXiv:1906.07649 (ID: 720290)
Toggle Abstract
We propose and discuss a method to engineer stroboscopically arbitrary onedimensional optical potentials with subwavelength resolution. Our approach is based on subwavelength optical potential barriers for atoms in the dark state in an optical \Lambda system, which we use as a stroboscopic drawing tool by controlling their amplitude and position by changing the amplitude and the phase of the control Rabi frequency in the \Lambda system. We demonstrate the ability of the method to engineer both smooth and comblike periodic potentials for atoms in the dark state, and establish the range of stroboscopic frequencies when the quasienergies of the stroboscopic Floquet system reproduce the band structure of the timeaveraged potentials. In contrast to usual stroboscopic engineering which becomes increasingly accurate with increasing the stroboscopic frequency, the presence of the bright states of the \Lambdasystem results in the upper bound on the frequency, above which the dynamics strongly mixes the dark and the bright channels, and the description in terms of a timeaveraged potential fails. For frequencies below this bound, the lowest Bloch band of quasienergies contains several avoidedcrossing coming from the coupling to high energy states, with widths decreasing with increasing stroboscopic frequency. We analyze the influence of these avoided crossings on the dynamics in the lowest band using Bloch oscillations as an example, and establish the parameter regimes when the population transfer from the lowest band into high bands is negligible. We also present protocols for loading atoms into the lowest band of the painted potentials starting from atoms in the lowest band of a standard optical lattice.

A. Elben, J. Yu, G. Zhu, M. Hafezi, F. Pollmann, P. Zoller, B. Vermersch Manybody topological invariants from randomized measurements,
(20190612),
arXiv:1906.05011 arXiv:1906.05011 (ID: 720289)
Toggle Abstract
The classification of symmetryprotected topological (SPT) phases in one dimension has been recently achieved, and had a fundamental impact in our understanding of quantum phases in condensed matter physics. In this framework, SPT phases can be identified by manybody topological invariants, which are quantized nonlocal correlators for the manybody wavefunction. While SPT phases can now be realized in interacting synthethic quantum systems, the direct measurement of quantized manybody topological invariants has remained so far elusive. Here, we propose measurement protocols for manybody topological invariants for all types of protecting symmetries of onedimensional interacting bosonic systems. Our approach relies on randomized measurements implemented with local random unitaries, and can be applied to any spin system with singlesite addressability and readout. Our scheme thus provides a versatile toolbox to experimentally classify interacting SPT phases.

D. Yang, A. Grankin, L. Sieberer, D. Vasilyev, P. Zoller Quantum Nondemolition Measurement of a ManyBody Hamiltonian,
(20190515),
arXiv:1905.06444v1 arXiv:1905.06444v1 (ID: 720277)
Toggle Abstract
An ideal quantum measurement collapses the wave function of a quantum system to an eigenstate of the measured observable, with the corresponding eigenvalue determining the measurement outcome. For a quantum nondemolition (QND) observable, i.e., one that commutes with the Hamiltonian generating the system's time evolution, repeated measurements yield the same result, corresponding to measurements with minimal disturbance. This concept applies universally to single quantum particles as well as to complex manybody systems. However, while QND measurements of systems with few degrees of freedom has been achieved in seminal quantum optics experiments, it is an open challenge to devise QND measurement of a complex manybody observable. Here, we describe how a QND measurement of the Hamiltonian of an interacting manybody system can be implemented in a trappedion analog quantum simulator. Through a single shot measurement, the manybody system is prepared in a narrow energy band of (highly excited) energy eigenstates, and potentially even a single eigenstate. Our QND scheme, which can be carried over to other platforms of quantum simulation, provides a novel framework to investigate experimentally fundamental aspects of equilibrium and nonequilibrium statistical physics including the eigenstate thermalization hypothesis (ETH) and quantum fluctuation relations.

A. Grankin, D. Vasilyev, P. Guimond, B. Vermersch, P. Zoller Freespace photonic quantum link and chiral quantum optics,
Phys. Rev. A 98 3825 (20181012),
http://dx.doi.org/10.1103/PhysRevA.98.043825 doi:10.1103/PhysRevA.98.043825 (ID: 719980)
Toggle Abstract
We present the design of a chiral photonic quantum link, where distant atoms interact by exchanging photons propagating in a single direction in free space. This is achieved by coupling each atom in a laserassisted process to an atomic array acting as a quantum phasedarray antenna. This provides a basic building block for quantum networks in free space, i.e., without requiring cavities or nanostructures, which we illustrate with highfidelity quantum state transfer protocols. Our setup can be implemented with neutral atoms using Rydbergdressed interactions.

D. Yang, D. Vasilyev, C. Laflamme, M. Baranov, P. Zoller Quantum Scanning Microscope for Cold Atoms,
Phys. Rev. A 98 23852 (20180827),
http://dx.doi.org/10.1103/PhysRevA.98.023852 doi:10.1103/PhysRevA.98.023852 (ID: 720027)
Toggle Abstract
We present a detailed theoretical description of an atomic scanning microscope in a cavity QED setup proposed in Phys. Rev. Lett. 120, 133601 (2018). The microscope continuously observes atomic densities with optical subwavelength resolution in a nondestructive way. The superresolution is achieved by engineering an internal atomic dark state with a sharp spatial variation of population of a ground level dispersively coupled to the cavity field. Thus, the atomic position encoded in the internal state is revealed as a phase shift of the light reflected from the cavity in a homodyne experiment. Our theoretical description of the microscope operation is based on the stochastic master equation describing the conditional time evolution of the atomic system under continuous observation as a competition between dynamics induced by the Hamiltonian of the system, decoherence effects due to atomic spontaneous decay, and the measurement backaction. Within our approach we relate the observed homodyne current with a local atomic density, and discuss the emergence of a quantum nondemolition measurement regime allowing continuous observation of spatial densities of quantum motional eigenstates without measurement backaction in a single experimental run.

M. Dalmonte, B. Vermersch, P. Zoller Quantum simulation and spectroscopy of entanglement Hamiltonians,
Nature Phys. 14 151 (20180521),
http://dx.doi.org/10.1038/s4156701801517 doi:10.1038/s4156701801517 (ID: 720028)
Toggle Abstract
The properties of a strongly correlated manybody quantum system, from the presence of topological order to the onset of quantum criticality, leave a footprint in its entanglement spectrum. The entanglement spectrum is composed by the eigenvalues of the density matrix representing a subsystem of the whole original system, but its direct measurement has remained elusive due to the lack of direct experimental probes. Here we show that the entanglement spectrum of the ground state of a broad class of Hamiltonians becomes directly accessible via the quantum simulation and spectroscopy of a suitably constructed entanglement Hamiltonian, building on the Bisognano–Wichmann theorem of axiomatic quantum field theory. This theorem gives an explicit physical construction of the entanglement Hamiltonian, identified as the Hamiltonian of the manybody system of interest with spatially varying couplings. On this basis, we propose a scalable recipe for the measurement of a system’s entanglement spectrum via spectroscopy of the corresponding Bisognano–Wichmann Hamiltonian realized in synthetic quantum systems, including atoms in optical lattices and trapped ions. We illustrate and benchmark this scenario on a variety of models, spanning phenomena as diverse as conformal field theories, topological order and quantum phase transitions.

E. Rico Ortega, M. Dalmonte, P. Zoller, D. Banerjee, M. Bögli, P. Stebler, U. Wiese SO(3) "Nuclear Physics" with ultracold Gases,
Ann. Phys. 393 483 (20180405),
http://dx.doi.org/10.1016/j.aop.2018.03.020 doi:10.1016/j.aop.2018.03.020 (ID: 719977)
Toggle Abstract
An ab initio calculation of nuclear physics from Quantum Chromodynamics (QCD), the fundamental SU(3) gauge theory of the strong interaction, remains an outstanding challenge. Here, we discuss the emergence of key elements of nuclear physics using an SO(3) lattice gauge theory as a toy model for QCD. We show that this model is accessible to stateoftheart quantum simulation experiments with ultracold atoms in an optical lattice. First, we demonstrate that our model shares characteristic manybody features with QCD, such as the spontaneous breakdown of chiral symmetry, its restoration at finite baryon density, as well as the existence of fewbody bound states. Then we show that in the onedimensional case, the dynamics in the gauge invariant sector can be encoded as a spin S=3/2 Heisenberg model, i.e., as quantum magnetism, which has a natural realization with bosonic mixtures in optical lattices, and thus sheds light on the connection between nonAbelian gauge theories and quantum magnetism.
(local copy)

D. Yang, C. Laflamme, D. Vasilyev, M. Baranov, P. Zoller Theory of a Quantum Scanning Microscope for Cold Atoms,
Phys. Rev. Lett. 120 133601 (20180330),
http://dx.doi.org/10.1103/PhysRevLett.120.133601 doi:10.1103/PhysRevLett.120.133601 (ID: 720010)
Toggle Abstract
We propose and analyze a scanning microscope to monitor “live” the quantum dynamics of cold atoms in a cavity QED setup. The microscope measures the atomic density with subwavelength resolution via dispersive couplings to a cavity and homodyne detection within the framework of continuous measurement theory. We analyze two modes of operation. First, for a fixed focal point the microscope records the wave packet dynamics of atoms with time resolution set by the cavity lifetime. Second, a spatial scan of the microscope acts to map out the spatial density of stationary quantum states. Remarkably, in the latter case, for a good cavity limit, the microscope becomes an effective quantum nondemolition device, such that the spatial distribution of motional eigenstates can be measured backaction free in single scans, as an emergent quantum nondemolition measurement.

Y. Wang, S. Subhankar, P. Bienias, M. Łącki, T. Tsui, M. Baranov, A. V. Gorshkov, P. Zoller, J. V. Porto, S. L. Rolston Dark state optical lattice with subwavelength spatial structure,
Phys. Rev. Lett. 120 83601 (20180220),
http://dx.doi.org/10.1103/PhysRevLett.120.083601 doi:10.1103/PhysRevLett.120.083601 (ID: 719917)
Toggle Abstract
We report on the experimental realization of a conservative optical lattice for cold atoms with subwavelength spatial structure. The potential is based on the nonlinear optical response of threelevel atoms in laserdressed dark states, which is not constrained by the diffraction limit of the light generating the potential. The lattice consists of a 1D array of ultranarrow barriers with widths less than 10~nm, well below the wavelength of the lattice light, physically realizing a KronigPenney potential. We study the band structure and dissipation of this lattice, and find good agreement with theoretical predictions. The observed lifetimes of atoms trapped in the lattice are as long as 60 ms, nearly 105 times the excited state lifetime, and could be further improved with more laser intensity. The potential is readily generalizable to higher dimension and different geometries, allowing, for example, nearly perfect box traps, narrow tunnel junctions for atomtronics applications, and dynamically generated lattices with subwavelength spacings.
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A. Elben, B. Vermersch, M. Dalmonte, J. I. Cirac, P. Zoller Rényi Entropies from Random Quenches in Atomic Hubbard and Spin Models,
Phys. Rev. Lett. 120 50406 (20180202),
http://dx.doi.org/10.1103/PhysRevLett.120.050406 doi:10.1103/PhysRevLett.120.050406 (ID: 719876)
Toggle Abstract
We present a scheme for measuring Rényi entropies in generic atomic Hubbard and spin models using single copies of a quantum state and for partitions in arbitrary spatial dimension. Our approach is based on the generation of random unitaries from random quenches, implemented using engineered timedependent disorder potentials, and standard projective measurements, as realized by quantum gas microscopes. By analyzing the properties of the generated unitaries and the role of statistical errors, with respect to the size of the partition, we show that the protocol can be realized in exisiting AMO quantum simulators, and used to measure for instance area law scaling of entanglement in twodimensional spin models or the entanglement growth in manybody localized systems.
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B. Vermersch, A. Elben, M. Dalmonte, J. I. Cirac, P. Zoller Unitary ndesigns via random quenches in atomic Hubbard and Spin models: Application to the measurement of Rényi entropies,
Phys. Rev. A 97 23604 (20180202),
http://dx.doi.org/10.1103/PhysRevA.97.023604 doi:10.1103/PhysRevA.97.023604 (ID: 719928)
Toggle Abstract
We present a general framework for the generation of random unitaries based on random quenches in atomic Hubbard and spin models, forming approximate unitary ndesigns, and their application to the measurement of R\'enyi entropies. We generalize our protocol presented in [Elben2017: arXiv:1709.05060, to appear in Phys. Rev. Lett.] to a broad class of atomic and spin lattice models. We further present an indepth numerical and analytical study of experimental imperfections, including the effect of decoherence and statistical errors, and discuss connections of our approach with manybody quantum chaos.
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P. Zoller Quantum Simulations in the Innsbruck Quantum Cloud,
WQC Inaugural Conference (Shanghai Jiao Tong University, 20181027) URL (20181027),
(ID: 720084)
Toggle Abstract
Hybrid classicalquantum algorithms aim at solving optimization problems, using a feedback loop between a classical computer and a quantum coprocessor, while benefitting from quantum resources. Here we present results from a theoryexperiment collaboration in Innsbruck [1], demonstrating selfverifying, hybrid, variational quantum simulation of lattice models in condensed matter and highenergy physics. Contrary to analog quantum simulation, this approach forgoes the requirement of realizing the targeted Hamiltonian directly in the laboratory, thus allowing the study of a wide variety of previously intractable target models. Our focus is the Lattice Schwinger model, a gauge theory of 1D quantum electrodynamics. Our quantum coprocessor is a programmable, trappedion analog quantum simulator with up to 20 qubits, capable of generating families of entangled trial states respecting symmetries of the target Hamiltonian. We determine ground states and, by measuring variances of the Schwinger Hamiltonian, we provide algorithmic error bars for energies, thus addressing the longstanding challenge of verifying quantum simulation.

P. Zoller Quantum Simulations in the Innsbruck Quantum Cloud,
DesOEQ Annual Meeting & DOQS Workshop (University of Strathclyde, Glasgow, 20181015) URL (20181018),
(ID: 720086)
Toggle Abstract
I will discuss recent developments in quantum simulation of quantum manybody systems with atomic platforms from a theory perspective. Systems of interests include atoms in optical lattices, Rydberg atoms in optical tweezer arrays, and trapped ions. Quantum simulation has so far been discussed as analog simulation, where we physically build a system with the desired Hamiltonian; or as digital quantum simulation, where time evolution of a manybody system is represented as a sequence of quantum gates on a quantum computer. I will add to this variation quantum simulation, where a quantum feedback loop between a classical computer and an analog quantum simulator, which acts as a quantum coprocessor. As an example I will present results from an ongoing theory [1]  experiment [2] collaboration in Innsbruck: here our quantum resource is a trappedion 20qubit analog quantum simulator, representing a transverse Ising model, and we compute on the quantum device the ground and excited states of the Lattice Schwinger Model as 1D Quantum Electrodynamics. Remarkably, variational quantum simulation allows "selfverification" of quantum results on the quantum machine, as assessment of the error. As a second topic we will discuss novel measurement protocols for Rényi entropies [3,4], and for outoftimeordered correlation functions (OTOCs) [5], which can be extracted from on statistical correlations between randomized measurements. I will show a recent experiment with an ion chain, demonstrating experimental observation of entanglement entropies in quench dynamics [4] in 10 and 20 qubit devices. I will conclude my talk with a brief outlook on possible future directions, including subwavelength optical lattices for atomic Hubbard models [6], and quantum chemistry [7].

P. Zoller Quantum Simulations in the Innsbruck Quantum Cloud,
Theorie Kolloquium (Universität Köln, 20181015) URL (20181015),
(ID: 720085)
Toggle Abstract
I will discuss recent developments in quantum simulation of quantum manybody systems with atomic platforms from a theory perspective. Systems of interests include atoms in optical lattices, Rydberg atoms in optical tweezer arrays, and trapped ions. Quantum simulation has so far been discussed as analog simulation, where we physically build a system with the desired Hamiltonian; or as digital quantum simulation, where time evolution of a manybody system is represented as a sequence of quantum gates on a quantum computer. I will add to this variation quantum simulation, where a quantum feedback loop between a classical computer and an analog quantum simulator, which acts as a quantum coprocessor. As an example I will present results from an ongoing theory [1]  experiment [2] collaboration in Innsbruck: here our quantum resource is a trappedion 20qubit analog quantum simulator, representing a transverse Ising model, and we compute on the quantum device the ground and excited states of the Lattice Schwinger Model as 1D Quantum Electrodynamics. Remarkably, variational quantum simulation allows "selfverification" of quantum results on the quantum machine, as assessment of the error. As a second topic we will discuss novel measurement protocols for Rényi entropies [3,4], and for outoftimeordered correlation functions (OTOCs) [5], which can be extracted from on statistical correlations between randomized measurements. I will show a recent experiment with an ion chain, demonstrating experimental observation of entanglement entropies in quench dynamics [4] in 10 and 20 qubit devices. I will conclude my talk with a brief outlook on possible future directions, including subwavelength optical lattices for atomic Hubbard models [6], and quantum chemistry [7].

P. Zoller Hybrid Quantum Simulators with Cold Atoms,
Jin Fest! (Boulder, 20180907) URL (20180908),
(ID: 720064)

P. Zoller Quantum Simulation with Cold Atoms and Ions,
MPS Conference on Ultra Quantum Matter II (New York City, 20180822) URL (20180823),
URL (ID: 720063)
Toggle Abstract
Zoller will discuss recent developments in quantum simulation of quantum manybody systems with atomic platforms from a theory perspective. Systems of interests include atoms in optical lattices, Rydberg atoms in optical tweezer arrays, and trapped ions. Quantum simulation has so far been discussed as analog simulation, where we physically build a system with the desired Hamiltonian; or as digital quantum simulation, where time evolution of a manybody system is represented as a sequence of quantum gates on a quantum computer. Zoller will add to this variational quantum simulation, based on a quantum feedback loop between a classical computer and an analog quantum simulator, which acts as a quantum coprocessor. As an example, he will present results from an ongoing theory – experiment collaboration in Innsbruck: here our quantum resource is an analog quantum simulator with trapped ions, representing a transverse Ising model, and we compute on the quantum device the ground and excited state wave functions of the Lattice Schwinger Model as 1D QED. Remarkably, variational quantum simulation allows "selfverification" of quantum results on the quantum machine. As a second topic we will discuss novel measurement protocols for Rényi entropies, and for outoftimeordered correlation functions (OTOCs), which can be extracted from on statistical correlations between randomized measurements. Zoller will show a recent experiment with a (single) ion chain demonstrating experimental observation of entanglement entropies in quench dynamics. He will conclude his talk with a brief outlook on possible future directions, including subwavelength optical lattices for atomic Hubbard models, and quantum chemistry.

P. Zoller Quantum Simulations in the Innsbruck Quantum Cloud,
ICAP 2018 (Barcelona, 20180722) URL (20180723),
(ID: 720061)
Toggle Abstract
We report results from a theory [1]  experiment [2]
collaboration on variational quantum simulations of
the ground state of the Schwinger Model, as a Lattice
Gauge Theory of 1D Quantum Electrodynamics.
The idea is to establish a feedback loop between
a classical computer and a programmable quantum
simulator, as familiar from quantum approximate op
timization algorithms, and quantum chemistry. The
quantum simulator is represented by a string of (up to
twenty) ions realizing a longrange Ising spin model
with transverse magnetic field. In combination with
single particle rotations this provides us with quan
tum resources to build highly entangled states on the
quantum machine depending on a set of variational
parameters. The feedback loop then allows us to min
imize the expectation value of the Schwinger Hamil
tonian to obtain an approximation to the ground
state wave function stored in quantum memory. The
two key elements for an eficient implementation are:
(i) the symmetries of the Schwinger problem are
matched by corresponding symmetries in quantum
resources generating the variational quantum state;
and (ii) a classical search algorithm, which for a given
number of maximum calls to the quantum simulator,
carefully allocates resources between a search for the
global minimum in the energy landscape of parame
ters, and number of measurements for given parame
ters to reduce projection noise (c.f. Fig. 1). Remark
ably, we can quantify energy error bars, and thus self
verify our quantum results experimentally by calcu
lating the variance of the Hamiltonian for the approx
imate quantum variational state (c.f. Fig. 2). Fur
thermore, we discuss computation of excited states,
convergence as scaling with system size, and crossing
a quantum phase transition. Our techniques should
be useful in establishing quantum simulations for con
densed matter and highenergy physics problems.
We conclude with brief remarks on several related
topics, in particular a protocol to measure Renyi en
tanglement entropies, extracted from statistical cor
relations in random measurements, and variational
entangled state preparation for optimal quantum pa
rameter estimation in quantum sensing.
[1] Theory collaborators: C. Kokail, R.M.W van Bijnen,
P. Silvi, C. Muschik, A. Elben and B. Vermersch
[2] Experimental collaborators: P. Jurcevic, C. Maier,
T. Brydges, P. Schindler, R. Stricker, E. Martinez,
T. Monz, B. P. Lanyon, C. Roos and R. Blatt.
− 0 100 200 300 400 500 600 700 800 900 4
−2
0
2
4
6
Number of Iterations
⟨ () ˆHS  ()⟩
Experimental data
Theoretically evaluated data
Ground state energy
1st excited state energy
Verification
Figure 1: Convergence of the energy expectation value to
wards the ground state of the Schwinger model ^H
S, as a
function of the number of iterations. We use a total bud
get of 3 104 projective measurements for the whole op
timisation. Orange connected points indicate experimen
tal data, whereas grey dots represent theoretically cal
culated expectation values. The triangles indicate points
for which we verify our quantum simulation by measuring
the Hamiltonian variance (see Figure 2)
0
2
4
6
⟨( ˆHS − EG)2⟩ /Δ2
Theory
Experiment
1 2 3 4 5 6
0
5
10
15
⟨( ˆHS − EG)2⟩
Experiment
Theory
Figure 2: Abinitio (self) verification of energy for the
variational ground state preparation of the Schwinger
model. For parameter vectors, corresponding to the
triangles indicated in Figure 1, we measure the Hamilto
nian variance h( ^H
S

P. Zoller Quantum Computing & Quantum Communication,
win² Zukunftskonferenz 2018 (Schloss Esterházy, Eisenstadt, 20180608) URL (20180610),
(ID: 720031)
Toggle Abstract
From Quantum Science to Quantum Technology

P. Zoller 2018 Norman F. Ramsey Prize in Atomic, Molecular and Optical Physics, and in Precision Tests of Fundamental Laws and Symmetries Recipient talk: Atomic Quantum Simulation 2.0,
49th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics APS Meeting (Ft. Lauderdale, Florida , 20180528) (20180529),
(ID: 720030)
Toggle Abstract
Atomic physics provides us with the realization of engineered quantum manybody lattice models as quantum simulators. This includes Hubbard models for bosonic and fermionic cold atoms in optical lattices, and spin models with Rydberg atoms and chains of trapped ions. Among the noticeable recent experimental advances are complete quantum control, and single shot measurements in lattice systems of atoms and ions achieving single site resolution, as illustrated by the quantum gas microscope. In this talk we focus from a theory perspective on new opportunities provided by these experimental advances, which blur the the traditional line between 'quantum computing' and 'quantum simulation'. Topics of interest are measurement protocols for Renyi entropies applicable across all atomic platforms, which quantifies entanglement through 'random measurements' on single copies of a quantum system. Furthermore, these developments open the door to implementing Quantum Approximate Optimization Algorithms (QAOA) on various atomic platforms, as a quantum simulator interacting with a classical computer in a feedback loop. We illustrate this approach with finding approximate ground states for high energy models of lattice gauge theories with trapped ions and Rydberg arrays. We conclude with remarks on applying QAOA to quantum metrology, including Ramsey spectroscopy.

P. Zoller Introduction to Quantum Optics ,
Modeling and Control of Open Quantum Systems (Centre International de Rencontres Mathématiques, CIRM Luminy, Marseille, 20180416) URL (20180417),
(ID: 720017)
Toggle Abstract
Quantum optical systems provides one of the best physical settings to engineer quantum manybody systems of atoms and photons, which can be controlled and measured on the level of single quanta. In this course we will provide an introduction to quantum optics from the perspective of control and measurement, and in light of possible applications including quantum computing and quantum communication.
The first part of the course will introduce the basic quantum optical systems and concepts as 'closed' (i.e. isolated) quantum systems. We start with laser driven twolevel atoms, the JaynesCummings model of Cavity Quantum Electrodynamics, and illustrate with the example of trapped ions control of the quantum motion of atoms via laser light. This will lead us to the model system of an ion trap quantum computer where we employ control ideas to design quantum gates.
In the second part of the course we will consider open quantum optical systems. Here the system of interest is coupled to a bosonic bath or environment (e.g. vacuum modes of the radiation field), providing damping and decoherence. We will develop the theory for the example of a radiatively damped twolevel atom, and derive the corresponding master equation, and discuss its solution and physical interpretation. On a more advanced level, and as link to the mathematical literature, we establish briefly the connection to topics like continuous measurement theory (of photon counting), the Quantum Stochastic Schrödinger Equation, and quantum trajectories (here as as time evolution of a radiatively damped atom conditional to observing a given photon count trajectory). As an example of the application of the formalism we discuss quantum state transfer in a quantum optical network.

P. Zoller Quantum Information Processing with Quantum Optical Systems,
2017 Dirac Medal Award Ceremony (ICTP, Trieste, 20180314) URL (20180314),
(ID: 720016)

P. Zoller Renyi Entropies from Random Quenches in Atomic Hubbard and Spin Models,
FINESS 2018, Finite Temperature Nonequilibrium Superfluid Systems (Wanaka, 20180219) URL (20180222),
(ID: 720014)

P. Zoller Renyi Entropies from Random Quenches in Atomic Hubbard and Spin Models,
Chaotic paths to cold atom physics, Kuba Zakrzewski 60th birthday celebration (Jagiellonian University, Krakow, 20180210) URL (20180210),
(ID: 720015)

P. Zoller FreeSpace’ Chiral Quantum Optics  From Quantum Antenna to 'Free Space' Quantum Links,
PQE2018 (Snowbird, 20180107) URL (20180108),
(ID: 719965)

J. Perczel, J. Borregaard, D. Chang, H. Pichler, S. F. Yelin, P. Zoller, M. Lukin Photonic band structure of twodimensional atomic lattices,
Phys. Rev. A 96 63801 (20171204),
http://dx.doi.org/10.1103/PhysRevA.96.063801 doi:10.1103/PhysRevA.96.063801 (ID: 719916)
Toggle Abstract
Twodimensional atomic arrays exhibit a number of intriguing quantum optical phenomena, including subradiance, nearly perfect reflection of radiation, and longlived topological edge states. Studies of emission and scattering of photons in such lattices require complete treatment of the radiation pattern from individual atoms, including longrange interactions. We describe a systematic approach to perform the calculations of collective energy shifts and decay rates in the presence of such longrange interactions for arbitrary twodimensional atomic lattices. As applications of our method, we investigate the topological properties of atomic lattices both in free space and near plasmonic surfaces.
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F. Reiter, A. Sorensen, P. Zoller, C. A. Muschik Autonomous Quantum Error Correction and Application to Quantum Sensing with Trapped Ions,
Nat. Commun. 8 (20171128),
http://dx.doi.org/10.1038/s41467017018955 doi:10.1038/s41467017018955 (ID: 719766)
Toggle Abstract
Quantumenhanced measurements hold the promise to improve highprecision sensing ranging from the definition of time standards to the determination of fundamental constants of nature. However, quantum sensors lose their sensitivity in the presence of noise. To protect them, the use of quantum error correcting codes has been proposed. Trapped ions are an excellent technological platform for both quantum sensing and quantum error correction. Here we present a quantum error correction scheme that harnesses dissipation to stabilize a trappedion qubit. In our approach, alwayson couplings to an engineered environment protect the qubit against spin or phase flips. Our dissipative error correction scheme operates in a fully autonomous manner without the need to perform measurements or feedback operations. We show that the resulting enhanced coherence time translates into a significantly enhanced precision for quantum measurements. Our work constitutes a stepping stone towards the paradigm of selfcorrecting quantum information processing.
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C. A. Muschik, M. Heyl, E. A. Martínez, T. Monz, P. Schindler, B. Vogell, M. Dalmonte, P. Hauke, R. Blatt, P. Zoller U(1) Wilson lattice gauge theories in digital quantum simulators,
New J. Phys. 19 103020 (20171020),
http://dx.doi.org/10.1088/13672630/aa89ab doi:10.1088/13672630/aa89ab (ID: 719936)
Toggle Abstract
Lattice gauge theories describe fundamental phenomena in nature, but calculating their realtime dynamics on classical computers is notoriously difficult. In a recent publication (Martinez et al 2016 Nature 534 516), we proposed and experimentally demonstrated a digital quantum simulation of the paradigmatic Schwinger model, a U(1)Wilson lattice gauge theory describing the interplay between fermionic matter and gauge bosons. Here, we provide a detailed theoretical analysis of the performance and the potential of this protocol. Our strategy is based on analytically integrating out the gauge bosons, which preserves exact gauge invariance but results in complicated longrange interactions between the matter fields. Trappedion platforms are naturally suited to implementing these interactions, allowing for an efficient quantum simulation of the model, with a number of gate operations that scales polynomially with system size. Employing numerical simulations, we illustrate that relevant phenomena can be observed in larger experimental systems, using as an example the production of particle–antiparticle pairs after a quantum quench. We investigate theoretically the robustness of the scheme towards generic error sources, and show that nearfuture experiments can reach regimes where finitesize effects are insignificant. We also discuss the challenges in quantum simulating the continuum limit of the theory. Using our scheme, fundamental phenomena of lattice gauge theories can be probed using a broad set of experimentally accessible observables, including the entanglement entropy and the vacuum persistence amplitude.

H. Pichler, S. Choi, P. Zoller, M. Lukin Universal photonic quantum computation via timedelayed feedback,
PNAS 114 (20171017),
http://dx.doi.org/10.1073/pnas.1711003114 doi:10.1073/pnas.1711003114 (ID: 720008)
Toggle Abstract
We propose and analyze a deterministic protocol to generate twodimensional photonic cluster states using a single quantum emitter via timedelayed quantum feedback. As a physical implementation, we consider a single atom or atomlike system coupled to a 1D waveguide with a distant mirror, where guided photons represent the qubits, while the mirror allows the implementation of feedback. We identify the class of manybody quantum states that can be produced using this approach and characterize them in terms of 2D tensor network states.

H. Pichler, S. Choi, P. Zoller, M. Lukin Photonic tensor networks produced by a single quantum emitter,
PNAS 114 11362 (20171010),
http://dx.doi.org/10.1073/pnas.1711003114 doi:10.1073/pnas.1711003114 (ID: 719751)
Toggle Abstract
We propose and analyze a protocol to generate two dimensional tensor network states using a single quantum system that sequentially interacts with a 1D string of qubits. This is accomplished by using parts of the string itself as a quantum queue memory. As a physical implementation, we consider a single atom or atom like system coupled to a 1D waveguide with a distant mirror, where guided photons represent the qubits while the mirror allows the implementation of the queue memory. We identify the class of manybody quantum states that can be produced using this approach. These include universal resources for measurement based quantum computation and states associated with topologically ordered phases. We discuss an explicit protocol to deterministically create a 2D cluster state in a quantum nanophotonic experiment, that allows for a realization of a quantum computer using a single atom coupled to light.
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A. Glätzle, K. Ender, D. S. Wild, S. Choi, H. Pichler, M. Lukin, P. Zoller Quantum Spin Lenses in Atomic Arrays,
Phys. Rev. X 7 31049 (20170920),
http://dx.doi.org/10.1103/PhysRevX.7.031049 doi:10.1103/PhysRevX.7.031049 (ID: 719790)
Toggle Abstract
We propose and discuss `quantum spin lenses', where quantum states of delocalized spin excitations in an atomic medium are `focused' in space in a coherent quantum process down to (essentially) single atoms. These can be employed to create controlled interactions in a quantum lightmatter interface, where photonic qubits stored in an atomic ensemble are mapped to a quantum register represented by single atoms. We propose Hamiltonians for quantum spin lenses as inhomogeneous spin models on lattices, which can be realized with Rydberg atoms in 1D, 2D and 3D, and with strings of trapped ions. We discuss both linear and nonlinear quantum spin lenses: in a nonlinear lens, repulsive spinspin interactions lead to focusing dynamics conditional to the number of spin excitations. This allows the mapping of quantum superpositions of delocalized spin excitations to superpositions of spatial spin patterns, which can be addressed by light fields and manipulated. Finally, we propose multifocal quantum spin lenses as a way to generate and distribute entanglement between distant atoms in an atomic lattice array.
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P. Guimond, M. Pletyukhov, H. Pichler, P. Zoller Delayed Coherent Quantum Feedback from a Scattering Theory and a Matrix Product State Perspective,
Quantum Sci. Technol. 2 44012 (20170908),
http://dx.doi.org/10.1088/20589565/aa7f03 doi:10.1088/20589565/aa7f03 (ID: 719814)
Toggle Abstract
We study the scattering of photons propagating in a semiinfinite waveguide terminated by a mirror and interacting with a quantum emitter. This paradigm constitutes an example of coherent quantum feedback, where light emitted towards the mirror gets redirected back to the emitter. We derive an analytical solution for the scattering of twophoton states, which is based on an exact resummation of the perturbative expansion of the scattering matrix, in a regime where the time delay of the coherent feedback is comparable to the timescale of the quantum emitter's dynamics. We compare the results with numerical simulations based on matrix product state techniques simulating the full dynamics of the system, and extend the study to the scattering of coherent states beyond the lowpower limit.
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D. T. Tran, A. Dauphin, A. G. Grushin, P. Zoller, N. Goldman Probing topology by “heating”: Quantized circular dichroism in ultracold atoms,
Sci. Adv. 3 e1701207 (20170818),
http://dx.doi.org/10.1126/sciadv.1701207 doi:10.1126/sciadv.1701207 (ID: 719782)
Toggle Abstract
We reveal an intriguing manifestation of topology, which appears in the depletion rate of topological states of matter in response to an external drive. This phenomenon is presented by analyzing the response of a generic 2D Chern insulator subjected to a circular timeperiodic perturbation: due to the system's chiral nature, the depletion rate is shown to depend on the orientation of the circular shake. Most importantly, taking the difference between the rates obtained from two opposite orientations of the drive, and integrating over a proper drivefrequency range, provides a direct measure of the topological Chern number of the populated band (ν ): this "differential integrated rate" is directly related to the strength of the driving field through the quantized coefficient η 0 =ν/ℏ 2 . Contrary to the integer quantum Hall effect, this quantized response is found to be nonlinear with respect to the strength of the driving field and it explicitly involves interband transitions. We investigate the possibility of probing this phenomenon in ultracold gases and highlight the crucial role played by edge states in this effect. We extend our results to 3D lattices, establishing a link between depletion rates and the nonlinear photogalvanic effect predicted for Weyl semimetals. The quantized effect revealed in this work designates depletionrate measurements as a universal probe for topological order in quantum matter.
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J. Perczel, J. Borregaard, D. Chang, H. Pichler, S. F. Yelin, P. Zoller, M. Lukin Topological Quantum Optics in TwoDimensional Atomic Arrays,
Phys. Rev. Lett. 119 23603 (20170714),
http://dx.doi.org/10.1103/PhysRevLett.119.023603 doi:10.1103/PhysRevLett.119.023603 (ID: 719767)
Toggle Abstract
We demonstrate that twodimensional atomic arrays with subwavelength spacing can be used to create topologically protected quantum optical systems where the photon propagation is robust against large imperfections while losses associated with free space emission are strongly suppressed. Breaking timereversal symmetry with a magnetic field results in gapped photonic bands with nontrivial Chern numbers. Such a system displays topologically protected bound states and unidirectional emission by individual atoms into longlived edge states. Possible experimental realizations and applications are discussed.
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A. Glätzle, R. van Bijnen, P. Zoller, W. Lechner A Coherent Quantum Annealer with Rydberg Atoms,
Nat. Commun. 8 15813 (20170622),
http://dx.doi.org/10.1038/ncomms15813 doi:10.1038/ncomms15813 (ID: 719686)
Toggle Abstract
There is a significant ongoing effort in realizing quantum annealing with different physical platforms. The challenge is to achieve a fully programmable quantum device featuring coherent adiabatic quantum dynamics. Here we show that combining the welldeveloped quantum simulation toolbox for Rydberg atoms with the recently proposed LechnerHaukeZoller~(LHZ) architecture allows one to build a prototype for a coherent adiabatic quantum computer with alltoall Ising interactions and, therefore, a novel platform for quantum annealing. In LHZ a infiniterange spinglass is mapped onto the low energy subspace of a spin1/2 lattice gauge model with quasilocal 4body parity constraints. This spin model can be emulated in a natural way with Rubidium and Cesium atoms in a bipartite optical lattice involving laserdressed RydbergRydberg interactions, which are several orders of magnitude larger than the relevant decoherence rates. This makes the exploration of coherent quantum enhanced optimization protocols accessible with stateoftheart atomic physics experiments.
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F. Iemini, L. Mazza, L. Fallani, P. Zoller, R. Fazio, M. Dalmonte Majorana QuasiParticles Protected by Z 2 Angular Momentum Conservation,
Phys. Rev. Lett. 118 200404 (20170519),
http://dx.doi.org/10.1103/PhysRevLett.118.200404 doi:10.1103/PhysRevLett.118.200404 (ID: 719762)
Toggle Abstract
We show how angular momentum conservation can stabilise a symmetryprotected quasitopological phase of matter supporting Majorana quasiparticles as edge modes in onedimensional cold atom gases. We investigate a numberconserving fourspecies Hubbard model in the presence of spinorbit coupling. The latter reduces the global spin symmetry to an angular momentum parity symmetry, which provides an extremely robust protection mechanism that does not rely on any coupling to additional reservoirs. The emergence of Majorana edge modes is elucidated using field theory techniques, and corroborated with densitymatrixrenormalizationgroup simulations. Our results pave the way toward the observation of Majorana edge modes with alkalineearthlike fermions in optical lattices, where all basic ingredients for our recipe  spinorbit coupling and strong interorbital interactions  have been experimentally realized over the last two years.
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C. Laflamme, D. Yang, P. Zoller Continuous Measurement of an Atomic Current,
Phys. Rev. A 95 43843 (20170428),
http://dx.doi.org/10.1103/PhysRevA.95.043843 doi:10.1103/PhysRevA.95.043843 (ID: 719758)
Toggle Abstract
We are interested in dynamics of quantum manybody systems under continuous observation, and its physical realizations involving cold atoms in lattices. In the present work we focus on continuous measurement of atomic currents in lattice models, including the Hubbard model. We describe a Cavity QED setup, where measurement of a homodyne current provides a faithful representation of the atomic current as a function of time. We employ the quantum optical description in terms of a diffusive stochastic Schr\"odinger equation to follow the time evolution of the atomic system conditional to observing a given homodyne current trajectory, thus accounting for the competition between the Hamiltonian evolution and measurement backaction. As an illustration, we discuss minimal models of atomic dynamics and continuous current measurement on rings with synthetic gauge fields, involving both real space and synthetic dimension lattices (represented by internal atomic states). Finally, by `not reading' the current measurements the time evolution of the atomic system is governed by a master equation, where  depending on the microscopic details of our CQED setups  we effectively engineer a current coupling of our system to a quantum reservoir. This provides novel scenarios of dissipative dynamics generating `dark' pure quantum manybody states.
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J. Budich, A. Elben, M. Łącki, A. Sterdyniak, M. Baranov, P. Zoller Coupled Atomic Wires in a Synthetic Magnetic Field,
Phys. Rev. A 95 43632 (20170424),
http://dx.doi.org/10.1103/PhysRevA.95.043632 doi:10.1103/PhysRevA.95.043632 (ID: 719753)
Toggle Abstract
We propose and study systems of coupled atomic wires in a perpendicular synthetic magnetic field as a platform to realize exotic phases of quantum matter. This includes (fractional) quantum Hall states in arrays of many wires inspired by the pioneering work [Kane et al. PRL {\bf{88}}, 036401 (2002)], as well as Meissner phases and Vortex phases in doublewires. With one continuous and one discrete spatial dimension, the proposed setup naturally complements recently realized discrete counterparts, i.e.~the HarperHofstadter model and the two leg flux ladder, respectively. We present both an indepth theoretical study and a detailed experimental proposal to make the unique properties of the semicontinuous HarperHofstadter model accessible with cold atom experiments. For the minimal setup of a doublewire, we explore how a subwavelength spacing of the wires can be implemented. This construction increases the relevant energy scales by at least an order of magnitude compared to ordinary optical lattices, thus rendering subtle manybody phenomena such as Lifshitz transitions in Fermi gases observable in an experimentally realistic parameter regime. For arrays of many wires, we discuss the emergence of Chern bands with readily tunable flatness of the dispersion and show how fractional quantum Hall states can be stabilized in such systems. Using for the creation of optical potentials LaguerreGauss beams that carry orbital angular momentum, we detail how the coupled atomic wire setups can be realized in nonplanar geometries such as cylinders, discs, and tori.
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B. Vermersch, P. Guimond, H. Pichler, P. Zoller Quantum State Transfer via Noisy Photonic and Phononic Waveguides,
Phys. Rev. Lett. 118 133601 (20170327),
http://dx.doi.org/10.1103/PhysRevLett.118.133601 doi:10.1103/PhysRevLett.118.133601 (ID: 719696)
Toggle Abstract
We describe a quantum state transfer protocol, where a quantum state of photons stored in a first cavity can be faithfully transferred to a second distant cavity via an infinite 1D waveguide, while being immune to arbitrary noise (e.g. thermal noise) injected into the waveguide. We extend the model and protocol to a cavity QED setup, where atomic ensembles, or single atoms representing quantum memory, are coupled to a cavity mode. We present a detailed study of sensitivity to imperfections, and develop a quantum error correction protocol to account for random losses (or additions) of photons in the waveguide. Our numerical analysis is enabled by Matrix Product State techniques to simulate the complete quantum circuit, which we generalize to include thermal input fields. Our discussion applies both to photonic and phononic quantum networks.
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J. Budich, Y. Hu, P. Zoller Helical Floquet Channels in 1D Lattices,
Phys. Rev. Lett. 118 105302 (20170307),
http://dx.doi.org/10.1103/PhysRevLett.118.105302 doi:10.1103/PhysRevLett.118.105302 (ID: 719644)
Toggle Abstract
We show how dispersionless channels exhibiting perfect spinmomentum locking can arise in a 1D lattice model. While such spectra are forbidden by fermion doubling in static 1D systems, here we demonstrate their appearance in the stroboscopic dynamics of a periodically driven system. Remarkably, this phenomenon does not rely on any adiabatic assumptions, in contrast to the well known Thouless pump and related models of adiabatic spin pumps. The proposed setup is shown to be experimentally feasible with state of the art techniques used to control ultracold alkaline earth atoms in optical lattices.
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P. Lodahl, S. Mahmoodian, S. Stobbe, P. Schneeweiss, J. Volz, A. Rauschenbeutel, H. Pichler, P. Zoller Chiral Quantum Optics,
Nature 541 473 (20170126),
http://dx.doi.org/10.1038/nature21037 doi:10.1038/nature21037 (ID: 719634)

C. Dlaska, B. Vermersch, P. Zoller Robust quantum state transfer via topologically protected edge channels in dipolar arrays,
Quantum Sci. Technol. 2 15001 (20170105),
http://dx.doi.org/10.1088/20589565/2/1/015001 doi:10.1088/20589565/2/1/015001 (ID: 719597)
Toggle Abstract
We show how to realize quantum state transfer between distant qubits using the chiral edge states of a twodimensional topological spin system. Our implementation based on Rydberg atoms allows to realize the quantum state transfer protocol in state of the art experimental setups. In particular, we show how to adapt the standard state transfer protocol to make it robust against dispersive and disorder effects.
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P. Zoller The second quantum revolution,
Winter Symposium: New Era of Quantum Science & Engineering (Seoul National University, 20171215) (20171215),
(ID: 720012)
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P. Zoller The second quantum revolution,
Symposium for New Era of Quantum Science and Engineering (Samsung Advanced Institute of Technology (SAIT), Seoul/Suwon, 20171214) (20171214),
(ID: 720013)

P. Zoller Quantum LightAtom Interfaces,
Symposium A. Aspect: The Second Quantum Revolution (Berlin, 20171109) (20171110),
(ID: 719912)

P. Zoller Quantum Simulation with Cold Atoms: How to Measure Renyi Entropies,
Workshop ”The Message of Quantum Science II (Bielefeld, 20171106) URL (20171107),
(ID: 719911)

P. Zoller Towards a Quantum leap in Computing and Communication,
2017 Altius Conference at Oxford (Oxford, 20170929) URL (20170929),
(ID: 719902)

A. Elben, B. Vermersch, P. Zoller Measurement of entanglement dynamics in the manybody localized phase: A random matrix approach,
Joint Annual Meeting of SPS and ÖPG (Geneve) URL (20170824),
(ID: 719933)

P. Zoller Quantum Simulation with Cold Atoms,
28th International Conference on Low Temperature Physics (Gothenburg, 20170816) URL (20170812),
(ID: 719856)

P. Zoller Spin Models with Rydberg Arrays,
Workshop on "Frontiers of Interacting Systems of Rydberg Atoms" (Cambridge, ITAMP, 20170601) URL (20170602),
(ID: 719807)
Toggle Abstract
We discuss recent work in Innsbruck on realization and application of spin models in arrays of Rydberg atoms. This includes work on a coherent quantum annealer [1] following the LHZ architecture [2]. In addition, we discuss our recent proposal for quantum spin lens [3] as a quantum light matter interface, where initially delocalized spin excitations (qubits) on an atomic array are focused down to single atoms in linear and nonlinear dynamics.
[1] A Coherent Quantum Annealer with Rydberg Atoms, A. W. Glaetzle, R. M. W. van Bijnen, P. Zoller, W. Lechner, Nature Communications, in press (2017); arXiv:1611.02594
[2] A quantum annealing architecture with alltoall connectivity from local interactions, W. Lechner, P. Hauke, and P. Zoller, Science Advances 1, e1500838 (2015)
[3] Quantum Spin Lenses in Atomic Arrays, A. W. Glaetzle, K. Ender, D. S. Wild, S. Choi, H. Pichler, M. D. Lukin, P. Zoller, arXiv:1704.08837

P. Zoller Linear and Nonlinear ‘Quantum SpinLenses’ in Atomic Arrays,
Quantum Fluids of Light and Matter (Cargese, 20170508) URL (20170508),
(ID: 719802)
Toggle Abstract
In quantum information processing with atoms, qubits are typically represented by internal
atomic states, e.g. as longlived spin excitations within the atomic ground state manifold. Ideally,
qubits are stored in single atoms, and for these qubits to be identifiable and addressable,
we typically require localization of the atoms in welldefined spatial regions. Spatial control,
and localization of single atoms is a prerequisite to implement single and twoqubit operations,
allowing addressing of individual qubits with laser light, and providing entangling operations
between adjacent qubits. Recent experiments have demonstrated in a remarkable way the basic
ingredients of single atom manipulation and addressing for trapped atoms and ions, and controlled
interaction and entanglement between atomic spinqubits with Rydberg atoms, trapped
ions, and cavity QED setups as lightmatter quantum interfaces.
In contrast to localized spinqubits, atomic ensembles provide us with qubits in form of
delocalized spinexcitations. Delocalized spinqubits arise naturally in lightatomic ensemble
interfaces in both free space and cavity assisted setups. Here incident photons representing a
‘flying qubit’ are absorbed in an atomic ensemble with enhanced interactions benefiting from a
large atom number N, as in an optically thick medium, and converted into a spinexcitation,
which may well be delocalized over the whole atomic cloud. The challenge is, therefore, to
convert delocalized spinqubits into localized qubits in the atomic array representing quantum
memory. Thus ideally we want operations – a lens for spin excitations – on the atomic array,
which allow in a coherent process ‘focusing’ of qubits to a welldefined and localized region, and
ultimately to a single atom.
It is the purpose of this talk to propose and discuss linear and nonlinear ‘quantum spinlenses’
and their physical realization with cold atoms and ions [1]. We will first of all identify
Hamiltonians to realize linear spinlenses, which map in a coherent process a delocalized to
localized spinexcitation, and vice versa. We discuss application as a (novel) quantum atomlight
interface, where incident photonic qubits are sequentially stored in an atomic array, and
focused to a quantum register of spatially localized spinqubits. More generally, we will discuss
the design of nonlinear spinlenses, adding finite range (repulsive) spinspin interactions to the
spinlens Hamiltonian. Thus focusing dynamics will be conditional to the number of initial spin
excitations, and an initial quantum superposition state of delocalized spins will be mapped to
superposition of spatial spin patterns. This provides a (novel) tool to manipulate quantum states
of spin excitations by spatial addressing in the atomic medium. Finally, we can generalize the
concept of a ‘quantum spinlens’ to multifocal lenses, with two or more foci. E.g. with a (linear)
multifocal lens a delocalized spinexcitation is mapped to spatial EPRlike superposition state,
providing a way to distribute or generate entanglement between (distant) atoms. As noted
above, the relevant spinmodels are naturally implemented in existing quantum optical setups.
This includes neutral atoms with (laser dressed) Rydbergmediated spinspin interactions in 1D,
2D and 3D atomic lattices, as well as with strings of trapped ions representing 1D spinmodels.
In a broader context, ‘Quantum spinlenses’ for spatial manipulation of spinexcitations
should also be of interest in quantum simulation, and as (coherent) quantum spintronics.
Acknowledgment: Work performed in collaboration with coauthors listed in [1].

P. Zoller Quantum Simulation with Quantum Optical Systems: Atomic physics meets condensed matter and high energy physics,
38th Heidelberg Physics Graduate Days (Heidelberg, 20170410) URL (20170412),
(ID: 719786)

P. Zoller Chiral Quantum Optics and Quantum Networks,
The first annual symposium on Quantum Science and Quantum Engineering (Würzburg, 20170404) URL (20170405),
(ID: 719785)

P. Zoller Quantum Simulation of Lattice Gauge Theories with Cold Atoms and Ions,
DPG Spring Meeting (Mainz, 20170306) URL (20170310),
(ID: 719784)

P. Zoller Synthetic Quantum Matter with Cold Atoms and Ions,
Conference on 90 Years of Quantum Mechanics (Singapore, 20170123) URL (20170125),
(ID: 719783)

M. Leib, P. Zoller, W. Lechner A Transmon Quantum Annealer: Decomposing ManyBody Ising Constraints Into Pair Interactions,
Quantum Sci. Technol. 1 15008 (20161216),
http://dx.doi.org/10.1088/20589565/1/1/015008 doi:10.1088/20589565/1/1/015008 (ID: 719543)
Toggle Abstract
Adiabatic quantum computing is an analog quantum computing scheme with various applications in solving optimization problems. In the parity picture of quantum optimization, the problem is encoded in local fields that act on qubits which are connected via local 4body terms. We present an implementation of a parity annealer with Transmon qubits with a specifically tailored Ising interaction from Josephson ring modulators.

M. Łącki, M. Baranov, H. Pichler, P. Zoller NanoScale `Dark State' Optical Potentials for Cold Atoms,
Phys. Rev. Lett. 117 233001 (20161130),
http://dx.doi.org/10.1103/PhysRevLett.117.233001 doi:10.1103/PhysRevLett.117.233001 (ID: 719619)
Toggle Abstract
We discuss generation of subwavelength optical barriers on the scale of tens of nanometers, as conservative optical potentials for cold atoms. These arise from nonadiabatic corrections to BornOppenheimer potentials from dressed `dark states' in atomic Λ configurations. We illustrate the concepts with a double layer potential for atoms obtained from inserting an optical subwavelength barrier into a well generated by an offresonant optical lattice, and discuss bound states of pairs of atoms interacting via magnetic dipolar interactions. The subwavelength optical barriers represent an optical `KronigPenney' potential. We present a detailed study of the bandstructure in optical `KronigPenney' potentials, including decoherence from spontaneous emission and atom loss to open `bright' channels.

D. Yang, G. S. Giri, M. Johannig, C. Wunderlich, P. Zoller, P. Hauke Analog Quantum Simulation of (1+1)D Lattice QED with Trapped Ions,
Phys. Rev. A 94 52321 (20161116),
http://dx.doi.org/10.1103/PhysRevA.94.052321 doi:10.1103/PhysRevA.94.052321 (ID: 719545)
Toggle Abstract
The prospect of quantum simulating lattice gauge theories opens exciting possibilities for understanding fundamental forms of matter. Here, we show that trapped ions represent a promising platform in this context when simultaneously exploiting internal pseudospins and external phonon vibrations. We illustrate our ideas with two complementary proposals for simulating latticeregularized quantum electrodynamics (QED) in (1+1) spacetime dimensions. The first scheme replaces the gauge fields by local vibrations with a high occupation number. By numerical finitesize scaling, we demonstrate that this model recovers Wilson's lattice gauge theory in a controlled way. Its implementation can be scaled up to tens of ions in an array of microtraps. The second scheme represents the gauge fields by spins 1/2, and thus simulates a quantum link model. As we show, this allows the fermionic matter to be replaced by bosonic degrees of freedom, permitting smallscale implementations in a linear Paul trap. Both schemes work on energy scales significantly larger than typical decoherence rates in experiments, thus enabling the investigation of phenomena such as string breaking, Coleman's quantum phase transition, and falsevacuum decay. The underlying ideas of the proposed analog simulation schemes may also be adapted to other platforms, such as superconducting qubits.

Y. Hu, P. Zoller, J. Budich Dynamical Buildup of a Quantized Hall Response from NonTopological States,
Phys. Rev. Lett. 117 126803 (20160916),
http://dx.doi.org/10.1103/PhysRevLett.117.126803 doi:10.1103/PhysRevLett.117.126803 (ID: 719515)
Toggle Abstract
Motivated by the current interest in dynamically preparing topological states in ultracold atomic gases, we consider a twodimensional system initialized in a topologically trivial state before its Hamiltonian is ramped into a Cherninsulator phase. Under coherent dynamics, the nonequilibrium Hall response is found to approach a topologically quantized time averaged value in the limit of slow parameter ramps, even though the Chern number of the state is constrained to remain trivial. Quite remarkably, the destruction of quantum coherence by dephasing stabilizes the Hall response towards its asymptotically quantized mean value by damping its oscillations. We demonstrate how this phenomenology generically arises from the interplay of LandauZener dynamics and dephasing. In the limit of a fast ramp (quench), we show how the presence of a cooling quantum bath enables a dynamical transition of the state's Chern number from trivial to nontrivial, accompanied by the onset of a quantized Hall response.
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P. Guimond, H. Pichler, A. Rauschenbeutel, P. Zoller Chiral quantum optics with Vlevel atoms and coherent quantum feedback,
Phys. Rev. A 94 33829 (20160916),
http://dx.doi.org/10.1103/PhysRevA.94.033829 doi:10.1103/PhysRevA.94.033829 (ID: 719590)
Toggle Abstract
We study the dissipative dynamics of an atom in a Vlevel configuration driven by lasers and coupled to a semiinfinite waveguide. The coupling to the waveguide is chiral, in that each transition interacts only with the modes propagating in a given direction, and this direction is opposite for the two transitions. The waveguide is terminated by a mirror which coherently feeds the photon stream emitted by one transition back to the atom. First, we are interested in the dynamics of the atom in the Markovian limit where the timedelay in the feedback is negligible. Specifically, we study the conditions under which the atom evolves towards a pure "dark" stationary state, where the photons emitted by both transitions interfere destructively thanks to the coherent feedback, and the overall emission vanishes. This is a singleatom analogue of the quantum dimer, where a pair of laserdriven twolevel atoms is coupled to a unidirectional waveguide and dissipates towards a pure entangled dark state. Our setup should be feasible with current stateoftheart experiments. Second, we extend our study to nonMarkovian regimes and investigate the effect of the feedback retardation on the steadystate.
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C. Laflamme, J. Budich, P. Zoller, M. Dalmonte Nonequilibrium 8π Josephson Effect in Atomic Kitaev Wires,
Nat. Commun. 7 12280 (20160802),
http://dx.doi.org/10.1038/ncomms12280 doi:10.1038/ncomms12280 (ID: 719504)
Toggle Abstract
We theoretically study a Kitaev wire interrupted by an extra site which gives rise to super exchange coupling between two Majorana bound states. We show that this system hosts a tunable, nonequlibrium Josephson effect with a characteristic 8π periodicity of the Josephson current. We elucidate the physical mechanism deriving a minimal model for the junction and confirm its quantitative accuracy by comparison to the numerical solution of the full model. The visibility of the 8π periodicity of the Josephson current is then studied using timedependent simulations including the effects of dephasing and particle losses. Our findings provide a novel signature of Majorana quasiparticles which is qualitatively different form the behavior of a conventional superconductor, and can be experimentally verified in cold atom systems using alkalineearthlike atoms.

E. A. Martínez, C. A. Muschik, P. Schindler, D. Nigg, A. Erhard, M. Heyl, P. Hauke, M. Dalmonte, T. Monz, P. Zoller, R. Blatt Realtime dynamics of lattice gauge theories with a fewqubit quantum computer,
Nature 534 519 (20160622),
http://dx.doi.org/10.1038/nature18318 doi:10.1038/nature18318 (ID: 719563)
Toggle Abstract
Gauge theories are fundamental to our understanding of interactions between the elementary constituents of matter as mediated by gauge bosons. However, computing the realtime dynamics in gauge theories is a notorious challenge for classical computational methods. In the spirit of Feynman's vision of a quantum simulator, this has recently stimulated theoretical effort to devise schemes for simulating such theories on engineered quantummechanical devices, with the difficulty that gauge invariance and the associated local conservation laws (Gauss laws) need to be implemented. Here we report the first experimental demonstration of a digital quantum simulation of a lattice gauge theory, by realising 1+1dimensional quantum electrodynamics (Schwinger model) on a fewqubit trappedion quantum computer. We are interested in the realtime evolution of the Schwinger mechanism, describing the instability of the bare vacuum due to quantum fluctuations, which manifests itself in the spontaneous creation of electronpositron pairs. To make efficient use of our quantum resources, we map the original problem to a spin model by eliminating the gauge fields in favour of exotic longrange interactions, which have a direct and efficient implementation on an ion trap architecture. We explore the Schwinger mechanism of particleantiparticle generation by monitoring the mass production and the vacuum persistence amplitude. Moreover, we track the realtime evolution of entanglement in the system, which illustrates how particle creation and entanglement generation are directly related. Our work represents a first step towards quantum simulating highenergy theories with atomic physics experiments, the longterm vision being the extension to realtime quantum simulations of nonAbelian lattice gauge theories.

B. Vermersch, T. Ramos, P. Hauke, P. Zoller Implementation of Chiral Quantum Optics with Rydberg and Trappedion Setups,
Phys. Rev. A 93 63830 (20160617),
http://dx.doi.org/10.1103/PhysRevA.93.063830 doi:10.1103/PhysRevA.93.063830 (ID: 719538)
Toggle Abstract
We propose two setups for realizing a chiral quantum network, where twolevel systems representing the nodes interact via directional emission into discrete waveguides, as introduced in Ref.~[T.\ Ramos \emph{et al.}, arXiv:1602.00926]. The first implementation realizes a spin waveguide via Rydberg states in a chain of atoms, whereas the second one realizes a phonon waveguide via the localized vibrations of a string of trapped ions. For both architectures, we show that strong chirality can be obtained by a proper design of synthetic gauge fields in the couplings from the nodes to the waveguide. In the Rydberg case, this is achieved via intrinsic spinorbit coupling in the dipoledipole interactions, while for the trapped ions it is obtained by engineered sideband transitions. We take longrange couplings into account that appear naturally in these implementations, discuss useful experimental parameters, and analyze potential error sources. Finally, we describe effects that can be observed in these implementations within stateoftheart technology, such as the drivendissipative formation of entangled dimer states.
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T. Ramos, B. Vermersch, P. Hauke, H. Pichler, P. Zoller NonMarkovian Dynamics in Chiral Quantum Networks with Spins and Photons,
Phys. Rev. A 93 62104 (20160602),
http://dx.doi.org/10.1103/PhysRevA.93.062104 doi:10.1103/PhysRevA.93.062104 (ID: 719497)
Toggle Abstract
We study the dynamics of chiral quantum networks consisting of nodes coupled by unidirectional or asymmetric bidirectional quantum channels. In contrast to the familiar photonic networks consisting of driven twolevel atoms exchanging photons via 1D photonic nanostructures, we propose and study a setup where interactions between the atoms are mediated by spin excitations (magnons) in 1D XXspin chains representing a spin waveguide. While Markovian quantum network theory eliminates quantum channels as structureless reservoirs in a BornMarkov approximation to obtain a master equation for the nodes, we are interested in nonMarkovian dynamics. This arises from the nonlinear character of the dispersion with bandedge effects, and from finite spin propagation velocities leading to time delays in interactions. To account for the nonMarkovian dynamics we treat the quantum degrees of freedom of the nodes and connecting channel as a composite spin system with the surrounding of the quantum network as a Markovian bath, allowing for an efficient solution with timedependent density matrix renormalization group techniques. We illustrate our approach showing nonMarkovian effects in the drivendissipative formation of quantum dimers, and we present examples for quantum information protocols involving quantum state transfer with engineered elements as basic building blocks of quantum spintronic circuits.
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N. Goldman, J. Budich, P. Zoller Topological quantum matter with ultracold gases in optical lattices,
Nature Phys. 12 645 (20160525),
http://dx.doi.org/10.1038/nphys3803 doi:10.1038/nphys3803 (ID: 719596)
Toggle Abstract
Since the discovery of topological insulators, many topological phases have been predicted and realized in a range of different systems, providing both fascinating physics and exciting opportunities for devices. And although new materials are being developed and explored all the time, the prospects for probing exotic topological phases would be greatly enhanced if they could be realized in systems that were easily tuned. The flexibility offered by ultracold atoms could provide such a platform. Here, we review the tools available for creating topological states using ultracold atoms in optical lattices, give an overview of the theoretical and experimental advances and provide an outlook towards realizing strongly correlated topological phases.

H. Pichler, G. Zhu, A. Seif, P. Zoller, M. Hafezi A Measurement Protocol for the Entanglement Spectrum of Cold Atoms,
Phys. Rev. X 6 41033 (20160517),
http://dx.doi.org/10.1103/PhysRevX.6.041033 doi:10.1103/PhysRevX.6.041033 (ID: 719569)
Toggle Abstract
Entanglement plays a major role in characterizing manybody quantum systems. In particular, the entanglement spectrum holds a great promise to characterize essential physics of quantum manybody systems. While there has been a surge of theoretical works on the subject, no experimental measurement has been performed to this date, due to the lack of an implementable measurement scheme. Here, we propose a measurement protocol to access the entanglement spectrum of manybody states in experiments with cold atoms in optical lattices. Our scheme effectively performs a Ramsey spectroscopy of the entanglement Hamiltonian, and is based on the ability to produce several copies of the state under investigation together with the possibility to perform a global swap gate between two copies conditioned on the state of an auxiliary qubit. We show how the required conditional swap gate can be implemented with cold atoms, either by using Rydberg interactions or coupling the atoms to a cavity mode. We illustrate these ideas on a simple (extended) BoseHubbard model where such a measurement protocol reveals topological features of the Haldane phase.
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C. Laflamme, W. Evans, M. Dalmonte, U. Gerber, H. MejiaDiaz, W. Bietenholz, U. Wiese, P. Zoller CP(N1) Quantum Field Theories with AlkalineEarth Atoms in Optical Lattices,
Ann. Phys. 370 127 (20160423),
http://dx.doi.org/10.1016/j.aop.2016.03.012 doi:10.1016/j.aop.2016.03.012 (ID: 719312)
Toggle Abstract
We propose a cold atom implementation to attain the continuum limit of (1+1)d CP(N1) quantum field theories. These theories share important features with (3+1)d QCD, such as asymptotic freedom and θ vacua. Moreover, their continuum limit can be accessed via the mechanism of dimensional reduction. In our scheme, the CP(N1) degrees of freedom emerge at low energies from a ladder system of SU(N) quantum spins, where the N spin states are embodied by the nuclear Zeeman states of alkalineearth atoms, trapped in an optical lattice. Based on Monte Carlo results, we establish that the continuum limit can be demonstrated by an atomic quantum simulation by employing the feature of asymptotic freedom. We discuss a protocol for the adiabatic state preparation of the ground state of the system, the realtime evolution of a false θvacuum state after a quench, and we propose experiments to unravel the phase diagram at nonzero density.
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S. Baier, M. J. Mark, D. Petter, K. Aikawa, L. Chomaz, Z. Cai, M. Baranov, P. Zoller, F. Ferlaino Extended BoseHubbard Models with Ultracold Magnetic Atoms,
Science 352 205 (20160408),
http://dx.doi.org/10.1126/science.aac9812 doi:10.1126/science.aac9812 (ID: 719290)
Toggle Abstract
The Hubbard model underlies our understanding of strongly correlated materials. While its standard form only comprises interaction between particles at the same lattice site, its extension to encompass longrange interaction, which activates terms acting between different sites, is predicted to profoundly alter the quantum behavior of the system. We realize the extended BoseHubbard model for an ultracold gas of strongly magnetic erbium atoms in a threedimensional optical lattice. Controlling the orientation of the atomic dipoles, we reveal the anisotropic character of the onsite interaction and hopping dynamics, and their influence on the superfluidtoMott insulator quantum phase transition. Moreover, we observe nearestneighbor interaction, which is a genuine consequence of the longrange nature of dipolar interactions. Our results lay the groundwork for future studies of novel exotic manybody quantum phases.
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P. Hauke, M. Heyl, L. Tagliacozzo, P. Zoller Measuring multipartite entanglement via dynamic susceptibilities,
Nature Phys. 12 778 (20160321),
http://dx.doi.org/10.1038/nphys3700 doi:10.1038/nphys3700 (ID: 719333)
Toggle Abstract
Entanglement plays a central role in our understanding of quantum many body physics, and is
fundamental in characterising quantum phases and quantum phase transitions. Developing protocols
to detect and quantify entanglement of manyparticle quantum states is thus a key challenge for
present experiments. Here, we show that the quantum Fisher information, representing a witness for
genuinely multipartite entanglement, becomes measurable for thermal ensembles via the dynamic
susceptibility, i.e., with resources readily available in present cold atomic gas and condensedmatter
experiments. This moreover establishes a fundamental connection between multipartite entanglement
and manybody correlations contained in response functions, with profound implications close
to quantum phase transitions. There, the quantum Fisher information becomes universal, allowing
us to identify strongly entangled phase transitions with a divergent multipartiteness of entanglement.
We illustrate our framework using paradigmatic quantum Ising models, and point out potential signaturesin opticallattice experiments.
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T. Pichler, M. Dalmonte, E. Rico Ortega, P. Zoller, S. Montangero Realtime Dynamics in U(1) Lattice Gauge Theories with Tensor Networks,
Phys. Rev. X 6 11023 (20160303),
http://dx.doi.org/10.1103/PhysRevX.6.011023 doi:10.1103/PhysRevX.6.011023 (ID: 719259)
Toggle Abstract
Tensor networks algorithms provide a suitable route to tackle realtime dependent problems in lattice gauge theories, enabling the investigation of outofequilibrium dynamics. We analyze a U(1) lattice gauge theory in (1+1) dimensions in the presence of dynamical matter for different mass and electric field couplings, a theory akin to quantumelectrodynamics in onedimension, which displays stringbreaking: the confining string between charges can spontaneously break during quench exper iments, giving rise to chargeanticharge pairs according to the Schwinger mechanism. We study the realtime spreading of excitations in the system by means of electric field and particle fluctuations: We determine a dynamical state diagram for string breaking, and we quantitatively evaluate the timescales for mass production. We show also that the time evolution of the quantum correlations can be detected via bipartite von Neumann entropies, which demonstrates that the Schwinger mech anism is tightly linked to entanglement spreading. To present the variety of possible applications of this simulation platform, we show how one could follow the realtime scattering processes between mesons and the creation of entanglement during scattering processes. Finally, we access the quality of quantum simulations of these dynamics quantifying the role of possible imperfections in cold atoms, trapped ions, and superconducting circuit systems. Our results demonstrate how entan glement properties can be used to deepen our understanding of basic phenomena in the realtime dynamics of gauge theories such as string breaking and collisions.

H. Pichler, P. Zoller Photonic Quantum Circuits with Time Delays,
Phys. Rev. Lett. 116 93601 (20160303),
http://dx.doi.org/10.1103/PhysRevLett.116.093601 doi:10.1103/PhysRevLett.116.093601 (ID: 719362)
Toggle Abstract
We study the dynamics of photonic quantum circuits consisting of nodes coupled by quantum channels. We are interested in the regime where the time delay in communication between the nodes is significant. This includes the problem of quantum feedback, where a quantum signal is fed back on a system with a time delay. We develop a matrix product state approach to solve the quantum stochastic Schrödinger equation with time delays, which accounts in an efficient way for the entanglement of nodes with the stream of emitted photons in the waveguide, and thus the nonMarkovian character of the dynamics. We illustrate this approach with two paradigmatic quantum optical examples: two coherently driven distant atoms coupled to a photonic waveguide with a time delay, and a driven atom coupled to its own output field with a time delay as an instance of a quantum feedback problem.
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M. Łącki, H. Pichler, A. Sterdyniak, A. Lyras, V. Lembessis, O. AlDossary, J. Budich, P. Zoller Quantum Hall Physics with Cold Atoms in Cylindrical Optical Lattices,
Phys. Rev. A 93 13604 (20160107),
http://dx.doi.org/10.1103/PhysRevA.93.013604 doi:10.1103/PhysRevA.93.013604 (ID: 719282)
Toggle Abstract
We propose and study various realizations of a HofstadterHubbard model on a cylinder geometry with fermionic cold atoms in optical lattices. The cylindrical optical lattice is created by counterpropagating LaguerreGauss beams, i.e. light beams carrying orbital angular momentum. By strong focusing of the light beams we create a real space optical lattice in the form of rings, which are offset in energy. A second set of LaguerreGauss beams then induces a Ramanhopping between these rings, imprinting phases corresponding to a synthetic magnetic field (artificial gauge field). In addition, by rotating the lattice potential, we achieve a slowly varying flux through the hole of the cylinder, which allows us to probe the Hall response of the system as a realization of Laughlin's thought experiment. We study how in the presence of interactions fractional quantum Hall physics could be observed in this setup.
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J. Budich, C. Laflamme, F. Tschirsich, S. Montangero, P. Zoller Synthetic Helical Liquids with Ultracold Atoms in Optical Lattices,
Phys. Rev. B 92 245121 (20151214),
http://dx.doi.org/10.1103/PhysRevB.92.245121 doi:10.1103/PhysRevB.92.245121 (ID: 719243)
Toggle Abstract
We develop a new platform for the synthetic realization of helical Tomonaga Luttinger liquids (HTLLs) with ultracold fermionic atoms in onedimensional optical lattices. The HTLL is a strongly correlated metallic state where spin polarization and propagation direction of the itinerant particles are locked to each other. We propose an unconventional onedimensional FermiHubbard model which, at quarter filling, resembles the HTLL in the long wavelength limit, as we demonstrate with a combination of analytical (bosonization) and numerical (density matrix renormalization group) methods. An experimentally feasible scheme is provided for the realization of this model with ultracold fermionic atoms in optical lattices. Finally, we discuss how the robustness of the HTLL against backscattering and imperfections, well known from its realization at the edge of a twodimensional topological insulators, is reflected in the present synthetic scenario.
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W. Lechner, P. Hauke, P. Zoller A quantum annealing architecture with alltoall connectivity from local interactions,
Sci. Adv. 1 e1500838 (20151023),
http://dx.doi.org/10.1126/sciadv.1500838 doi:10.1126/sciadv.1500838 (ID: 719363)
Toggle Abstract
Quantum annealers are physical devices that aim at solving NPcomplete optimization problems by exploiting quantum mechanics. The basic principle of quantum annealing is to encode the optimization problem in Ising interactions between quantum bits (qubits). A fundamental challenge in building a fully programmable quantum annealer is the competing requirements of full controllable alltoall connectivity and the quasilocality of the interactions between physical qubits. We present a scalable architecture with full connectivity, which can be implemented with local interactions only. The input of the optimization problem is encoded in local fields acting on an extended set of physical qubits. The output is—in the spirit of topological quantum memories—redundantly encoded in the physical qubits, resulting in an intrinsic fault tolerance. Our model can be understood as a lattice gauge theory, where longrange interactions are mediated by gauge constraints. The architecture can be realized on various platforms with local controllability, including superconducting qubits, NVcenters, quantum dots, and atomic systems.
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Y. Hu, Z. Cai, M. Baranov, P. Zoller Majoranas in Noisy Kitaev Wires,
Phys. Rev. B 92 165118 (20151016),
http://dx.doi.org/10.1103/PhysRevB.92.165118 doi:10.1103/PhysRevB.92.165118 (ID: 719277)
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Robustness of edge states and nonAbelian excitations of topological states of matter promises quantum memory and quantum processing, which is naturally immune against microscopic imperfections such as static disorder. However, topological properties will not in general protect quantum system from timedependent disorder or noise. Here we take the example of a network of Kitaev wires with Majorana edge modes storing qubits to investigate the effects of classical noise in the crossover from the quasistatic to the fast fluctuation regime. We present detailed results for the Majorana edge correlations, and fidelity of braiding operations for both global and local noise sources preserving parity symmetry, such as random chemical potentials and phase fluctuations. While in general noise will induce heating and dephasing, we identify examples of longlived quantum correlations in presence of fast noise due to motional narrowing, where external noise drives the system rapidly between the topological and nontopological phases.
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M. Mancini, G. Pagano, G. Cappellini, L. Livi, M. Rider, J. Catani, C. Sias, P. Zoller, M. Inguscio, M. Dalmonte, L. Fallani Observation of chiral edge states with neutral fermions in synthetic Hall ribbons,
Science 349 1513 (20150925),
http://dx.doi.org/10.1126/science.aaa8736 doi:10.1126/science.aaa8736 (ID: 719172)
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Chiral edge states are a hallmark of quantum Hall physics. In electronic systems, they appear as a macroscopic consequence of the cyclotron orbits induced by a magnetic field, which are naturally truncated at the physical boundary of the sample. Here we report on the experimental realization of chiral edge states in a ribbon geometry with an ultracold gas of neutral fermions subjected to an artificial gauge field. By imaging individual sites along a synthetic dimension, we detect the existence of the edge states, investigate the onset of chirality as a function of the bulkedge coupling, and observe the edgecyclotron orbits induced during a quench dynamics. The realization of fermionic chiral edge states is a fundamental achievement, which opens the door towards experiments including edge state interferometry and the study of nonAbelian anyons in atomic systems.
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W. Lechner, P. Zoller Spatial Patterns of Rydberg Excitations from Logarithmic Pair Interactions,
Phys. Rev. Lett. 115 125301 (20150918),
http://dx.doi.org/10.1103/PhysRevLett.115.125301 doi:10.1103/PhysRevLett.115.125301 (ID: 719185)
Toggle Abstract
The collective excitations in ensembles of dissipative, laser driven ultracold atoms exhibit crystallike patterns, a manybody effect of the Rydberg blockade mechanism. These crystalline structure are revealed in experiment from a postselection of configurations with fixed numbers of excitations. Here, we show that these subensemble can be well represented by ensembles of effective particles that interact via logarithmic pair potentials. This allows one to study the emergent patterns with a small number of effective particles to determine the phases of Rydberg crystals and to systematically study contributions from N body terms.
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R. Nath, M. Dalmonte, A. Glätzle, P. Zoller, F. SchmidtKaler, R. Gerritsma Hexagonal Plaquette Spinspin Interactions and Quantum Magnetism in a Twodimensional Ion Crystal,
New J. Phys. 17 065018 (20150625),
http://dx.doi.org/10.1088/13672630/17/6/065018 doi:10.1088/13672630/17/6/065018 (ID: 719216)
Toggle Abstract
We propose a trapped ion scheme en route to realize spin Hamiltonians on a Kagome lattice which, at low energies, are described by emergent Z2 gauge fields, and support a topological quantum spin liquid ground state. The enabling element in our scheme is the hexagonal plaquette spinspin interactions in a 2D ion crystal. For this, the phononmode spectrum of the crystal is engineered by standingwave optical potentials or by using Rydberg excited ions, thus generating localized phononmodes around a hexagon of ions selected out of the entire twodimensional crystal. These tailored modes can mediate spinspin interactions between ionqubits on a hexagonal plaquette when subject to statedependent optical dipole forces. We discuss how these interactions can be employed to emulate a generalized BalentsFisherGirvin model in minimal instances of one and two plaquettes. This model is an archetypical Hamiltonian in which gauge fields are the emergent degrees of freedom on top of the classical ground state manifold. Under realistic situations, we show the emergence of a discrete Gauss's law as well as the dynamics of a deconfined charge excitation on a gaugeinvariant background using the twoplaquettes trapped ions spinsystem. The proposed scheme in principle allows further scaling in a future trapped ion quantum simulator, and we conclude that our work will pave the way towards the simulation of emergent gauge theories and quantum spin liquids in trapped ion systems.
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A. Glätzle, M. Dalmonte, R. Nath, C. Gross, I. Bloch, P. Zoller Designing Frustrated Quantum Magnets with LaserDressed Rydberg Atoms,
Phys. Rev. Lett. 114 173002 (20150428),
http://dx.doi.org/10.1103/PhysRevLett.114.173002 doi:10.1103/PhysRevLett.114.173002 (ID: 719027)
Toggle Abstract
We show how a broad class of lattice spin1/2 models with angular and distancedependent couplings can be realized with cold alkali atoms stored in optical or magnetic trap arrays. The effective spin1/2 is represented by a pair of atomic ground states, and spinspin interactions are obtained by admixing van der Waals interactions between finestructure split Rydberg states with laser light. The strengths of the diagonal spin interactions as well as the "flipflop", and "flipflip" and "flopflop" interactions can be tuned by exploiting quantum interference, thus realizing different spin symmetries. The resulting energy scales of interactions compare well with typical temperatures and decoherence timescales, making the exploration of exotic forms of quantum magnetism, including emergent gauge theories and compass models, accessible within stateoftheart experiments.
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B. Vogell, T. Kampschulte, M. Rakher, A. Faber, P. Treutlein, K. Hammerer, P. Zoller Long Distance Coupling of a Quantum Mechanical Oscillator to the Internal States of an Atomic Ensemble,
New J. Phys. 17 043044 (20150422),
http://dx.doi.org/10.1088/13672630/17/4/043044 doi:10.1088/13672630/17/4/043044 (ID: 719098)
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We propose and investigate a hybrid optomechanical system consisting of a micromechanical oscillator coupled to the internal states of a distant ensemble of atoms. The interaction between the systems is mediated by a light eld which allows to couple the two systems in a modular way over long distances. Coupling to internal degrees of freedom of atoms opens up the possibility to employ highfrequency mechanical resonators in the MHz to GHz regime, such as optomechanical crystal structures, and to benet from the rich toolbox of quantum control over internal atomic states. Previous schemes involving atomic motional states are rather limited in both of these aspects. We derive a full quantum model for the eective coupling including the main sources of decoherence. As an application we show that sympathetic groundstate cooling and strong coupling between the two systems is possible.
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J. Budich, P. Zoller, S. Diehl Dissipative preparation of Chern Insulators,
Phys. Rev. A 91 042117 (20150414),
http://dx.doi.org/10.1103/PhysRevA.91.042117 doi:10.1103/PhysRevA.91.042117 (ID: 719006)
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Engineered dissipation can be employed to efficiently prepare interesting quantum many body states in a nonequilibrium fashion. Here, we study the open quantum system dynamics of fermions on a 2D lattice in the framework of a Lindblad master equation. In particular, we propose a novel mechanism to dissipatively prepare a topological state with nonzero Chern number as the unique steady state by means of shortrange system bath interaction. This provides a genuine open quantum system approach to the preparation of topological states, which, quite remarkably, gives rise to a stable topological phase in a nonequilibrium phase diagram. We demonstrate how our theoretical construction can be implemented in a microscopic model that is experimentally feasible with cold atoms in optical lattices.
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H. Pichler, T. Ramos, A. J. Daley, P. Zoller Quantum Optics of Chiral Spin Networks,
Phys. Rev. A 91 042116 (20150414),
http://dx.doi.org/10.1103/PhysRevA.91.042116 doi:10.1103/PhysRevA.91.042116 (ID: 719040)
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We study the drivendissipative dynamics of a network of spin1/2 systems coupled to one or more chiral 1D bosonic waveguides within the framework of a Markovian master equation. We determine how the interplay between a coherent drive and collective decay processes can lead to the formation of pure multipartite entangled steady states. The key ingredient for the emergence of these manybody dark states is an asymmetric coupling of the spins to left and right propagating guided modes. Such systems are motived by experimental possibilities with internal states of atoms coupled to optical fibers, or motional states of trapped atoms coupled to a spinorbit coupled BoseEinstein condensate. We discuss the characterization of the emerging multipartite entanglement in this system in terms of the Fisher information.
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Z. Xu, X. Li, P. Zoller, W. Liu Spontaneous quantum Hall effect in an atomic spinor BoseFermi mixture,
Phys. Rev. Lett. 114 125303 (20150327),
http://dx.doi.org/10.1103/PhysRevLett.114.125303 doi:10.1103/PhysRevLett.114.125303 (ID: 718957)
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We study a mixture of spin1 bosonic and spin1/2 fermionic cold atoms, e.g., 87 Rb and 6 Li, confined in a triangular optical lattice. With fermions at 3/4 filling, Fermi surface nesting leads to spontaneous formation of various spin textures of bosons in the ground state, such as collinear, coplanar and even noncoplanar spin orders. The phase diagram is mapped out with varying boson tunneling and BoseFermi interactions. Most significantly, in one noncoplanar state the mixture is found to exhibit a spontaneous quantum Hall effect in fermions and crystalline superfluidity in bosons, both driven by interaction.
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B. Vermersch, A. Glätzle, P. Zoller Magic distances in the blockade mechanism of Rydberg p and d states,
Phys. Rev. A 91 023411 (20150211),
http://dx.doi.org/10.1103/PhysRevA.91.023411 doi:10.1103/PhysRevA.91.023411 (ID: 719085)
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We show that the Rydberg blockade mechanism, which is well known in the case of s states, can be significantly different for p and d states due to the van der Waals couplings between different Rydberg Zeeman sublevels and the presence of a magneticfield. We show, in particular, the existence of magic distances corresponding to the laserexcitation of a superposition of doubly excited states.
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M. Dalmonte, S. Mirzaei, P. R. Muppalla, D. Marcos, P. Zoller, G. Kirchmair Dipolar Spin Models with Arrays of Superconducting Qubits,
Phys. Rev. B 92 174507 (20150113),
http://dx.doi.org/10.1103/PhysRevB.92.174507 doi:10.1103/PhysRevB.92.174507 (ID: 719116)
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We propose a novel platform for quantum many body simulations of dipolar spin models using current circuit QED technology. Our basic building blocks are 3D Transmon qubits where we use the naturally occurring dipolar interactions to realize interacting spin systems. This opens the way toward the realization of a broad class of tunable spin models in both two and onedimensional geometries. We illustrate the potential offered by these systems in the context of dimerized MajumdarGhoshtype phases, archetypical examples of quantum magnetism, showing how such phases are robust against disorder and decoherence, and could be observed within stateoftheart experiments.
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B. Vermersch, M. Punk, A. Glätzle, C. Gross, P. Zoller Dynamical preparation of laserexcited anisotropic Rydberg crystals in 2D optical lattices,
New J. Phys. 17 013008 (20150109),
http://dx.doi.org/10.1088/13672630/17/1/013008 doi:10.1088/13672630/17/1/013008 (ID: 718969)
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We describe the dynamical preparation of anisotropic crystalline
phases obtained by laserexciting ultracold Alkali atoms to Rydberg pstates
where they interact via anisotropic van derWaals interactions. We develop a timedependent
variational mean field ansatz to model large, but finite twodimensional
systems in experimentally accessible parameter regimes, and we present numerical
simulations to illustrate the dynamical formation of anisotropic Rydberg crystals.
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T. Ramos, H. Pichler, A. J. Daley, P. Zoller Quantum SpinDimers from Chiral Dissipation in Cold Atom Chains,
Phys. Rev. Lett. 113 237203 (20141203),
http://dx.doi.org/10.1103/PhysRevLett.113.237203 doi:10.1103/PhysRevLett.113.237203 (ID: 718977)
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We consider the nonequilibrium dynamics of a driven dissipative spin chain with chiral coupling to a 1D bosonic bath, and its atomic implementation with a twospecies mixture of cold quantum gases. The reservoir is represented by a spinorbit coupled 1D quasicondensate of atoms in a magnetized phase, while the spins are identified with motional states of a separate species of atoms in an optical lattice. The chirality of reservoir excitations allows the spins to couple differently to left and right moving modes, which in our atomic setup can be tuned from bidirectional to purely unidirectional. Remarkably, this leads to a pure steady state in which pairs of neighboring spins form dimers that decouple from the remainder of the chain. Our results also apply to current experiments with twolevel emitters coupled to photonic waveguides.
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D. Marcos, P. Widmer, E. Rico Ortega, M. Hafezi, P. Rabl, U. Wiese, P. Zoller Twodimensional Lattice Gauge Theories with Superconducting Quantum Circuits,
Ann. Phys. 351 654 (20140910),
http://dx.doi.org/10.1016/j.aop.2014.09.011 doi:10.1016/j.aop.2014.09.011 (ID: 718973)
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A quantum simulator of U(1) lattice gauge theories can be implemented with superconducting circuits. This allows the investigation of confined and deconfined phases in quantum link models, and of valence bond solid and spin liquid phases in quantum dimer models. Fractionalized confining strings and the realtime dynamics of quantum phase transitions are accessible as well. Here we show how stateoftheart superconducting technology allows us to simulate these phenomena in relatively small circuit lattices. By exploiting the strong nonlinear couplings between quantized excitations emerging when superconducting qubits are coupled, we show how to engineer gauge invariant Hamiltonians, including ringexchange and fourbody Ising interactions. We demonstrate that, despite decoherence and disorder effects, minimal circuit instances allow us to investigate properties such as the dynamics of electric flux strings, signaling confinement in gauge invariant field theories. The experimental realization of these models in larger superconducting circuits could address open questions beyond current computational capability.
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J. Budich, J. Eisert, E. Bergholtz, S. Diehl, P. Zoller Search for localized Wannier functions of topological band structures via compressed sensing,
Phys. Rev. B 90 115110 (20140904),
http://dx.doi.org/10.1103/PhysRevB.90.115110 doi:10.1103/PhysRevB.90.115110 (ID: 718935)
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We approach the problem of finding maximally localized Wannier functions from a viewpoint of compressed sensing. A practical toolbox is developed to search for maximally localized Wannier functions that exactly obey the underlying physical symmetries of a translationally invariant quantum lattice system under investigation. Each step of the iteration is an O(NlogN) algorithm in the number of lattice sites N . Most interestingly, we are able to generalize our construction to systematically identify the most localized representative of a topological equivalence class of band structures, i.e., the most localized set of Wannier functions that is adiabatically connected to a generic initial representative. We present benchmark results on onedimensional topological superconductors demonstrating the power of these tools. Furthermore, we employ our method to address the open question whether compact Wannier functions can exist for symmetry protected topological states like topological insulators in two dimensions. The existence of such functions would imply exact flat band models with strictly finite range hopping. Here, we find strong numerical evidence for the absence of such functions. We briefly discuss applications in dissipative state preparation and in devising variational sets of states for tensor network methods.
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X. Zhang, M. Bishof, S. L. Bromley, C. Kraus, M. Safronova, P. Zoller, A. M. Rey, J. Ye Spectroscopic observation of SU(N)symmetric interactions in Sr orbital magnetism,
Science 1254978 (20140821),
http://dx.doi.org/10.1126/science.1254978 doi:10.1126/science.1254978 (ID: 718864)
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Symmetries play a fundamental role in the laws of nature. SU(N) symmetry can emerge in a quantum system with N singleparticle spin states when the spin degree of freedom is decoupled from interactions. Such a system is anticipated to exhibit large degeneracy and exotic manybody behaviors. Owing to the strong decoupling between electronicorbital and nuclearspin degrees of freedom, alkalineearth atoms (AEAs), prepared in the two lowest electronic states (clock states), are predicted to obey an accurate SU(N=2I+1) symmetry arising from the nuclear spin (I). So far, only indirect evidence for this symmetry exists, and the scattering parameters remain largely unknown. Here we report the first direct observation of SU(N=10) symmetry in 87Sr (I=9/2) using the stateoftheart measurement precision offered by an ultrastable laser. By encoding the electronic orbital degree of freedom in the two clock states, while keeping the system open to all 10 nuclear spin sublevels, we probe the nonequilibrium twoorbital SU(N) magnetism via Ramsey spectroscopy of atoms confined in an array of 2D optical traps. The motional degrees of freedom are frozen during the spin dynamics, which allows probing generic models of quantum magnetism at high temperatures. Compared with lattice models of spatially localized atoms coupled with shortrange interactions, our system shows two key differences: the lattice is spanned by energy eigenvalues and the underlying interactions are longrange and nuclear spin independent. To elucidate the microscopic mechanism for SU(N) orbital physics, we model the spinorbital dynamics and determine all relevant interaction parameters with an analytic relation between s and pwave scattering lengths. This work prepares for using AEAs as testbeds for iconic orbital models expected to describe transition metal oxides, heavy fermion materials, and spin liquid phases.
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P. Jurcevic, B. P. Lanyon, P. Hauke, C. Hempel, P. Zoller, R. Blatt, C. F. Roos Quasiparticle engineering and entanglement propagation in a quantum manybody system,
Nature 511 202 (20140710),
http://dx.doi.org/10.1038/nature13461 doi:10.1038/nature13461 (ID: 718717)
Toggle Abstract
The key to explaining a wide range of quantum phenomena is understanding how entanglement propagates around manybody systems. Furthermore, the controlled distribution of entanglement is of fundamental importance for quantum communication and computation. In many situations, quasiparticles are the carriers of information around a quantum system and are expected to distribute entanglement in a fashion determined by the system interactions. Here we report on the observation of magnon quasiparticle dynamics in a onedimensional manybody quantum system of trapped ions representing an Ising spin model. Using the ability to tune the effective interaction range, and to prepare and measure the quantum state at the individual particle level, we observe new quasiparticle phenomena. For the first time, we reveal the entanglement distributed by quasiparticles around a manybody system. Second, for longrange interactions we observe the divergence of quasiparticle velocity and breakdown of the lightcone picture that is valid for shortrange interactions. Our results will allow experimental studies of a wide range of phenomena, such as quantum transport, thermalisation, localisation and entanglement growth, and represent a first step towards a new quantumoptical regime with ondemand quasiparticles with tunable nonlinear interactions.
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W. Lechner, H. Büchler, P. Zoller Role of Quantum Fluctuations in the Hexatic Phase of Cold Polar Molecules,
Phys. Rev. Lett. 112 255301 (20140625),
http://dx.doi.org/10.1103/PhysRevLett.112.255301 doi:10.1103/PhysRevLett.112.255301 (ID: 718725)
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Two dimensional crystals melt via an intermediate \textit{hexatic} phase which is characterized by an anomalous scaling of spatial and orientational correlation functions and the absence of an attraction between dislocations. We propose a protocol to study the role of quantum fluctuations on the nature of this phase with a system of strongly correlated polar molecules in a parameter regime where thermal and quantum fluctuations are of the same order of magnitude. The dislocations can be located in experiment from local energy differences which induce internal stark shifts in dislocation molecules. We present a criterium to identify the hexatic phase from the statistics of the end points of topological defect strings and find a hexatic phase, which is dominated by quantum fluctuations, between crystal and superfluid phase.
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A. Carmele, B. Vogell, K. Stannigel, P. Zoller OptoNanomechanics Strongly Coupled to a Rydberg Superatom: Coherent vs. Incoherent Dynamics,
New J. Phys. 16 063042 (20140619),
http://dx.doi.org/10.1088/13672630/16/6/063042 doi:10.1088/13672630/16/6/063042 (ID: 718701)
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We propose a hybrid optomechanical quantum system consisting of a moving membrane strongly coupled to an ensemble of N atoms with a Rydberg state. Due to the strong vanderWaals interaction between the atoms, the ensemble forms an effective twolevel system, a Rydberg superatom, with a collectively enhanced atomlight coupling. Using this superatom imposed collective enhancement strong coupling between membrane and superatom is feasible for parameters within the range of current experiments. The quantum interface to couple the membrane and the superatom can be a pumped single mode cavity, or a laser field in free space, where the Rydberg superatom and the membrane are spatially separated. In addition to the coherent dynamics, we study in detail the impact of the typical dissipation processes, in particular the radiative decay as a source for incoherent superpositions of atomic excitations. We identify the conditions to suppress these incoherent dynamics and thereby a parameter regime for strong coupling. The Rydberg superatom in this hybrid system serves as a toolbox for the nanomechanical resonator allowing for a wide range of applications such as state transfer, sympathetic cooling and nonclassical state preparation. As an illustration, we show that a thermally occupied membrane can be prepared in a nonclassical state without the necessity of ground state cooling.
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E. Rico Ortega, T. Pichler, M. Dalmonte, P. Zoller, S. Montangero Tensor networks for Lattice Gauge Theories and Atomic Quantum Simulation,
Phys. Rev. Lett. 112 201601 (20140523),
http://dx.doi.org/10.1103/PhysRevLett.112.201601 doi:10.1103/PhysRevLett.112.201601 (ID: 718702)
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We show that gauge invariant quantum link models, Abelian and nonAbelian, can be exactly described in terms of tensor networks states. Quantum link models represent an ideal bridge between highenergy to cold atom physics, as they can be used in coldatoms in optical lattices to study lattice gauge theories. In this framework, we characterize the phase diagram of a (1+1)d quantum link version of the Schwinger model in an external classical background electric field: the quantum phase transition from a charge and parity ordered phase with nonzero electric flux to a disordered one with a net zero electric flux configuration is described by the Ising universality class.
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A. Glätzle, M. Dalmonte, R. Nath, I. Rousochatzakis, R. Mössner, P. Zoller Quantum Spin Ice and dimer models with Rydberg atoms,
Phys. Rev. X 4 041037 (20140421),
http://dx.doi.org/10.1103/PhysRevX.4.041037 doi:10.1103/PhysRevX.4.041037 (ID: 718901)
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Quantum spin ice represents a paradigmatic example on how the physics of frustrated magnets is related to gauge theories. In the present work we address the problem of approximately realizing quantum spin ice in two dimensions with cold atoms in optical lattices. The relevant interactions are obtained by weakly admixing van der Waals interactions between laser admixed Rydberg states to the atomic ground state atoms, exploiting the strong angular dependence of interactions between Rydberg pstates together with the possibility of designing steplike potentials. This allows us to implement Abelian gauge theories in a series of geometries, which could be demonstrated within state of the art atomic Rydberg experiments. We numerically analyze the family of resulting microscopic Hamiltonians and find that they exhibit both classical and quantum order by disorder, the latter yielding a quantum plaquette valence bond solid. We also present strategies to implement Abelian gauge theories using both s and pRydberg states in exotic geometries, e.g. on a 48 lattice.
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K. Stannigel, P. Hauke, D. Marcos, M. Hafezi, S. Diehl, M. Dalmonte, P. Zoller Constrained dynamics via the Zeno effect in quantum simulation: Implementing nonAbelian lattice gauge theories with cold atoms,
Phys. Rev. Lett. 112 120406 (20140326),
http://dx.doi.org/10.1103/PhysRevLett.112.120406 doi:10.1103/PhysRevLett.112.120406 (ID: 718579)
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We show how engineered classical noise can be used to generate constrained Hamiltonian dynamics in atomic quantum simulators of manybody systems, taking advantage of the continuous Zeno effect. After discussing the general theoretical framework, we focus on applications in the context of lattice gauge theories, where imposing exotic, quasilocal constraints is usually challenging. We demonstrate the effectiveness of the scheme for both Abelian and nonAbelian gauge theories, and discuss how engineering dissipative constraints substitutes complicated, nonlocal interaction patterns by global coupling to laser fields.
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C. Laflamme, M. Baranov, P. Zoller, C. Kraus Hybrid Topological Quantum Computation with Majorana Fermions: A Cold Atom Setup,
Phys. Rev. A 89 022319 (20140214),
http://dx.doi.org/10.1103/PhysRevA.89.022319 doi:10.1103/PhysRevA.89.022319 (ID: 718700)
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In this paper we present a hybrid scheme for topological quantum computation in a system of cold atoms trapped in an atomic lattice. A topological qubit subspace is defined using Majorana fermions which emerge in a network of atomic Kitaev onedimensional wires. We show how braiding can be implemented in this setup and introduce a proposal for the efficient, robust and reversible mapping of the topological qubits to a conventional qubit stored in a single atom. There, wellcontrolled standard techniques can be used to implement the missing gates required for universal computation. Our setup is complemented with an efficient nondestructive protocol to check for errors in the mapping.
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P. Zoller Manybody quantum dynamics and phases of driven twolevel atoms coupled via a chiral bath,
Quantum Optics VII (Mar del Plata, 20141027) URL (20141031),
(ID: 719034)
Toggle Abstract
Quantum Optics allows the realization of unique and unconventional quantum many body systems, and thus novel scenarios of quantum dynamics and phases for the formation of strongly correlated quantum states. Here we will be interested in the manybody dynamics and phases of driven twolevel systems, which are coupled to, and interact via a “chiral 1D wave guide” representing a quantum reservoir [1,2]. By “chirality” we mean that there is a asymmetry to generate left and rightmoving excitations in the bath from the decay of the twolevel atoms. We will derive a quantum optical master equation for this spinchain, eliminating the chiral reservoir in a BornMarkov approximation [1]. Remarkably, this master equation predicts a steady state as a “pure state” in the form of quantum spindimers, where pairs of neighboring spins decouple by quantum interference from the remainder of the chain. This “cooling” to quantum spindimers is the generic steady state for a wide range of parameters, providing an example for “quantum magnetism by dissipation”.
Quantum optical implementation of the driven dissipative spinchain coupled to a chiral reservoir are provided first of all by twolevel emitters coupled (strongly) to a fiber, or photonic band gap material, as discussed in [3,4]. Another realization is with a twospecies mixture of atomic quantum gases [1], where the reservoir is represented by a spinorbit coupled 1D quasicondensate of atoms in a magnetized phase, while the spins are are identified with motional states of atoms in an optical lattice.
[1] T. Ramos, H. Pichler, A. Daley, and P. Zoller, unpublished
[2] K. Stannigel, P. Rabl and P. Zoller, New J. Phys. 14, 063014 (2012).
[3] R. Mitsch, C. Sayrin, B. Albrecht, P. Schneeweiss and A. Rauschenbeutel, arXiv:1406.0896 (2014).
[4] I. Söllner, S. Mahmoodian, A. Javadi and P. Lodahl, arXiv:1406.4295 (2014).

P. Zoller Quantum Simulation of Dynamical Gauge Fields with Cold Atoms,
ICAP 2014 (Washington, 20140803) URL (20140808),
(ID: 718976)
Toggle Abstract
Abelian and nonAbelian gauge theories play a central role in physics. In condensed matter physics lattice gauge theories arise in the context of quantum spin liquids, and in high energy physics quantum chromodynamics is a nonAbelian SU(3) gauge theory describing the strong interactions between quarks and gluons. In this talk we show that cold bosonic and fermionic atoms in optical lattices, and strings of cold ions provide a toolbox for quantum simulation of Abelian and nonAbelian lattice gauge theories, and we discuss various physical phenomena, which could be observed in such experiments [14]. Our discussion will focus in particular on the paradigmatic example of quantum spin ice in 2D, and its realization with Rydberg atoms [5]. We conclude with a general outlook on quantum simulation, touching various aspects of equilibrium and nonequilibrium dynamics, and phases and phase transitions in setups of cold atoms in optical lattices.
[ 1] D. Banerjee, M. Dalmonte, M. Müller, E. Rico, P. Stebler, U.J. Wiese, and P. Zoller, Phys. Rev. Lett. 109, 175302 (2012)
[2] D. Banerjee, M. Bögli, M. Dalmonte, E. Rico, P. Stebler, U.J. Wiese, and P. Zoller, Phys. Rev. Lett. 110, 125303 (2013)
[3] P. Hauke, D. Marcos, M. Dalmonte, and P. Zoller, Phys. Rev. X 3, 041018 (2013)
[4] K. Stannigel, P. Hauke, D. Marcos, M. Hafezi, S. Diehl, M. Dalmonte, and P. Zoller, Phys. Rev. Lett. 112, 120406 (2014)
[5] A.W. Glaetzle, M. Dalmonte, R. Nath, I. Rousochatzakis, R. Moessner, P. Zoller, arXiv:1404.5326

P. Zoller Quantum Ice and dimer models with ultracold Rydberg atoms,
Frontiers in the Quantum World, Symposium in Honour of Ennio Arimondo on the Occasion of his Retirement (Firenze, 20140310) (20140310),
(ID: 718858)

P. Hauke, D. Marcos, M. Dalmonte, P. Zoller Quantum simulation of a lattice Schwinger model in a chain of trapped ions,
Phys. Rev. X 3 041018 (20131122),
http://dx.doi.org/10.1103/PhysRevX.3.041018 doi:10.1103/PhysRevX.3.041018 (ID: 718532)
Toggle Abstract
We discuss how a lattice Schwinger model can be realized in a linear ion trap, allowing a detailed study of the physics of Abelian lattice gauge theories related to onedimensional quantum electrodynamics. Relying on the rich quantumsimulation toolbox available in stateoftheart trappedion experiments, we show how one can engineer an effectively gaugeinvariant dynamics by imposing energetic constraints, provided by strong Isinglike interactions. Applying exact diagonalization to groundstate and timedependent properties, we study the underlying microscopic model, and discuss undesired interaction terms and other imperfections. As our analysis shows, the proposed scheme allows for the observation in realistic setups of spontaneous parity and chargesymmetry breaking, as well as falsevacuum decay. Besides an implementation aimed at larger ion chains, we also discuss a minimal setting, consisting of only four ions in a simpler experimental setup, which enables to probe basic physical phenomena related to the full manybody problem. The proposal opens a new route for analog quantum simulation of highenergy and condensedmatter models where gauge symmetries play a prominent role.
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C. Kraus, P. Zoller, M. Baranov Braiding of Atomic Majorana Fermions in Wire Networks and Implementation of the DeutschJosza Algorithm,
Phys. Rev. Lett. 111 203001 (20131111),
http://dx.doi.org/10.1103/PhysRevLett.111.203001 doi:10.1103/PhysRevLett.111.203001 (ID: 718425)
Toggle Abstract
We propose an efficient protocol for braiding atomic Majorana fermions in wire networks with AMO techniques and demonstrate its robustness against experimentally relevant errors. Based on this protocol we provide a topologically protected implementation of the DeutschJosza algorithm.
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W. Lechner, P. Zoller From Classical to Quantum Glasses with Ultracold Polar Molecules,
Phys. Rev. Lett. 111 185306 (20131030),
http://dx.doi.org/10.1103/PhysRevLett.111.185306 doi:10.1103/PhysRevLett.111.185306 (ID: 718563)
Toggle Abstract
Using experiments with single particle resolution and computer simulations we study the collective behaviour of multiple vacancies injected into twodimensional crystals. We find that the defects assemble into linear strings that propagate through the crystal in a succession of rapid onedimensional gliding phases and rare rotations, during which the direction of motion changes. At both ends, strings are terminated by dislocations with antiparallel Burgers vectors. By monitoring the separation of the dislocations, we measure their effective interactions with high precision, for the first time beyond spontaneous formation and annihilation, and explain the doublewell form of the dislocation interaction in terms of continuum elasticity theory. Our results give a detailed picture of the motion and interaction of dislocations in two dimensions and enhance our understanding of topological defects in twodimensional nanomaterials.
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C. Kraus, M. Dalmonte, M. Baranov, A. Läuchli, P. Zoller Majorana edge states in two atomic wires coupled by pairhopping,
Phys. Rev. Lett. 111 173004 (20131023),
http://dx.doi.org/10.1103/PhysRevLett.111.173004 doi:10.1103/PhysRevLett.111.173004 (ID: 718420)
Toggle Abstract
We present evidence for the existence of Majorana edge states in a number conserving theory describing a system of spinless fermions on two wires that are coupled by a pair hopping. Our analysis is based on the combination of a qualitative low energy approach and numerical techniques using the Density Matrix Renormalization Group. We also discuss an experimental realization of pairhopping interactions in cold atom gases confined in optical lattices, and its possible alternative applications to quantum simulation.
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O. RomeroIsart, C. Navau, A. Sanchez, P. Zoller, J. I. Cirac Superconducting Vortex Lattices for Ultracold Atoms,
Phys. Rev. Lett. 111 145304 (20131004),
http://dx.doi.org/10.1103/PhysRevLett.111.145304 doi:10.1103/PhysRevLett.111.145304 (ID: 718430)
Toggle Abstract
The ability to trap and manipulate ultracold atoms in lattice structures has lead to a remarkable experimental progress to build quantum simulators for Hubbard models. A prominent example is atoms in optical lattices where lasers are used to create lattices with spacing set by the laser wavelength as well as to control and measure the manybody states. In contrast, here we propose and analyze a nanoengineered vortex array in a thinfilm typeII superconductor as a magnetic lattice for ultracold atoms. This proposal addresses several of the key questions in the development of atomic quantum simulators. By trapping atoms close to the surface, tools of nanofabrication and structuring of lattices on the scale of few tens of nanometers become available with a corresponding benefit in energy scales and temperature requirements. This can be combined with the possibility of magnetic single site addressing and manipulation together with a favorable scaling of superconducting surface induced decoherence.
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D. Marcos, P. Rabl, E. Rico Ortega, P. Zoller Superconducting circuits for quantum simulation of dynamical gauge fields,
Phys. Rev. Lett. 111 110504 (20130913),
http://dx.doi.org/10.1103/PhysRevLett.111.110504 doi:10.1103/PhysRevLett.111.110504 (ID: 718537)
Toggle Abstract
We describe a superconductingcircuit lattice design for the implementation and simulation of dynamical lattice gauge theories. We illustrate our proposal by analyzing a onedimensional U(1) quantumlink model, where superconducting qubits play the role of matter fields on the lattice sites and the gauge fields are represented by two coupled microwave resonators on each link between neighboring sites. A detailed analysis of a minimal experimental protocol for probing the physics related to string breaking effects shows that despite the presence of decoherence in these systems, distinctive phenomena from condensedmatter and highenergy physics can be visualized with stateoftheart technology in small superconductingcircuit arrays.
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C. Bardyn, M. Baranov, C. Kraus, E. Rico Ortega, A. Imamoglu, P. Zoller, S. Diehl Topology by dissipation,
New J. Phys. 15 085001 (20130801),
http://dx.doi.org/10.1088/13672630/15/8/085001 doi:10.1088/13672630/15/8/085001 (ID: 718440)
Toggle Abstract
Topological states of fermionic matter can be induced by means of a suitably engineered dissipative dynamics. Dissipation then does not occur as a perturbation, but rather as the main resource for manybody dynamics, providing a targeted cooling into a topological phase starting from an arbitrary initial state. We explore the concept of topological order in this setting, developing and applying a general theoretical framework based on the system density matrix which replaces the wave function appropriate for the discussion of Hamiltonian groundstate physics. We identify key analogies and differences to the more conventional Hamiltonian scenario. Differences mainly arise from the fact that the properties of the spectrum and of the state of the system are not as tightly related as in a Hamiltonian context. We provide a symmetrybased topological classification of bulk steady states and identify the classes that are achievable by means of quasilocal dissipative processes driving into superfluid paired states. We also explore the fate of the bulkedge correspondence in the dissipative setting, and demonstrate the emergence of Majorana edge modes. We illustrate our findings in one and twodimensional models that are experimentally realistic in the context of cold atoms.
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H. Pichler, L. Bonnes, A. J. Daley, A. Läuchli, P. Zoller Thermal versus entanglement entropy: a measurement protocol for fermionic atoms with a quantum gas microscope,
New J. Phys. 15 063003 (20130606),
http://dx.doi.org/10.1088/13672630/15/6/063003 doi:10.1088/13672630/15/6/063003 (ID: 718531)
Toggle Abstract
We show how to measure the ordertwo Renyi entropy of manybody states of spinful fermionic atoms in an optical lattice in equilibrium and nonequilibrium situations. The proposed scheme relies on the possibility to produce and couple two copies of the state under investigation, and to measure the occupation number in a site and spinresolved manner, e.g. with a quantum gas microscope. Such a protocol opens the possibility to measure entanglement and test a number of theoretical predictions, such as area laws and their corrections. As an illustration we discuss the interplay between thermal and entanglement entropy for a one dimensional Fermi–Hubbard model at finite temperature, and its possible measurement in an experiment using the present scheme.
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P. Schindler, M. Müller, D. Nigg, J. T. Barreiro, E. A. Martínez, M. Hennrich, T. Monz, S. Diehl, P. Zoller, R. Blatt Quantum simulation of opensystem dynamical maps with trapped ions,
Nature Phys. 9 367 (20130519),
http://dx.doi.org/10.1038/nphys2630 doi:10.1038/nphys2630 (ID: 718325)
Toggle Abstract
Dynamical maps describe general transformations of the state of a physical system, and their iteration can be interpreted as generating a discrete time evolution. Prime examples include classical nonlinear systems undergoing transitions to chaos. Quantum mechanical counterparts show intriguing phenomena such as dynamical localization on the single particle level. Here we extend the concept of dynamical maps to an opensystem, manyparticle context: We experimentally explore the stroboscopic dynamics of a complex manybody spin model by means of a universal quantum simulator using up to five ions. In particular, we generate longrange phase coherence of spin by an iteration of purely dissipative quantum maps. We also demonstrate the characteristics of competition between combined coherent and dissipative nonequilibrium evolution. This opens the door for studying manyparticle nonequilibrium physics and associated dynamical phase transitions with no immediate counterpart in equilibrium condensed matter systems. An error detection and reduction toolbox that facilitates the faithful quantum simulation of larger systems is developed as a first step in this direction.
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S. Habraken, W. Lechner, P. Zoller Resonances in dissipative optomechanics with nanoparticles: Sorting, speed rectification and transverse cooling,
Phys. Rev. A 87 053808 (20130506),
http://dx.doi.org/10.1103/PhysRevA.87.053808 doi:10.1103/PhysRevA.87.053808 (ID: 718463)
Toggle Abstract
The interaction between dielectric particles and a laserdriven optical cavity gives rise to both conservative and dissipative dynamics, which can be used to levitate, trap and cool nanoparticles. We analytically and numerically study a twomode setup in which the optical potentials along the cavity axis cancel, so that the resulting dynamics is almost purely dissipative. For appropriate detunings of the laserdrives, this dissipative optomechanical dynamics can be used to sort particles according to their size, to rectify their velocities and to enhance transverse cooling.
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S. Bennett, N. Y. Yao, J. Otterbach, P. Zoller, P. Rabl, M. Lukin Phononinduced spinspin interactions in diamond nanostructures: application to spin squeezing,
Phys. Rev. Lett. 110 156402 (20130409),
http://dx.doi.org/10.1103/PhysRevLett.110.156402 doi:10.1103/PhysRevLett.110.156402 (ID: 718345)
Toggle Abstract
We propose and analyze a novel mechanism for longrange spinspin interactions in diamond nanostructures. The interactions between electronic spins, associated with nitrogenvacancy centers in diamond, are mediated by their coupling via strain to the vibrational mode of a diamond mechanical nanoresonator. This coupling results in phononmediated effective spinspin interactions that can be used to generate squeezed states of a spin ensemble. We show that spin dephasing and relaxation can be largely suppressed, allowing for substantial spin squeezing under realistic experimental conditions. Our approach has implications for spinensemble magnetometry, as well as phononmediated quantum information processing with spin qubits.
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N. Goldman, J. Dalibard, A. Dauphin, F. Gerbier, M. Lewenstein, P. Zoller, I. Spielman Direct imaging of topological edges states with cold atoms,
Proc. Natl. Acad. U.S.A. 6 (20130408),
http://dx.doi.org/10.1073/pnas.1300170110 doi:10.1073/pnas.1300170110 (ID: 718336)
Toggle Abstract
Detecting topological order in coldatom experiments is an outstanding challenge, the resolution of which offers novel perspectives on topological matter. In material systems, unambiguous signatures of topological order exist for topological insulators and quantum Hall devices. In quantum Hall systems, the quantized conductivity and the associated robust propagating edge modes  guaranteed by the existence of nontrivial topological invariants  have been observed through transport and spectroscopy measurements. Here, we show that opticallatticebased experiments can be tailored to directly visualize the propagation of topological edge modes. Our method is rooted in the unique capability for initially shaping the atomic gas, and imaging its timeevolution after suddenly removing the shaping potentials. Our scheme, applicable to an assortment of atomic topological phases, provides the first method for imaging the dynamics of topological edge modes, directly revealing their angular velocity and spin structure.
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W. Lechner, S. Habraken, N. Kiesel, M. Aspelmeyer, P. Zoller Cavity Optomechanics of Levitated NanoDumbbells: NonEquilibrium Phases and SelfAssembly,
Phys. Rev. Lett. 110 143604 (20130405),
http://dx.doi.org/10.1103/PhysRevLett.110.143604 doi:10.1103/PhysRevLett.110.143604 (ID: 718335)
Toggle Abstract
Levitated nanospheres in optical cavities open a novel route to study manybody systems out of solution and highly isolated from the environment. We show that properly tuned optical parameters allow for the study of the nonequilibrium dynamics of composite nanoparticles with nonisotropic optical friction. We find friction induced ordering and nematic transitions with nonequilibrium analogs to liquid crystal phases for ensembles of dimers.
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D. Banerjee, M. Bögli, M. Dalmonte, E. Rico Ortega, P. Stebler, U. Wiese, P. Zoller Atomic Quantum Simulation of U(N) and SU(N) NonAbelian Lattice Gauge Theories,
Phys. Rev. Lett. 110 125303 (20130321),
http://dx.doi.org/10.1103/PhysRevLett.110.125303 doi:10.1103/PhysRevLett.110.125303 (ID: 718423)
Toggle Abstract
Using ultracold alkalineearth atoms in optical lattices, we construct a quantum simulator for U(N) and SU(N) lattice gauge theories with fermionic matter based on quantum link models. These systems share qualitative features with QCD, including chiral symmetry breaking and restoration at nonzero temperature or baryon density. Unlike classical simulations, a quantum simulator does not suffer from sign problems and can address the corresponding chiral dynamics in real time.
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N. Y. Yao, C. R. Laumann, A. V. Gorshkov, H. Weimer, L. Jiang, I. J. Cirac, P. Zoller, M. Lukin Topologically Protected Quantum State Transfer in a Chiral Spin Liquid,
Nat. Commun. 4 1585 (20130312),
http://dx.doi.org/10.1038/ncomms2531 doi:10.1038/ncomms2531 (ID: 717921)
Toggle Abstract
Topology plays a central role in ensuring the robustness of a wide variety of physical phenomena.
Notable examples range from the robust current carrying edge states associated with the quantum
Hall and the quantum spin Hall effects to proposals involving topologically protected quantum
memory and quantum logic operations. Here, we propose and analyze a topologically protected
channel for the transfer of quantum states between remote quantum nodes. In our approach, state
transfer is mediated by the edge mode of a chiral spin liquid. We demonstrate that the proposed
method is intrinsically robust to realistic imperfections associated with disorder and decoherence.
Possible experimental implementations and applications to the detection and characterization of
spin liquid phases are discussed.
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H. Pichler, J. Schachenmayer, A. J. Daley, P. Zoller Heating dynamics of bosonic atoms in a noisy optical lattice,
Phys. Rev. A 87 033606 (20130305),
http://dx.doi.org/10.1103/PhysRevA.87.033606 doi:10.1103/PhysRevA.87.033606 (ID: 718344)
Toggle Abstract
We analyze the heating of interacting bosonic atoms in an optical lattice due to intensity fluctuations of the lasers forming the lattice. We focus in particular on fluctuations at low frequencies below the band gap frequency, such that the dynamics is restricted to the lowest band. We derive stochastic equations of motion, and analyze the effects on different manybody states, characterizing heating processes in both strongly and weakly interacting regimes. In the limit where the noise spectrum is flat at low frequencies, we can derive an effective Master equation describing the dynamics. We compute heating rates and changes to characteristic correlation functions both in the perturbation theory limit, and using a full timedependent calculation of the stochastic manybody dynamics in 1D based on timedependent densitymatrixrenormalizationgroup methods.
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B. Vogell, K. Stannigel, P. Zoller, K. Hammerer, M. T. Rakher, M. Korppi, A. Jöckel, P. Treutlein CavityEnhanced LongDistance Coupling of an Atomic Ensemble to a Micromechanical Membrane,
Phys. Rev. A 87 023816 (20130214),
http://dx.doi.org/10.1103/PhysRevA.87.023816 doi:10.1103/PhysRevA.87.023816 (ID: 718342)
Toggle Abstract
We discuss a hybrid quantum system where a dielectric membrane situated inside an optical cavity is coupled to a distant atomic ensemble trapped in an optical lattice. The coupling is mediated by the exchange of sideband photons of the lattice laser, and is enhanced by the cavity finesse as well as the square root of the number of atoms. In addition to observing coherent dynamics between the two systems, one can also switch on a tailored dissipation by laser cooling the atoms, thereby allowing for sympathetic cooling of the membrane. The resulting cooling scheme does not require resolved sideband conditions for the cavity, which relaxes a constraint present in standard optomechanical cavity cooling. We present a quantum mechanical treatment of this modular open system which takes into account the dominant imperfections, and identify optimal operation points for both coherent dynamics and sympathetic cooling. In particular, we find that ground state cooling of a cryogenically precooled membrane is possible for realistic parameters.
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T. Ramos, V. Sudhir, K. Stannigel, P. Zoller, T. J. Kippenberg Nonlinear Quantum Optomechanics via Individual Intrinsic TwoLevel Defects,
Phys. Rev. Lett. 110 193602 (20130207),
http://dx.doi.org/10.1103/PhysRevLett.110.193602 doi:10.1103/PhysRevLett.110.193602 (ID: 718426)
Toggle Abstract
We propose to use the intrinsic twolevel system (TLS) defect states found naturally in integrated optomechanical devices for exploring cavity QEDlike phenomena with localized phonons. The JaynesCummingstype interaction between TLS and mechanics can reach the strong coupling regime for existing nanooptomechanical systems, observable via clear signatures in the optomechanical output spectrum. These signatures persist even at finite temperature, and we derive an explicit expression for the temperature at which they vanish. Further, the ability to drive the defect with a microwave field allows for realization of phonon blockade, and the available controls are sufficient to deterministically prepare nonclassical states of the mechanical resonator.
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P. Komar, S. D. Bennett, K. Stannigel, S. Habraken, P. Rabl, P. Zoller, M. Lukin Singlephoton nonlinearities in twomode optomechanics,
Phys. Rev. A 87 013839 (20130128),
http://dx.doi.org/10.1103/PhysRevA.87.013839 doi:10.1103/PhysRevA.87.013839 (ID: 718245)
Toggle Abstract
We present a detailed theoretical analysis of a weakly driven multimode optomechanical system, in which two optical modes are strongly and nearresonantly coupled to a single mechanical mode via a threewave mixing interaction. We calculate one and twotime intensity correlations of the two optical fields and compare them to analogous correlations in atomcavity systems. Nonclassical photon correlations arise when the optomechanical coupling $g$ exceeds the cavity decay rate $\kappa$, and we discuss signatures of one and twophoton resonances as well as quantum interference. We also find a longlived correlation that decays slowly with the mechanical decay rate $\gamma$, reflecting the heralded preparation of a single phonon state after detection of a photon. Our results provide insight into the quantum regime of multimode optomechanics, with potential applications for quantum information processing with photons and phonons.
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N. Y. Yao, C. R. Laumann, A. V. Gorshkov, S. D. Bennett, E. Demler, P. Zoller, M. Lukin Topological Flat Bands from Dipolar Spin Systems,
Phys. Rev. Lett. 109 266804 (20121226),
http://dx.doi.org/10.1103/PhysRevLett.109.266804 doi:10.1103/PhysRevLett.109.266804 (ID: 718171)
Toggle Abstract
We propose and analyze a physical system that naturally admits twodimensional topological nearly flat bands. Our approach utilizes an array of threelevel dipoles (effective S = 1 spins) driven by inhomogeneous electromagnetic fields. The dipolar interactions produce arbitrary uniform background gauge fields for an effective collection of conserved hardcore bosons, namely, the dressed spinflips. These gauge fields result in topological band structures, whose bandgap can be larger than the corresponding bandwidth. Exact diagonalization of the full interacting Hamiltonian at halffilling reveals the existence of superfluid, crystalline, and supersolid phases. An experimental realization using either ultracold polar molecules or spins in the solid state is considered.
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M. Gullans, T. Tiecke, S. Chang, J. Feist, J. D. Thompson, I. J. Cirac, P. Zoller, M. Lukin Nanoplasmonic Lattices for Ultracold Atoms,
Phys. Rev. Lett. 109 235309 (20121207),
http://dx.doi.org/10.1103/PhysRevLett.109.235309 doi:10.1103/PhysRevLett.109.235309 (ID: 718190)
Toggle Abstract
We propose to use subwavelength confinement of light associated with the near field of plasmonic systems to create nanoscale optical lattices for ultracold atoms. Our approach combines the unique coherence properties of isolated atoms with the subwavelength manipulation and strong lightmatter interaction associated with nanoplasmonic systems. It allows one to considerably increase the energy scales in the realization of Hubbard models and to engineer effective longrange interactions in coherent and dissipative manybody dynamics. Realistic imperfections and potential applications are discussed.
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C. Kraus, S. Diehl, P. Zoller, M. Baranov Preparing and probing atomic Majorana fermions and topological order in optical lattices,
New J. Phys. 14 113036 (20121127),
http://dx.doi.org/10.1088/13672630/14/11/113036 doi:10.1088/13672630/14/11/113036 (ID: 717882)
Toggle Abstract
We introduce a onedimensional system of fermionic atoms in an optical lattice whose phase diagram includes topological states of different symmetry classes. These states can be identified by their zeroenergy edge modes which are Majorana fermions. We propose several universal methods of detecting the Majorana edge states, based on their genuine features: zeroenergy, localized character of the wave functions, and induced nonlocal fermionic correlations.
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H. Pichler, J. Schachenmayer, J. Simon, P. Zoller, A. J. Daley Noise and disorderresilient optical lattices,
Phys. Rev. A 86 051605 (20121116),
http://dx.doi.org/10.1103/PhysRevA.86.051605 doi:10.1103/PhysRevA.86.051605 (ID: 718093)
Toggle Abstract
We show how a dressed lattice scheme can provide control over certain types of noise for atomic quantum gases in the lowest band of an optical lattice, removing the effects of global lattice amplitude noise to first order for particular choices of the dressing field parameters. We investigate the nonequilibrium manybody dynamics of bosons and fermions induced by noise away from this parameter regime, and show how the same technique can reduce spatial disorder in projected lattice potentials.
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S. Habraken, K. Stannigel, M. Lukin, P. Zoller, P. Rabl Continuous mode cooling and phonon routers for phononic quantum networks,
New J. Phys. 24 115004 (20121105),
http://dx.doi.org/10.1088/13672630/14/11/115004 doi:10.1088/13672630/14/11/115004 (ID: 718101)
Toggle Abstract
We study the implementation of quantum state transfer protocols in phonon networks, where in analogy to optical networks, quantum information is transmitted through propagating phonons in extended mechanical resonator arrays or phonon waveguides. We describe how the problem of a nonvanishing thermal occupation of the phononic quantum channel can be overcome by implementing optomechanical multi and continuous mode cooling schemes to create a 'cold' frequency window for transmitting quantum states. In addition, we discuss the implementation of phonon circulators and switchable phonon routers, which rely on strong coherent optomechanical interactions only, and do not require strong magnetic fields or specific materials. Both techniques can be applied and adapted to various physical implementations, where phonons coupled to spin or charge based qubits are used for onchip networking applications.
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D. Banerjee, M. Dalmonte, M. Müller, E. Rico Ortega, P. Stebler, U. Wiese, P. Zoller Atomic Quantum Simulation of Dynamical Gauge Fields coupled to Fermionic Matter: From String Breaking to Evolution after a Quench,
Phys. Rev. Lett. 109 175302 (20121023),
http://dx.doi.org/10.1103/PhysRevLett.109.175302 doi:10.1103/PhysRevLett.109.175302 (ID: 718094)
Toggle Abstract
Using a FermiBose mixture of ultracold atoms in an optical lattice, we construct a quantum simulator for a U(1) gauge theory coupled to fermionic matter. The construction is based on quantum links which realize continuous gauge symmetry with discrete quantum variables. At low energies, quantum link models with staggered fermions emerge from a Hubbardtype model which can be quantum simulated. This allows us to investigate string breaking as well as the realtime evolution after a quench in gauge theories, which are inaccessible to classical simulation methods.
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A. Glätzle, R. Nath, B. Zhao, G. Pupillo, P. Zoller Drivendissipative dynamics of a strongly interacting Rydberg gas,
Phys. Rev. A 86 043403 (20121002),
http://dx.doi.org/10.1103/PhysRevA.86.043403 doi:10.1103/PhysRevA.86.043403 (ID: 718134)
Toggle Abstract
We study the nonequilibrium manybody dynamics of a cold gas of ground state alkali atoms weakly admixed by Rydberg states with laser light. On a timescale shorter than the lifetime of the dressed states, effective dipoledipole or van der Waals interactions between atoms can lead to the formation of strongly correlated phases, such as atomic crystals. Using a semiclassical approach, we study the longtime dynamics where decoherence and dissipative processes due to spontaneous emission and blackbody radiation dominate, leading to heating and melting of atomic crystals as well as particle losses. These effects can be substantially mitigated by performing active laser cooling in the presence of atomic dressing.
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C. Bardyn, M. Baranov, E. Rico Ortega, A. Imamoglu, P. Zoller, S. Diehl Majorana Modes in DrivenDissipative Atomic Superfluids With Zero Chern Number,
Phys. Rev. Lett. 109 130402 (20120925),
http://dx.doi.org/10.1103/PhysRevLett.109.130402 doi:10.1103/PhysRevLett.109.130402 (ID: 717881)
Toggle Abstract
We investigate dissipationinduced pwave paired states of fermions in two dimensions and show that dissipation can break the bulkedge correspondence present in Hamiltonian systems in a way that leads to the appearance of spatially separated Majorana zero modes in a phase with vanishing Chern number. We construct an explicit model of a dissipative vortex that traps a single of these modes and establish its topological origin by mapping it to a onedimensional wire where we observe a nonequilibrium topological phase transition characterized by an abrupt change of a topological invariant (winding number). Engineered dissipation opens up possibilities for experimentally realizing such states with no Hamiltonian counterpart.
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A. Tomadin, S. Diehl, M. Lukin, P. Rabl, P. Zoller Reservoir engineering and dynamical phase transitions in optomechanical arrays,
Phys. Rev. A 86 033821 (20120914),
http://dx.doi.org/10.1103/PhysRevA.86.033821 doi:10.1103/PhysRevA.86.033821 (ID: 718117)
Toggle Abstract
We study the drivendissipative dynamics of photons interacting with an array of micromechanical membranes in an optical cavity. Periodic membrane driving and phonon creation result in an effective photonnumber conserving nonunitary dynamics, which features a steady state with longrange photonic coherence. If the leakage of photons out of the cavity is counteracted by incoherent driving of the photonic modes, we show that the system undergoes a dynamical phase transition to the state with longrange coherence. A minimal system, composed of two micromechanical membranes in a cavity, is studied in detail, and it is shown to be a realistic setup where the key processes of the drivendissipative dynamics can be seen.
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M. Baranov, M. Dalmonte, G. Pupillo, P. Zoller Condensed Matter Theory of Dipolar Quantum Gases,
Chem. Rev. (20120809),
http://dx.doi.org/10.1021/cr2003568 doi:10.1021/cr2003568 (ID: 718140)
Toggle Abstract
Recent experimental breakthroughs in trapping, cooling and controlling ultracold gases of polar molecules, magnetic and Rydberg atoms have paved the way toward the investigation of highly tunable quantum systems, where anisotropic, longrange dipolar interactions play a prominent role at the manybody level. In this article we review recent theoretical studies concerning the physics of such systems. Starting from a general discussion on interaction design techniques and microscopic Hamiltonians, we provide a summary of recent work focused on manybody properties of dipolar systems, including: weakly interacting Bose gases, weakly interacting Fermi gases, multilayer systems, strongly interacting dipolar gases and dipolar gases in 1D and quasi1D geometries. Within each of these topics, purely dipolar effects and connections with experimental realizations are emphasized.
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A. J. Daley, H. Pichler, J. Schachenmayer, P. Zoller Measuring entanglement growth in quench dynamics of bosons in an optical lattice,
Phys. Rev. Lett. 109 020505 (20120712),
http://dx.doi.org/10.1103/PhysRevLett.109.020505 doi:10.1103/PhysRevLett.109.020505 (ID: 718070)
Toggle Abstract
We discuss a scheme to measure the manybody entanglement growth during quench dynamics with bosonic atoms in optical lattices. By making use of a 1D or 2D setup in which two copies of the same state are prepared, we show how arbitrary order Renyi entropies can be extracted using tunnelcoupling between the copies and measurement of the parity of onsite occupation numbers, as has been performed in recent experiments. We illustrate these ideas for a SuperfluidMott insulator quench in the BoseHubbard model, and also for hardcore bosons, and show that the scheme is robust against imperfections in the measurements.
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K. Stannigel, P. Komar, S. Habraken, S. D. Bennett, M. Lukin, P. Zoller, P. Rabl Optomechanical quantum information processing with photons and phonons,
Phys. Rev. Lett. 109 013603 (20120706),
http://dx.doi.org/10.1103/PhysRevLett.109.013603 doi:10.1103/PhysRevLett.109.013603 (ID: 717990)
Toggle Abstract
We describe how strong resonant interactions in multimode optomechanical systems can be used to induce controlled nonlinear couplings between single photons and phonons. Combined with linear mapping schemes between photons and phonons, these techniques provide a universal building block for various classical and quantum information processing applications. Our approach is especially suited for nanooptomechanical devices, where strong optomechanical coupling on a single photon level is within experimental reach.
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K. Stannigel, P. Rabl, P. Zoller Drivendissipative preparation of entangled states in cascaded quantumoptical networks,
New J. Phys. 14 063014 (20120614),
http://dx.doi.org/10.1088/13672630/14/6/063014 doi:10.1088/13672630/14/6/063014 (ID: 717830)
Toggle Abstract
We study the dissipative dynamics and the formation of entangled states in driven cascaded quantum networks, where multiple systems are coupled to a common unidirectional bath. Specifically, we identify the conditions under which emission and coherent reabsorption of radiation drives the whole network into a pure stationary state with nontrivial quantum correlations between the individual nodes. For the case of cascaded twolevel systems, we present an explicit preparation scheme that allows for tuning the network into different classes of multipartite entangled states. We discuss potential realizations of such cascaded networks with optical and microwave photons, where these effects could be used, for example, for new longdistance entanglement distribution protocols.
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B. Zhao, A. Glätzle, G. Pupillo, P. Zoller Atomic Rydberg Reservoirs for Polar Molecules,
Phys. Rev. Lett. 108 193007 (20120511),
http://dx.doi.org/10.1103/PhysRevLett.108.193007 doi:10.1103/PhysRevLett.108.193007 (ID: 717841)
Toggle Abstract
We discuss laser dressed dipolar and Van der Waals interactions between atoms and polar
molecules, so that a cold atomic gas with laser admixed Rydberg levels acts as a designed reservoir
for both elastic and inelastic collisional processes. The elastic scattering channel is characterized by
large elastic scattering cross sections and repulsive shields to protect from close encounter collisions.
In addition, we discuss a dissipative (inelastic) collision where a spontaneously emitted photon carries
away (kinetic) energy of the collision partners, thus providing a significant energy loss in a single
collision. This leads to the scenario of rapid thermalization and cooling of a molecule in the mK
down to the uK regime by cold atoms.
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W. Yi, S. Diehl, A. J. Daley, P. Zoller Drivendissipative manybody pairing states for cold fermionic atoms in an optical lattice,
New J. Phys. 14 055002 (20120501),
http://dx.doi.org/10.1088/13672630/14/5/055002 doi:10.1088/13672630/14/5/055002 (ID: 717858)
Toggle Abstract
We discuss the preparation of manybody states of cold fermionic atoms in an optical lattice via controlled dissipative processes induced by coupling the system to a reservoir. Based on a mechanism combining Pauli blocking and phase locking between adjacent sites, we construct complete sets of jump operators describing coupling to a reservoir that leads to dissipative preparation of pairing states for fermions with various symmetries in the absence of direct interparticle interactions. We discuss the uniqueness of these states, and demonstrate it with smallscale numerical simulations. In the late time dissipative dynamics, we identify a "dissipative gap" that persists in the thermodynamic limit. This gap implies exponential convergence of all manybody observables to their steady state values. We then investigate how these pairing states can be used as a starting point for the preparation of the ground state of FermiHubbard Hamiltonian via an adiabatic state preparation process also involving the parent Hamiltonian of the pairing state. We also provide a proofofprinciple example for implementing these dissipative processes and the parent Hamiltonians of the pairing states, based on Yb171 atoms in optical lattice potentials.
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I. Cirac, P. Zoller Goals and opportunities in quantum simulation,
Nature Phys. 8 266 (20120402),
http://dx.doi.org/10.1038/nphys2275 doi:10.1038/nphys2275 (ID: 718063)
Toggle Abstract
Richard Feynman's presentation 'There's plenty of room at the bottom'1 at the American Physical Society meeting in 1959 is widely acknowledged as one of the main inspirations for the development of nanotechnologies. In the same talk, Feynman also anticipated the possibilities that quantum mechanics can offer us in the microscopic world: “When we get to the very, very small world we have a lot of new things that would happen that represent completely new opportunities for design. Atoms on a small scale behave like nothing on a large scale, for they satisfy the laws of quantum mechanics. So, as we go down and fiddle around with the atoms down there, we are working with different laws, and we can expect to do different things.”
The field of quantum information explores such 'different things' and tries to exploit the laws of quantum mechanics that involve the superposition principle to carry out computational tasks in a more efficient way than is possible with devices governed by classical physics. Experimental progress during the past decades has been extraordinary, and has enabled us to isolate single microscopic particles, to manipulate and control their internal quantum states, and to detect them with almost perfect fidelity. Those advances notwithstanding, a quantum computer is still a longterm goal, requiring the full control of a manybody system and, eventually, the implementation of sophisticated errorcorrection protocols to achieve fault tolerance. So, as we still have to wait for a fully fledged universal quantuminformation processor, is it possible to build on the experimental advances made so far, and construct a device not quite at the level of complexity of a quantum computer, but one that still could perform some tasks that classical devices cannot?
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K. Stannigel, P. Rabl, A. Sorensen, M. Lukin, P. Zoller Optomechanical transducers for quantum information processing,
Phys. Rev. A 84 042341 (20111031),
http://dx.doi.org/10.1103/PhysRevA.84.042341 doi:10.1103/PhysRevA.84.042341 (ID: 717703)
Toggle Abstract
We discuss the implementation of optical quantum networks where the interface between stationary and photonic qubits is realized by optomechanical transducers [K. Stannigel et al., PRL 105, 220501 (2010)]. This approach does not rely on the optical properties of the qubit and thereby enables optical quantum communication applications for a wide range of solidstate spin and chargebased systems. We present an effective description of such networks for many qubits and give a derivation of a state transfer protocol for longdistance quantum communication. We also describe how to mediate local onchip interactions by means of the optomechanical transducers that can be used for entangling gates. We finally discuss experimental systems for the realization of our proposal.
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R. Sandner, M. Müller, A. J. Daley, P. Zoller Spatial Pauli blocking of spontaneous emission in optical lattices,
Phys. Rev. A 84 043825 (20111014),
http://dx.doi.org/10.1103/PhysRevA.84.043825 doi:10.1103/PhysRevA.84.043825 (ID: 717732)
Toggle Abstract
Spontaneous emission by an excited fermionic atom can be suppressed due to the Pauli exclusion principle if the relevant final states after the decay are already occupied by identical atoms in the ground state. Here we discuss a setup where a single atom is prepared in the first excited state on a single site of an optical lattice under conditions of very tight trapping. We investigate these phenomena in the context of two experimental realizations: (1) with alkali atoms, where the decay rate of the excited state is large and (2) with alkaline earthlike atoms, where the decay rate from metastable states can be tuned in experiments. This phenomenon has potential applications towards reservoir engineering and dissipative manybody state preparation in an optical lattice.
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M. Ortner, Y. Zhou, P. Rabl, P. Zoller Quantum information processing in selfassembled crystals of cold polar molecules,
Quant. Inf. Proc. 10 819 (20111013),
http://dx.doi.org/10.1007/s1112801103017 doi:10.1007/s1112801103017 (ID: 717726)
Toggle Abstract
We discuss the implementation of quantum gate operations in a selfassembled dipolar crystal of polar molecules. Here qubits are encoded in longlived spin states of the molecular ground state and stabilized against collisions by repulsive dipoledipole interactions. To overcome the single site addressability problem in this high density crystalline phase, we describe a new approach for implementing controlled single and twoqubit operations based on resonantly enhanced spinspin interactions mediated by a localized phonon mode. This local mode is created at a specified lattice position with the help of an additional marker molecule such that individual qubits can be manipulated by using otherwise global static and microwave fields only. We present a general strategy for generating state and time dependent dipole moments to implement a universal set of gate operations for molecular qubits and we analyze the resulting gate fidelities under realistic conditions. Our analysis demonstrates the experimental feasibility of this approach for scalable quantum computing or digital quantum simulation schemes with polar molecules.
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M. Dalmonte, P. Zoller, G. Pupillo Trimer liquids and crystals of polar molecules in coupled wires,
Phys. Rev. Lett. 107 163202 (20111011),
http://dx.doi.org/10.1103/PhysRevLett.107.163202 doi:10.1103/PhysRevLett.107.163202 (ID: 717651)
Toggle Abstract
We investigate the pairing and crystalline instabilities of bosonic and fermionic polar molecules confined to a ladder geometry. By means of analytical and quasiexact numerical techniques, we show that gases of composite molecular dimers as well as trimers can be stabilized as a function of the density difference between the wires. A shallow optical lattice can pin both liquids, realizing crystals of composite bosons or fermions. We show that these exotic quantum phases should be realizable under current experimental conditions in finitesize confining potentials.
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S. Diehl, E. Rico Ortega, M. Baranov, P. Zoller Topology by Dissipation in Atomic Quantum Wires,
Nature Phys. 7 971 (20111002),
http://dx.doi.org/10.1038/nphys2106 doi:10.1038/nphys2106 (ID: 717686)
Toggle Abstract
Robust edge states and nonAbelian excitations are the trademark of topological states of matter, with promising applications such as "topologically protected" quantum memory and computing. While so far topological phases have been exclusively discussed in a Hamiltonian context, we show that such phases and the associated topological protection and phenomena also emerge in open quantum systems with engineered dissipation. The specific system studied here is a quantum wire of spinless atomic fermions in an optical lattice coupled to a bath. The key feature of the dissipative dynamics described by a Lindblad master equation is the existence of Majorana edge modes, representing a nonlocal decoherence free subspace. The isolation of the edge states is enforced by a dissipative gap in the pwave paired bulk of the wire. We describe dissipative nonAbelian braiding operations within the Majorana subspace, and we illustrate the insensitivity to imperfections. Topological protection is granted by a nontrivial winding number of the system density matrix.
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B. P. Lanyon, C. Hempel, D. Nigg, M. Müller, R. Gerritsma, F. Zähringer, P. Schindler, J. T. Barreiro, M. Rambach, G. Kirchmair, M. Hennrich, P. Zoller, R. Blatt, C. F. Roos Universal Digital Quantum Simulation with Trapped Ions,
Science 334 57 (20110901),
http://dx.doi.org/10.1126/science.1208001 doi:10.1126/science.1208001 (ID: 717768)
Toggle Abstract
A digital quantum simulator is an envisioned quantum device that can be programmed to efficiently simulate any other local system. We demonstrate and investigate the digital approach to quantum simulation in a system of trapped ions. Using sequences of up to 100 gates and 6 qubits, the fulltime dynamics of a range of spin systems are digitally simulated. Interactions beyond those naturally present in our simulator are accurately reproduced, and quantitative bounds are provided for the overall simulation quality. Our results demonstrate the key principles of digital quantum simulation and provide evidence that the level of control required for a fullscale device is within reach.
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M. Müller, K. Hammerer, Y. Zhou, C. F. Roos, P. Zoller Simulating open quantum systems: from manybody interactions to stabilizer pumping,
New J. Phys. 13 085007 (20110810),
http://dx.doi.org/10.1088/13672630/13/8/085007 doi:10.1088/13672630/13/8/085007 (ID: 717750)
Toggle Abstract
In a recent experiment, Barreiro et al (2011 Nature 470 486) demonstrated the fundamental building blocks of an opensystem quantum simulator with trapped ions. Using up to five ions, dynamics were realized by sequences that combined single and multiqubit entangling gate operations with optical pumping. This enabled the implementation of both coherent manybody dynamics and dissipative processes by controlling the coupling of the system to an artificial, suitably tailored environment. This engineering was illustrated by the dissipative preparation of entangled two and fourqubit states, the simulation of coherent fourbody spin interactions and the quantum nondemolition measurement of a multiqubit stabilizer operator. In this paper, we present the theoretical framework of this gatebased ('digital') simulation approach for opensystem dynamics with trapped ions. In addition, we discuss how within this simulation approach, minimal instances of spin models of interest in the context of topological quantum computing and condensed matter physics can be realized in stateoftheart linear iontrap quantum computing architectures. We outline concrete simulation schemes for Kitaev's toric code Hamiltonian and a recently suggested color code model. The presented simulation protocols can be adapted to scalable and twodimensional iontrap architectures, which are currently under development.
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A. J. Daley, J. Ye, P. Zoller Statedependent lattices for quantum computing with alkalineearthmetal atoms,
Eur. Phys. J. D 65 217 (20110729),
http://dx.doi.org/10.1140/epjd/e2011200952 doi:10.1140/epjd/e2011200952 (ID: 717608)
Toggle Abstract
Recent experimental progress with AlkalineEarth atoms has opened the door to quantum computing schemes in which qubits are encoded in longlived nuclear spin states, and the metastable electronic states of these species are used for manipulation and readout of the qubits. Here we discuss a variant of these schemes, in which gate operations are performed in nuclearspindependent optical lattices, formed by nearresonant coupling to the metastable excited state. This provides an alternative to a previous scheme [A. J. Daley, M. M. Boyd, J. Ye, and P. Zoller, Phys. Rev. Lett 101, 170504 (2008)], which involved independent lattices for different electronic states. As in the previous case, we show how existing ideas for quantum computing with Alkali atoms such as entanglement via controlled collisions can be freed from important technical restrictions. We also provide additional details on the use of collisional losses from metastable states to perform gate operations via a lossy blockade mechanism.
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L. Jiang, T. Kitagawa, J. Alicea, A. Akhmerov, D. Pekker, G. Refael, I. J. Cirac, E. Demler, M. Lukin, P. Zoller Majorana Fermions in Equilibrium and in Driven ColdAtom Quantum Wires,
Phys. Rev. Lett. 106 220402 (20110602),
http://dx.doi.org/10.1103/PhysRevLett.106.220402 doi:10.1103/PhysRevLett.106.220402 (ID: 717687)
Toggle Abstract
We introduce a new approach to create and detect Majorana fermions using optically trapped 1D fermionic atoms. In our proposed setup, two internal states of the atoms couple via an optical Raman transition—simultaneously inducing an effective spinorbit interaction and magnetic field—while a background molecular BEC cloud generates swave pairing for the atoms. The resulting coldatom quantum wire supports Majorana fermions at phase boundaries between topologically trivial and nontrivial regions, as well as “Floquet Majorana fermions” when the system is periodically driven. We analyze experimental parameters, detection schemes, and various imperfections.
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J. Schachenmayer, A. J. Daley, P. Zoller Atomic matterwave revivals with definite atom number in an optical lattice,
Phys. Rev. A 83 043614 (20110418),
http://dx.doi.org/10.1103/PhysRevA.83.043614 doi:10.1103/PhysRevA.83.043614 (ID: 717393)
Toggle Abstract
We study the collapse and revival of interference patterns in the momentum distribution of atoms in optical lattices, using a projection technique to properly account for the fixed total number of atoms in the system. We consider the common experimental situation in which weakly interacting bosons are loaded into a shallow lattice, which is suddenly made deep. The collapse and revival of peaks in the momentum distribution is then driven by interactions in a lattice with essentially no tunnelling. The projection technique allows to us to treat inhomogeneous (trapped) systems exactly in the case that noninteracting bosons are loaded into the system initially, and we use timedependent density matrix renormalization group techniques to study the system in the case of finite tunnelling in the lattice and finite initial interactions. For systems of more than a few sites and particles, we find good agreement with results calculated via a naive approach, in which the state at each lattice site is described by a coherent state in the particle occupation number. However, for systems on the order of 10 lattice sites, we find experimentally measurable discrepancies to the results predicted by this standard approach.
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F. SchmidtKaler, T. Feldker, D. Kolbe, J. Walz, M. Müller, P. Zoller, W. Li, I. Lesanovsky Rydberg excitation of trapped cold ions: A detailed case study,
New J. Phys. 13 075014 (20110415),
http://dx.doi.org/10.1088/13672630/13/7/075014 doi:10.1088/13672630/13/7/075014 (ID: 717667)
Toggle Abstract
We provide a detailed theoretical and conceptual study of a planned
experiment to excite Rydberg states of ions trapped in a Paul trap. The ultimate
goal is to exploit the strong state dependent interactions between Rydberg ions to
implement quantum information processing protocols and to simulate the dynamics of
strongly interacting spin systems. We highlight the promises of this approach when
combining the high degree of control and readout of quantum states in trapped ion
crystals with the novel and fast gate schemes based on interacting giant Rydberg atomic
dipole moments. We discuss anticipated theoretical and experimental challenges on the
way towards its realization.
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J. T. Barreiro, M. Müller, P. Schindler, D. Nigg, T. Monz, M. Chwalla, M. Hennrich, C. F. Roos, P. Zoller, R. Blatt An opensystem quantum simulator with trapped ions,
Nature 470 491 (20110224),
http://dx.doi.org/10.1038/nature09801 doi:10.1038/nature09801 (ID: 717617)
Toggle Abstract
The control of quantum systems is of fundamental scientific interest and promises powerful applications and
technologies. Impressive progress has been achieved in isolating quantum systems from the environment and
coherently controlling their dynamics, as demonstrated by the creation and manipulation of entanglement in various
physical systems. However, for open quantum systems, engineering the dynamics of many particles by a controlled
coupling to an environment remains largely unexplored. Here we realize an experimental toolbox for simulating an open
quantum system with up to five quantum bits (qubits). Using a quantum computing architecture with trapped ions, we
combine multiqubit gates with optical pumping to implement coherent operations and dissipative processes. We
illustrate our ability to engineer the opensystem dynamics through the dissipative preparation of entangled states,
the simulation of coherent manybody spin interactions, and the quantum nondemolition measurement of multiqubit
observables. By adding controlled dissipation to coherent operations, this work offers novel prospects for opensystem
quantum simulation and computation.
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A. Tomadin, S. Diehl, P. Zoller Nonequilibrium phase diagram of a driven and dissipative manybody system,
Phys. Rev. A 83 013611 (20110118),
http://dx.doi.org/10.1103/PhysRevA.83.013611 doi:10.1103/PhysRevA.83.013611 (ID: 717765)
Toggle Abstract
We study the nonequilibrium dynamics of a manybody bosonic system on a lattice, subject to driving and dissipation. The time evolution is described by a master equation, which we treat within a generalized Gutzwiller mean field approximation for density matrices. The dissipative processes are engineered such that the system, in the absence of interaction between the bosons, is driven into a homogeneous steady state with offdiagonal longrange order. We investigate how the coherent interaction affects the properties of the steady state of the system qualitatively and derive a nonequilibrium phase diagram featuring a phase transition into a steady state without longrange order. The phase diagram also exhibits an extended domain where an instability of the homogeneous steady state gives rise to a persistent density pattern with spontaneously broken translational symmetry. In the limit of low particle density, we provide a precise analytical description of the time evolution during the instability. Moreover, we investigate the transient following a quantum quench of the dissipative processes and we elucidate the prominent role played by collective topological variables in this regime.
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P. Zoller Quantum Optics of Many Body Systems,
International Symposium on Frontiers in Quantum Photon Science (Hamburg, Germany, 20111123) URL (20111125),
(ID: 717857)
Toggle Abstract
Among the unique achievements of theoretical quantum optics are control of quantum systems, the development of a theory of quantum noise, and in particular how we "think" of open system dynamics in close relation to experiment. While historically many of these concepts and applications have been developed in the context of single or few particle physics, we will focus on recent developments of a quantum optics with emphasis on many body systems.The talk will discuss examples for systems of cold atom, molecules and ions, to solid state quantum optics, where engineered couplings to an environment lead to a new nonequilibrium condensed matter physics, and to new tools in quantum information to manipulate and prepare entangled states.

P. Zoller Open System Quantum Simulations with Quantum Optical Systems,
ERC meeting on Quantum Optics (Shanghai, China, 20111102) (20111102),
(ID: 717825)
Toggle Abstract
Abstract: We discuss engineering of open system dynamics in systems of cold atoms and ions. We are interested in controlled coupling to an environment both as a tool for quantum information processing and quantum simulation, as wells as in the context of nonequilibrium condensed matter physics. Our discussion will start with a brief review of a "digital" open system Rydberg quantum simulator [1], and related experiments with cold trapped ions demonstrating entangled state preparation by dissipation [2]. The main part of the talk will focus on "analog" open system quantum simulation, including the examples of dwave pairing induced by dissipation [3], and topology via dissipation in an atomic quantum wire [4].
[1] H. Weimer, M. Müller, I. Lesanovsky, P. Zoller and H.P. Büchler, Nature Physics 6, 382 (2010)
[2] J. T. Barreiro, M. Müller, P. Schindler, D. Nigg, T. Monz, M. Chwalla, M. Hennrich, C. F. Roos, P. Zoller, and R. Blatt, Nature 470, 486491 (2011).
[3] S. Diehl, W. Yi, A. Daley, and P. Zoller, Phys Rev Lett 105, (2010).
[4] E. Rico, S. Diehl, M. Baranov, and P. Zoller, Nature Physics in press (2010)

P. Zoller Quantum Information with Engineered Dissipation in Quantum Optical Systems,
QIPC 2011 (Zurich, Switzerland, 20110905) URL (20110905),
(ID: 717761)

P. Zoller Engineered Dissipation for Quantum Information and Many Body Physics,
Symposium on "Pushing Frontiers in Quantum Information with Atoms and Photons" (Heidelberg, 20110901) URL (20110903),
(ID: 717762)

P. Zoller Engineered Dissipation and Quantum Many Body Physics,
Frontiers of Quantum and Mesoscopic Thermodynamics (Prague, Czech Republic, 20110725) URL (20110726),
(ID: 717733)
Toggle Abstract
We discuss engineering of open system dynamics in systems of cold atoms and ions. We are
interested in controlled coupling to an environment both as a tool for quantum information
processing and quantum simulation, as wells as in the context of nonequilibrium condensed
matter physics. Our discussion will start with a description of an open system Rydberg quantum
simulator [1], and we present a related experiment with cold trapped ions demonstrating
entangled state preparation by dissipation [2]. We then present examples of dwave pairing
induced by dissipation [3], and a dissipative version of Majorana edge states [4].
[1] H. Weimer, M. M¨uller, I. Lesanovsky, P. Zoller, and H. P. B¨uchler, Nature Physics 6, 382
(2010)
[2] J. T. Barreiro, M. M¨uller, P. Schindler, D. Nigg, T. Monz, M. Chwalla, M. Hennrich, C. F.
Roos, P. Zoller, and R. Blatt, Nature 470, 486491 (2011)
[3] S. Diehl, W. Yi, A. Daley, and P. Zoller, Phys Rev Lett 105, 227001 (2010)
[4] E. Rico Ortega, S. Diehl, M. Baranov, and P. Zoller, in preparation

P. Zoller Engineered Dissipation and Quantum Many Body Physics,
Gordon Research Conference on Atomic Physics (West Dover, Vermont, USA, 20110626) URL (20110701),
(ID: 717705)

P. Zoller Open System Quantum Simulations with Cold Atoms, Molecules and Ions,
DAMOP11 meeting of The American Physical Society (Atlanta, USA, 20110613) URL (20110617),
(ID: 717702)
Toggle Abstract
We discuss concepts and possible implementations of open system quantum simulation with quantum optical
systems of cold atoms, molecules and ions. We first explain the general concepts of coherent control in open quantum systems, and we relate these ideas to
quantum information and nonequilibirium condensed matter physics. The specific systems to be discussed include cold atoms in optical lattices coupled coupled
to a BEC as a phonon reservoir, and an open system Rydberg quantum simulator. We finally discuss theory as well as recent experiments with trapped ions
which have demonstrated the basic elements of a such an open system quantum simulator.

P. Zoller Engineered Dissipation for Quantum Information and Many Body Physics,
International Conference on Quantum Information (ICQI) (Ottawa, Canada) URL (20110606),
(ID: 717697)
Toggle Abstract
We discuss engineering of open system dynamics in systems of cold atoms and ions. We are interested in controlled coupling to an environment both as a tool for quantum information processing and quantum simulation, as wells as in the context of nonequilibrium condensed matter physics. Our discussion will start with a description of an open system Rydberg quantum simulator [1], and we present a related experiment with cold trapped ions demonstrating entangled state preparation by dissipation [2]. We then present examples of dwave pairing induced by dissipation [3], and a dissipative version of Majorana edge states [4].
[1] H. Weimer, M. Müller, I. Lesanovsky, P. Zoller and H.P. Büchler, Nature Physics 6, 382 (2010)
[2] J. T. Barreiro, M. Müller, P. Schindler, D. Nigg, T. Monz, M. Chwalla, M. Hennrich, C. F. Roos, P. Zoller, and R. Blatt, Nature 470, 486491 (2011).
[3] S. Diehl, W. Yi, A. Daley, and P. Zoller, Phys Rev Lett 105, (2010).
[4] E. Ortega, S. Diehl, M. Baranov, and P. Zoller, in preparation

P. Zoller Open System Quantum Simulations with Cold Atoms, Molecules and Ions,
ICOLS 2011 (Hameln, Germany, 20110529) URL (20110531),
(ID: 717714)
Toggle Abstract
A universal quantum simulator is a controlled quantum device that reproduces the dynamics of any other many‐particle quantum system with short‐range interactions. This dynamics can refer to both coherent Hamiltonian and dissipative open‐system evolution. While impressive progress has been reported in isolating the systems from the environment and coherently controlling their many body dynamics in both quantum computing and quantum simulation, we will focus here on the engineering the /open system/ dynamics of many particles by a controlled coupling to an environment. We will discuss both the basic theoretical concepts [1,2] as well as their physical implementation with cold Rydberg atoms [3] and ions [4]. In particular, for a system of trapped ions we report the first realization of a toolbox for simulating an open quantum system with up to five qubits [4]. Using a quantum computing architecture with trapped ions, we combine multi‐qubit gates with optical pumping to implement coherent operations and dissipative processes. We illustrate this engineering by the dissipative preparation of entangled states, the simulation of coherent many‐body spin interactions and the quantum non‐demolition measurement of multi‐qubit observables. We conclude with an outlook on prospects for open‐system quantum simulation and computation.
[1] S. Diehl A. Micheli, A. Kantian, B. Kraus, H. P. Büchler and P. Zoller, /Nature Physics/ *4*, 878 (2008) [2] B. Kraus, H. P. Büchler, S. Diehl, A. Kantian, A. Micheli, and P. Zoller, /Phys. Rev. A /*78*, 042307 (2008) [3] H. Weimer, M. Müller, I. Lesanovsky, P. Zoller and H.P. Büchler, /Nature Physics/ *6*, 382 (2010) [4] J. T. Barreiro, M. Müller, P. Schindler, D. Nigg, T. Monz, M. Chwalla, M. Hennrich, C. F. Roos, P. Zoller and R. Blatt, submitted for publication

P. Zoller Open System Quantum Coherent Control and Quantum Simulation and Implementation,
Workshop on Ion Trap Technology (Boulder, USA, 20110216) URL (20110216),
(ID: 717612)

P. Zoller Hybrid Systems: Atoms and OptoNanomechanics,
Workshop on "Optomechanics and Macroscopic Cooling" (Cambridge, USA, 20110207) URL (20110207),
(ID: 717618)
Toggle Abstract
We discuss a range of problems considering the interaction of atomic ensembles in free space and
inside cavities with optonanomechanical oscillators. Topics to be discussed include: Optical
lattices with micromechanical mirrors [1], strong coupling of a single atom to an oscillator in
CQED [2,3], and establishing continuous variable EPR correlations between atomic ensembles and
a optonanomechanical system [4].
References:
[1] K. Hammerer, K. Stannigel, C. Genes, P. Zoller, P. Treutlein, S. Camerer, D. Hunger, and T. W.
Hänsch, Phys. Rev. A 82, 021803(R) (2010)
[2] M. Wallquist, K. Hammerer, P. Zoller, C. Genes, M. Ludwig, F. Marquardt, P. Treutlein, J. Ye, and
H. J. Kimble, Phys. Rev. A 81, 023816 (2010)
[3] K. Hammerer, M. Wallquist, C. Genes, M. Ludwig, F. Marquardt, P. Treutlein, P. Zoller, J. Ye, and
H. J. Kimble Phys. Rev. Lett. 103, 063005 (2009)
[4] K. Hammerer, M. Aspelmeyer, E. S. Polzik, and P. Zoller, Phys. Rev. Lett. 102, 020501 (2009)

P. Zoller Many Body Quantum Simulations with Rydberg Atoms,
Workshop "Quantum simulators with ultracold atoms" (Paris, France, 20110203) (20110203),
(ID: 717606)

P. Zoller Quantum Computing and Quantum Simulation with Quantum Optical Systems,
Erwin Schrödinger Symposium (Wien, Austria, 20110113) (20110115),
(ID: 717394)

P. Zoller Quantum Information and Condensed Matter Physics with Cold Atoms, Molecules and Ions,
International Workshop on Quantum Physics of LowDimensional Systems and Materials (Stellenbosch, South Africa, 20110103) URL (20110107),
(ID: 717388)

H. Pichler, A. J. Daley, P. Zoller Nonequilibrium dynamics of bosonic atoms in optical lattices: Decoherence of manybody states due to spontaneous emission,
Phys. Rev. A 82 063605 (20101206),
http://dx.doi.org/10.1103/PhysRevA.82.063605 doi:10.1103/PhysRevA.82.063605 (ID: 717301)
Toggle Abstract
We analyze in detail the heating of bosonic atoms in an optical lattice due to incoherent scattering of light from the lasers forming the lattice. Because atoms scattered into higher bands do not thermalize on the time scale of typical experiments, this process cannot be described by the total energy increase in the system alone (which is determined by singleparticle effects). The heating instead involves an important interplay between the atomic physics of the heating process and the manybody physics of the state. We characterize the effects on manybody states for various system parameters, where we observe important differences in the heating for strongly and weakly interacting regimes, as well as a strong dependence on the sign of the laser detuning from the excited atomic state. We compute heating rates and changes to characteristic correlation functions based on both perturbationtheory calculations and a timedependent calculation of the dissipative manybody dynamics. The latter is made possible for onedimensional systems by combining timedependent densitymatrixrenormalizationgroup methods with quantum trajectory techniques.
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K. Stannigel, P. Rabl, A. Sorensen, P. Zoller, M. Lukin Optomechanical Transducers for LongDistance Quantum Communication,
Phys. Rev. Lett. 105 220501 (20101123),
http://dx.doi.org/10.1103/PhysRevLett.105.220501 doi:10.1103/PhysRevLett.105.220501 (ID: 717347)
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We describe a new scheme to interconvert stationary and photonic qubits which is based on indirect qubitlight interactions mediated by a mechanical resonator. This approach does not rely on the specific optical response of the qubit and thereby enables optical quantum interfaces for a wide range of solid state spin and charge based systems. We discuss the implementation of state transfer protocols between distant nodes of a quantum network and show that high transfer fidelities can be achieved under realistic experimental conditions.
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S. Diehl, W. Yi, A. J. Daley, P. Zoller DissipationInduced dWave Pairing of Fermionic Atoms in an Optical Lattice,
Phys. Rev. Lett. 105 227001 (20101122),
http://dx.doi.org/10.1103/PhysRevLett.105.227001 doi:10.1103/PhysRevLett.105.227001 (ID: 717276)
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We show how dissipative dynamics can give rise to pairing for twocomponent fermions on a lattice. In particular, we construct a \\\\\\\"parent\\\\\\\" Liouvillian operator so that a BCStype state of a given symmetry, e.g. a dwave state, is reached for arbitrary initial states in the absence of conservative forces. The systembath couplings describe singleparticle, number conserving and quasilocal processes. The pairing mechanism crucially relies on Fermi statistics. We show how such Liouvillians can be realized via reservoir engineering with cold atoms representing a driven dissipative dynamics.
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M. Dalmonte, G. Pupillo, P. Zoller OneDimensional Quantum Liquids with PowerLaw Interactions: The Luttinger Staircase,
Phys. Rev. Lett. 105 140401 (20100928),
http://dx.doi.org/10.1103/PhysRevLett.105.140401 doi:10.1103/PhysRevLett.105.140401 (ID: 717312)
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We study onedimensional fermionic and bosonic gases with repulsive powerlaw interactions 1/xβ, with β>1, in the framework of TomonagaLuttinger liquid (TLL) theory. We obtain an accurate analytical expression linking the TLL parameter to the microscopic Hamiltonian, for arbitrary β and strength of the interactions. In the presence of a small periodic potential, powerlaw interactions make the TLL unstable towards the formation of a cascade of lattice solids with fractional filling, a “Luttinger staircase.” Several of these quantum phases and phase transitions are realized with ground state polar molecules and weakly bound magnetic Feshbach molecules.
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F. Cinti, P. Jain, M. Boninsegni, A. Micheli, P. Zoller, G. Pupillo Supersolid Droplet Crystal in a DipoleBlockaded Gas,
Phys. Rev. Lett. 105 135301 (20100921),
http://dx.doi.org/10.1103/PhysRevLett.105.135301 doi:10.1103/PhysRevLett.105.135301 (ID: 717214)
Toggle Abstract
A novel supersolid phase is predicted for an ensemble of Rydberg atoms in the dipoleblockade regime, interacting via a repulsive dipolar potential \"softened\" at short distances. Using exact numerical techniques, we study the low temperature phase diagram of this system, and observe an intriguing phase consisting of a crystal of mesoscopic superfluid droplets. At low temperature, phase coherence throughout the whole system, and the ensuing bulk superfluidity, are established through tunnelling of identical particles between neighbouring droplets.
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K. Hammerer, K. Stannigel, C. Genes, P. Zoller, P. Treutlein, S. Camerer, D. Hunger, T. W. Hänsch Optical Lattices with Micromechanical Mirrors,
Phys. Rev. A 82 021803(R) (20100825),
http://dx.doi.org/10.1103/PhysRevA.82.021803 doi:10.1103/PhysRevA.82.021803 (ID: 717187)
Toggle Abstract
We investigate a setup where a cloud of atoms is trapped in an optical lattice potential of a standingwave laser field which is created by retroreflection on a micromembrane. The membrane vibrations itself realize a quantum mechanical degree of freedom. We show that the centerofmass mode of atoms can be coupled to the vibrational mode of the membrane in free space. Via laser cooling of atoms a significant sympathetic cooling effect on the membrane vibrations can be achieved. Switching off laser cooling brings the system close to a regime of strong coherent coupling. This setup provides a controllable segregation between the cooling and coherent dynamics regimes, and allows one to keep the membrane in a cryogenic environment and atoms at a distance in a vacuum chamber.
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S. Diehl, M. Baranov, A. J. Daley, P. Zoller Quantum Field Theory for the ThreeBody Constrained Lattice Bose Gas  Part I: Formal Developments,
Phys. Rev. B 82 064509 (20100813),
http://dx.doi.org/10.1103/PhysRevB.82.064509 doi:10.1103/PhysRevB.82.064509 (ID: 716839)
Toggle Abstract
We develop a quantum field theoretical framework to analytically study the threebody constrained BoseHubbard model beyond mean field and noninteracting spin wave approximations. It is based on an exact mapping of the constrained model to a theory with two coupled bosonic degrees of freedom with polynomial interactions, which have a natural interpretation as single particles and twoparticle states. The procedure can be seen as a proper quantization of the Gutzwiller mean field theory. The theory is conveniently evaluated in the framework of the quantum effective action, for which the usual symmetry principles are now supplemented with a ``constraint principle'' operative on short distances. We test the theory via investigation of scattering properties of few particles in the limit of vanishing density, and we address the complementary problem in the limit of maximum filling, where the low lying excitations are holes and diholes on top of the constraint induced insulator. This is the first of a sequence of two papers. The application of the formalism to the manybody problem, which can be realized with atoms in optical lattices with strong threebody loss, is performed in a related work [13].
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S. Diehl, M. Baranov, A. J. Daley, P. Zoller Quantum Field Theory for the ThreeBody Constrained Lattice Bose Gas  Part II: Application to the ManyBody Problem,
Phys. Rev. B 82 064510 (20100813),
http://dx.doi.org/10.1103/PhysRevB.82.064510 doi:10.1103/PhysRevB.82.064510 (ID: 716840)
Toggle Abstract
We analyze the ground state phase diagram of attractive lattice bosons, which are stabilized by a threebody onsite hardcore constraint. A salient feature of this model is an Ising type transition from a conventional atomic superfluid to a dimer superfluid with vanishing atomic condensate. The study builds on an exact mapping of the constrained model to a theory of coupled bosons with polynomial interactions, proposed in a related paper [11]. In this framework, we focus by analytical means on aspects of the phase diagram which are intimately connected to interactions, and are thus not accessible in a mean field plus spin wave approach. First, we determine shifts in the mean field phase border, which are most pronounced in the low density regime. Second, the investigation of the strong coupling limit reveals the existence of a new collective mode, which emerges as a consequence of enhanced symmetries in this regime. Third, we show that the Ising type phase transition, driven first order via the competition of long wavelength modes at generic fillings, terminates into a true Ising quantum critical point in the vicinity of half filling.
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A. Micheli, Z. Idziaszek, G. Pupillo, M. Baranov, P. Zoller, P. S. Julienne Universal rates for reactive ultracold polar molecules in reduced dimensions,
Phys. Rev. Lett. 105 073202 (20100813),
http://dx.doi.org/10.1103/PhysRevLett.105.073202 doi:10.1103/PhysRevLett.105.073202 (ID: 717207)
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Analytic expressions describe universal elastic and reactive rates of quasitwodimensional and quasionedimensional collisions of highly reactive ultracold molecules interacting by a van der Waals potential. Exact and approximate calculations for the example species of KRb show that stability and evaporative cooling can be realized for spinpolarized fermions at moderate dipole and trapping strength, whereas bosons or unlike fermions require significantly higher dipole or trapping strengths.
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Z. Idziaszek, T. Calarco, P. Zoller Ionassisted groundstate cooling of a trapped polar molecule,
Phys. Rev. A 83 053413 (20100811),
http://dx.doi.org/10.1103/PhysRevA.83.053413 doi:10.1103/PhysRevA.83.053413 (ID: 717300)
Toggle Abstract
We propose and analyze a scheme for sympathetic cooling of the translational motion of polar molecules in an optical lattice, interacting one by one with lasercooled ions in a radiofrequency trap. The energy gap between the excitation spectra of the particles in their respective trapping potentials is bridged by means of a parametric resonance, provided by the additional modulation of the RF field. We analyze two scenarios: simultaneous laser cooling and energy exchange between the ion and the molecule, and a scheme when these two processes take place separately. We calculate the lowest final energy of the molecule and the cooling rate depending on the amplitude of the parametric modulation. For small parametric modulation, the dynamics can be solved analytically within the rotating wave approximation.
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S. Diehl, A. Tomadin, A. Micheli, R. Fazio, P. Zoller Dynamical Phase Transitions and Instabilities in Open Atomic ManyBody Systems,
Phys. Rev. Lett. 105 015702 (20100701),
http://dx.doi.org/10.1103/PhysRevLett.105.015702 doi:10.1103/PhysRevLett.105.015702 (ID: 717159)
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We discuss an open drivendissipative manybody system, in which the competition of unitary Hamiltonian and dissipative Liouvillian dynamics leads to a nonequilibrium phase transition. It shares features of a quantum phase transition in that it is interaction driven, and of a classical phase transition, in that the ordered phase is continuously connected to a thermal state. Within a generalized Gutzwiller approach which includes the description of mixed state density matrices, we characterize the complete phase diagram and the critical behavior at the phase transition approached as a function of time. We find a novel fluctuation induced dynamical instability, which occurs at long wavelength as a consequence of a subtle dissipative renormalization effect on the speed of sound.
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G. Pupillo, A. Micheli, M. Boninsegni, I. Lesanovsky, P. Zoller Strongly correlated gases of Rydbergdressed atoms: quantum and classical dynamics,
Phys. Rev. Lett. 104 223002 (20100601),
http://dx.doi.org/10.1103/PhysRevLett.104.223002 doi:10.1103/PhysRevLett.104.223002 (ID: 716985)
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We discuss techniques to generate longrange interactions in a gas of ground state alkali atoms, by weakly admixing excited Rydberg states with laser light. This provides a tool to engineer strongly correlated phases with reduced decoherence from inelastic collisions and spontaneous emission. As an illustration, we discuss the quantum phases of dressed atoms with dipoledipole interactions confined in a harmonic potential, as relevant to experiments. We show that residual spontaneous emission from the Rydberg state acts as a heating mechanism, leading to a quantumclassical crossover.
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P. Rabl, S. J. Kolkowitz, F. H. Koppens, J. Harris, P. Zoller, M. Lukin A quantum spin transducer based on nanoelectromechanical resonator arrays,
Nature Phys. 608 (20100530),
http://dx.doi.org/10.1038/nphys1679 doi:10.1038/nphys1679 (ID: 702633)
Toggle Abstract
Isolated electronic and nuclear spins in solids are at present being actively explored for potential quantumcomputing applications. Spin degrees of freedom provide an excellent quantum memory, owing to their weak magnetic interactions with the environment. For the same reason, however, it is difficult to achieve controlled interactions of spins over distances larger than tens of nanometres. Here we propose a new realization of a quantum data bus for spin qubits where spins are coupled to the motion of magnetized mechanical resonators through magneticfield gradients. Provided that the mechanical system is charged, the magnetic moments associated with spin qubits can be effectively amplified to enable a coherent spin–spin coupling over long distances through Coulomb forces. Our approach is applicable to a wide class of electronic spin qubits, which can be localized near magnetized tips and can be used for the implementation of hybrid quantumcomputing architectures.
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B. Zhao, K. Hammerer, M. Müller, P. Zoller Efficient quantum repeater based on deterministic Rydberg gates,
Phys. Rev. A 81 052329 (20100521),
http://dx.doi.org/10.1103/PhysRevA.81.052329 doi:10.1103/PhysRevA.81.052329 (ID: 717186)
Toggle Abstract
We propose an efficient quantum repeater architecture with mesoscopic atomic ensembles, where the Rydberg blockade is employed for deterministic local entanglement generation, entanglement swapping, and entanglement purification. Compared to a conventional atomicensemblebased quantum repeater, the entanglement distribution rate is improved by up to two orders of magnitude with the help of the deterministic Rydberg gate. This quantum repeater scheme is robust and fast, and thus opens up a way for practical longdistance quantum communication.
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A. Kantian, A. J. Daley, P. Zoller An etacondensate of fermionic atom pairs via adiabatic state preparation,
Phys. Rev. Lett. 104 (20100519),
http://dx.doi.org/10.1103/PhysRevLett.104.240406 doi:10.1103/PhysRevLett.104.240406 (ID: 716970)
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We discuss how an $\\eta$condensate, corresponding to an exact excited eigenstate of the FermiHubbard model, can be produced with cold atoms in an optical lattice. Using timedependent density matrix renormalisation group methods, we analyse a state preparation scheme beginning from a band insulator state in an optical superlattice. This state can act as an important test case, both for adiabatic preparation methods and the implementation of the manybody Hamiltonian, and measurements on the final state can be used to help detect associated errors.
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S. Diehl, M. Baranov, A. J. Daley, P. Zoller Observability of Quantum Criticality and a Continuous Supersolid in Atomic Gases,
Phys. Rev. Lett. 104 165301 (20100420),
http://dx.doi.org/10.1103/PhysRevLett.104.165301 doi:10.1103/PhysRevLett.104.165301 (ID: 716766)
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We analyze the BoseHubbard model with threebody onsite hardcore constraint, which stabilizes the system for an attractive interparticle interaction and allows, in particular, the formation of a superfluid phase of bosonic dimers. Our approach is based on an exact mapping of the constrained Hamiltonian to a theory of two coupled bosonic degrees of freedom. We demonstrate that the phase transition between atomic and dimer superfluidity is generically of the first order as a result of the ColemanWeinberg phenomenon, while at unit filling we identify an Ising quantum critical point. At this filling, furthermore, a symmetry enhancement in the strong coupling limit leads to a continuous supersolid phase for deeply bound dimers, observable in experiments.
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A. Glätzle, K. Hammerer, A. J. Daley, R. Blatt, P. Zoller A single trapped atom in front of an oscillating mirror,
Opt. Com. 283 765 (20100315),
http://dx.doi.org/10.1016/j.optcom.2009.10.063 doi:10.1016/j.optcom.2009.10.063 (ID: 717009)
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We investigate the Wigner–Weisskopf decay of a twolevel atom in front of an oscillating mirror. This work builds on and extends previous theoretical and experimental studies of the effects of a static mirror on spontaneous decay and resonance fluorescence. The spontaneously emitted field is inherently nonstationary due to the timedependent boundary conditions and in order to study its spectral distribution we employ the operational definition of the spectrum of nonstationary light due to the seminal work by Eberly and Wódkiewicz. We find a rich dependence of this spectrum as well as of the effective decay rates and level shifts on the mirror–atom distance and on the amplitude and frequency of the mirror’s oscillations. The results presented here provide the basis for future studies of more complex setups, where the motion of the atom and/or the mirror are included as quantum degrees of freedom.
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H. Weimer, M. Müller, I. Lesanovsky, P. Zoller, H. Büchler A Rydberg Quantum Simulator,
Nature Phys. 6 388 (20100314),
http://dx.doi.org/10.1038/nphys1614 doi:10.1038/nphys1614 (ID: 695077)
Toggle Abstract
Following Feynman and as elaborated on by Lloyd, a universal quantum simulator (QS) is a controlled quantum device which reproduces the dynamics of any other many particle quantum system with short range interactions. This dynamics can refer to both coherent Hamiltonian and dissipative open system evolution. We investigate how laser excited Rydberg atoms in large spacing optical or magnetic lattices can provide an efficient implementation of a universal QS for spin models involving (high order) nbody interactions. This includes the simulation of Hamiltonians of exotic spin models involving nparticle constraints such as the Kitaev toric code, color code, and lattice gauge theories with spin liquid phases. In addition, it provides the ingredients for dissipative preparation of entangled states based on engineering nparticle reservoir couplings. The key basic building blocks of our architecture are efficient and highfidelity nqubit entangling gates via auxiliary Rydberg atoms, including a possible dissipative time step via optical pumping. This allows to mimic the time evolution of the system by a sequence of fast, parallel and highfidelity nparticle coherent and dissipative Rydberg gates.
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A. V. Gorshkov, M. Hermele, V. Gurarie, C. Xu, P. S. Julienne, J. Ye, P. Zoller, E. Demler, M. Lukin, A. M. Rey Twoorbital SU(N) magnetism with ultracold alkalineearth atoms,
Nature Phys. 6 295 (20100228),
http://dx.doi.org/10.1038/nphys1535 doi:10.1038/nphys1535 (ID: 681277)
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Fermionic alkalineearth atoms have unique properties that make them attractive candidates for the realization of atomic clocks and degenerate quantum gases. At the same time, they are attracting considerable theoretical attention in the context of quantum information processing. Here we demonstrate that when such atoms are loaded in optical lattices, they can be used as quantum simulators of unique manybody phenomena. In particular, we show that the decoupling of the nuclear spin from the electronic angular momentum can be used to implement manybody systems with an unprecedented degree of symmetry, characterized by the SU(N) group with N as large as 10. Moreover, the interplay of the nuclear spin with the electronic degree of freedom provided by a stable optically excited state should enable the study of physics governed by the spin–orbital interaction. Such systems may provide valuable insights into the physics of strongly correlated transitionmetal oxides, heavyfermion materials and spinliquid phases.
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M. Baranov, A. Micheli, S. Ronen, P. Zoller Bilayer superfluidity of fermionic polar molecules: many body effects,
Phys. Rev. A 83 043602 (20100224),
http://dx.doi.org/10.1103/PhysRevA.83.043602 doi:10.1103/PhysRevA.83.043602 (ID: 717481)
Toggle Abstract
We study the BCS superfluid transition in a singlecomponent fermionic gas of dipolar particles loaded in a tight bilayer trap, with the electric dipole moments polarized perpendicular to the layers. Based on the detailed analysis of the interlayer scattering, we calculate the critical temperature of the interlayer superfluid pairing transition when the layer separation is both smaller (dilute regime) and of the order or larger (dense regime) than the mean interparticle separation in each layer. Our calculations go beyond the standard BCS approach and include the manybody contributions resulting in the mass renormalization, as well as additional contributions to the pairing interaction. We find that the manybody effects have a pronounced effect on the critical temperature, and can either decrease (in the very dilute limit) or increase (in the dense and moderately dilute limits) the transition temperature as compared to the BCS approach.
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B. CapogrossoSansone, C. Trefzger, M. Lewenstein, P. Zoller, G. Pupillo Quantum Phases of Cold Polar Molecules in 2D Optical Lattices,
Phys. Rev. Lett. 104 125301 (20100223),
http://dx.doi.org/10.1103/PhysRevLett.104.125301 doi:10.1103/PhysRevLett.104.125301 (ID: 686428)
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We study the quantum phases of hardcore bosonic polar molecules on a twodimensional square lattice interacting via repulsive dipoledipole interactions. In the limit of small tunneling, we find evidence for a devil’s staircase, where Mott solids appear at rational fillings of the lattice. For finite tunneling, we establish the existence of extended regions of parameters where the ground state is a supersolid, obtained by doping the solids either with particles or vacancies. We discuss the effects of finite temperature and finitesize confining potentials as relevant to experiments.
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M. Wallquist, K. Hammerer, P. Zoller, C. Genes, M. Ludwig, F. Marquardt, P. Treutlein, J. Ye, H. Kimble Singleatom cavity QED and optomicromechanics,
Phys. Rev. A 81 023816 (20100218),
http://dx.doi.org/10.1103/PhysRevA.81.023816 doi:10.1103/PhysRevA.81.023816 (ID: 716968)
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In a recent publication [K. Hammerer, M. Wallquist, C. Genes, M. Ludwig, F. Marquardt, P. Treutlein, P. Zoller, J. Ye, and H. J. Kimble, Phys. Rev. Lett. 103, 063005 (2009)] we have shown the possibility to achieve strong coupling of the quantized motion of a micronsized mechanical system to the motion of a single trapped atom. In the proposed setup the coherent coupling between a SiN membrane and a single atom is mediated by the field of a high finesse cavity and can be much larger than the relevant decoherence rates. This makes the welldeveloped tools of cavity quantum electrodynamics with single atoms available in the realm of cavity optomechanics. In this article we elaborate on this scheme and provide detailed derivations and technical comments. Moreover, we give numerical as well as analytical results for a number of possible applications for transfer of squeezed or Fock states from atom to membrane as well as entanglement generation, taking full account of dissipation. In the limit of strongcoupling the preparation and verification of nonclassical states of a mesoscopic mechanical system is within reach.
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P. Zoller, M. Wallquist, K. Hammerer, P. Rabl, M. Lukin Hybrid quantum devices and quantum engineering,
Physica Scripta 014001 (20091214),
http://dx.doi.org/10.1088/00318949/2009/T137/014001 doi:10.1088/00318949/2009/T137/014001 (ID: 716806)
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We discuss prospects of building hybrid quantum devices involving elements of atomic and molecular physics, quantum optics and solidstate elements with the attempt to combine advantages of the respective systems in compatible experimental setups. In particular, we summarize our recent work on quantum hybrid devices and briefly discuss recent ideas for quantum networks. These include interfacing of molecular quantum memory with circuit QED, and using nanomechanical elements strongly coupled to qubits represented by electronic spins, as well as single atoms or atomic ensembles.
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A. Kantian, M. Dalmonte, S. Diehl, W. Hofstetter, P. Zoller, A. J. Daley Atomic Color Superfluid via ThreeBody Loss,
Phys. Rev. Lett. 103 240401 (20091209),
http://dx.doi.org/10.1103/PhysRevLett.103.240401 doi:10.1103/PhysRevLett.103.240401 (ID: 707092)
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Large threebody loss rates in a threecomponent Fermi gas confined in an optical lattice can dynamically prevent atoms from tunneling so as to occupy a lattice site with three atoms. This effective constraint not only suppresses the occurrence of actual loss events, but stabilizes BCSpairing phases by suppressing the formation of trions. We study the effect of the constraint on the manybody physics using bosonization and density matrix renormalization group techniques, and also investigate the full dissipative dynamics including loss for the example of 6Li.
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D. Chang, C. A. Regal, S. B. Papp, D. J. Wilson, J. Ye, O. Painter, H. Kimble, P. Zoller Cavity optomechanics using an optically levitated nanosphere,
Proc. Natl. Acad. U.S.A. 107 1010 (20091110),
http://dx.doi.org/10.1073/pnas.0912969107 doi:10.1073/pnas.0912969107 (ID: 719878)

D. Chang, C. A. Regal, S. B. Papp, D. J. Wilson, J. Ye, O. Painter, H. J. Kimble, P. Zoller Cavity optomechanics using an optically levitated nanosphere,
Proc. Natl. Acad. U.S.A. 107 1010 (20091017),
URL (ID: 708587)
Toggle Abstract
Recently, remarkable advances have been made in coupling a number of highQ modes of nanomechanical systems to highfinesse optical cavities, with the goal of reaching regimes in which quantum behavior can be observed and leveraged toward new applications. To reach this regime, the coupling between these systems and their thermal environments must be minimized. Here we propose a novel approach to this problem, in which optically levitating a nanomechanical system can greatly reduce its thermal contact, while simultaneously eliminating dissipation arising from clamping. Through the long coherence times allowed, this approach potentially opens the door to groundstate cooling and coherent manipulation of a single mesoscopic mechanical system or entanglement generation between spatially separate systems, even in roomtemperature environments. As an example, we show that these goals should be achievable when the mechanical mode consists of the centerofmass motion of a levitated nanosphere.
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D. Chang, J. D. Thompson, H. Park, V. Vuletic, P. Zoller, A. Zibrov, M. Lukin Trapping and manipulation of isolated atoms using nanoscale plasmonic structures,
Phys. Rev. Lett. 103 123004 (20090918),
http://dx.doi.org/10.1103/PhysRevLett.103.123004 doi:10.1103/PhysRevLett.103.123004 (ID: 686216)
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We propose and analyze a scheme to interface individual neutral atoms with nanoscale solidstate systems. The interface is enabled by optically trapping the atom via the strong nearfield generated by a sharp metallic nanotip. We show that under realistic conditions, a neutral atom can be trapped with position uncertainties of just a few nanometers, and within tens of nanometers of other surfaces. Simultaneously, the guided surface plasmon modes of the nanotip allow the atom to be optically manipulated, or for fluorescence photons to be collected, with very high efficiency. Finally, we analyze the surface forces, heating and decoherence rates acting on the trapped atom.
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Y. Han, Y. Chan, W. Yi, A. J. Daley, S. Diehl, P. Zoller, L. Duan Stabilization of the pwave superfluid state in an optical lattice,
Phys. Rev. Lett. 103 070404 (20090814),
http://dx.doi.org/10.1103/PhysRevLett.103.070404 doi:10.1103/PhysRevLett.103.070404 (ID: 707070)
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It is hard to stabilize the pwave superfluid state of cold atomic gas in free space due to inelastic collisional losses. We consider the pwave Feshbach resonance in an optical lattice, and show that it is possible to have a stable pwave superfluid state where the multiatom collisional loss is suppressed through the quantum Zeno effect. We derive the effective Hamiltonian for this system, and calculate its phase diagram in a onedimensional optical lattice. The results show rich phase transitions between the pwave superfluid state and different types of insulator states induced either by interaction or by dissipation.

K. Hammerer, M. Wallquist, C. Genes, M. Ludwig, F. Marquardt, P. Treutlein, P. Zoller, J. Ye, H. J. Kimble Strong coupling of a mechanical oscillator and a single atom,
Phys. Rev. Lett. 103 063005 (20090806),
http://dx.doi.org/10.1103/PhysRevLett.103.063005 doi:10.1103/PhysRevLett.103.063005 (ID: 680115)
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We propose and analyze a setup to achieve strong coupling between a single trapped atom and a mechanical oscillator. The interaction between the motion of the atom and the mechanical oscillator is mediated by a quantized light field in a laser driven highfinesse cavity. In particular, we show that high fidelity transfer of quantum states between the atom and the mechanical oscillator is in reach for existing or near future experimental parameters. Our setup provides the basic toolbox for coherent manipulation, preparation and measurement of micro and nanomechanical oscillators via the tools of atomic physics.
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K. Jähne, C. Genes, K. Hammerer, M. Wallquist, E. Polzik, P. Zoller Cavityassisted squeezing of a mechanical oscillator,
Phys. Rev. A 79 063819 (20090611),
http://dx.doi.org/10.1103/PhysRevA.79.063819 doi:10.1103/PhysRevA.79.063819 (ID: 679748)
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We investigate the creation of squeezed states of a vibrating membrane or a movable mirror in an optomechanical system. An optical cavity is driven by squeezed light and couples via radiation pressure to the membrane/mirror, effectively providing a squeezed heatbath for the mechanical oscillator. Under the conditions of laser cooling to the ground state, we find an efficient transfer of squeezing with roughly 60% of light squeezing conveyed to the membrane/mirror (on a dB scale). We determine the requirements on the carrier frequency and the bandwidth of squeezed light. Beyond the conditions of ground state cooling, we predict mechanical squashing to be observable in current systems.
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M. Ortner, A. Micheli, G. Pupillo, P. Zoller Quantum Simulations of Extended Hubbard Models with Dipolar Crystals,
New J. Phys. 11 055045 (20090514),
http://dx.doi.org/10.1088/13672630/11/5/055045 doi:10.1088/13672630/11/5/055045 (ID: 665436)
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In this paper we study the realization of lattice models in mixtures of atomic and dipolar molecular quantum gases. We consider a situation where polar molecules form a selfassembled dipolar lattice, in which atoms or molecules of a second species can move and scatter. We describe the system dynamics in a master equation approach in the Brownian motion limit of slow particles and fast phonons, which we find appropriate for our system. In a wide regime of parameters, the reduced dynamics of the particles leads to physical realizations of extended Hubbard models with tuneable longrange interactions mediated by crystal phonons. This extends the notion of quantum simulation of strongly correlated systems with cold atoms and molecules to include phonondynamics, where all coupling parameters can be controlled by external fields.
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M. Müller, I. Lesanovsky, H. Weimer, H. Büchler, P. Zoller Mesoscopic Rydberg Gate based on Electromagnetically Induced Transparency,
Phys. Rev. Lett. 102 170502 (20090428),
http://dx.doi.org/10.1103/PhysRevLett.102.170502 doi:10.1103/PhysRevLett.102.170502 (ID: 628410)
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We demonstrate theoretically a parallelized CNOT gate which allows to entangle a mesoscopic ensemble of atoms with a single control atom in a single step, with high fidelity and on a microsecond timescale. Our scheme relies on the strong and longranged interaction between Rydberg atoms triggering Electromagnetically Induced Transparency (EIT). By this we can robustly implement a conditional transfer of all ensemble atoms among two logical states, depending on the state of the control atom. We outline a many body interferometer which allows a comparison of two manybody quantum states by performing a measurement of the control atom.
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C. Schwenke, T. Calarco, P. Zoller, C. Koch Collective Rydberg excitations of an atomic gas confined in a ring lattice,
Phys. Rev. A 79 043419 (20090424),
http://dx.doi.org/10.1103/PhysRevA.79.043419 doi:10.1103/PhysRevA.79.043419 (ID: 650348)
Toggle Abstract
We study the excitation dynamics of Rydberg atoms in a onedimensional lattice with periodic boundary conditions where the atomic Rydberg states are resonantly excited from the electronic ground state. Our description of the corresponding dynamics is numerically exact within the perfect blockade regime, i.e. no two atoms in a given range can be excited. The timeevolution of the mean Rydberg density, densitydensity correlations as well as entanglement properties are analyzed in detail. We demonstrate that the short time dynamics is universal and dominated by quantum phenomena, while for larger time the characteristics of the lattice become important and the classical features determine the dynamics. The results of the perfect blockade approach are compared to the predictions of an effective Hamiltonian which includes the interaction of two neighboring Rydberg atoms up to second order perturbation theory.
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A. V. Gorshkov, A. M. Rey, A. J. Daley, M. M. Boyd, J. Ye, P. Zoller, M. Lukin AlkalineEarth Atoms as FewQubit Quantum Registers,
Phys. Rev. Lett. 102 110503 (20090318),
http://dx.doi.org/10.1103/PhysRevLett.102.110503 doi:10.1103/PhysRevLett.102.110503 (ID: 644448)
Toggle Abstract
We propose and analyze a novel approach to quantum information processing, in which multiple qubits can be encoded and manipulated using electronic and nuclear degrees of freedom associated with individual alkalineearthmetal atoms trapped in an optical lattice. Specifically, we describe how the qubits within each register can be individually manipulated and measured with subwavelength optical resolution. We also show how such fewqubit registers can be coupled to each other in optical superlattices via conditional tunneling to form a scalable quantum network. Finally, potential applications to quantum computation and precision measurements are discussed.
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S. Montangero, R. Fazio, P. Zoller, G. Pupillo Dipole oscillations of confined lattice bosons in one dimension,
Phys. Rev. A 79 041602(R) (20090310),
http://dx.doi.org/10.1103/PhysRevA.79.041602 doi:10.1103/PhysRevA.79.041602 (ID: 620456)
Toggle Abstract
We study the dynamics of a nonintegrable system comprising interacting cold bosons trapped in an optical lattice in onedimension by means of exact timedependent numerical DMRG techniques. Particles are confined by a parabolic potential, and dipole oscillations are induced by displacing the trap center of a few lattice sites. Depending on the system parameters this motion can vary from undamped to overdamped. We study the dipole oscillations as a function of the lattice displacement, the particle density and the strength of interparticle interactions. These results explain the recent experiment C.D. Fertig et al., Phys. Rev. Lett. 94, 120403 (2005).
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A. J. Daley, J. Taylor, S. Diehl, M. Baranov, P. Zoller Atomic threebody loss as a dynamical threebody interaction,
Phys. Rev. Lett. 102 040402 (20090130),
http://dx.doi.org/10.1103/PhysRevLett.102.040402 doi:10.1103/PhysRevLett.102.040402 (ID: 627388)
Toggle Abstract
We discuss how large threebody loss of atoms in an optical lattice can give rise to effective hardcore threebody interactions. For bosons, in addition to the usual atomic superfluid, a dimer superfluid can then be observed for attractive twobody interactions. The nonequilibrium dynamics of preparation and stability of these phases are studied in 1D by combining timedependent Density Matrix Renormalisation Group techniques with a quantum trajectories method.
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I. Lesanovsky, M. Müller, P. Zoller Trap assisted creation of giant molecules and Rydbergmediated coherent charge transfer in a Penning trap,
Phys. Rev. A 79 010701(R) (20090114),
http://dx.doi.org/10.1103/PhysRevA.79.010701 doi:10.1103/PhysRevA.79.010701 (ID: 620459)
Toggle Abstract
We study two ions confined in a Penning trap. We show that electronically highly excited states exist in which an electron is delocalized among the two ions forming a giant molecule of several micrometer size. At energies close to the top of the Coulomb barrier these molecular states can be regarded as superpositions of Rydberg states of individual ions. We illuminate the possibility to observe coherent charge transfer between the ions. Beyond a critical principal quantum number the electron can coherently tunnel through the Coulomb barrier to an adjacent doubly charged ion. The tunneling occurs on timescales on which the dynamics of the nuclei can be considered frozen and radiative decay can be neglected.
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K. Hammerer, M. Aspelmeyer, E. Polzik, P. Zoller Establishing EinsteinPoldoskyRosen Channels between Nanomechanics and Atomic Ensembles,
Phys. Rev. Lett. 102 020501 (20090112),
http://dx.doi.org/10.1103/PhysRevLett.102.020501 doi:10.1103/PhysRevLett.102.020501 (ID: 637866)
Toggle Abstract
We suggest interfacing nanomechanical systems via an optical quantum bus to atomic ensembles, for which means of high precision state preparation, manipulation, and measurement are available. This allows, in particular, for a quantum nondemolition Bell measurement, projecting the coupled system, atomicensemble–nanomechanical resonator, into an entangled EPR state. The entanglement is observable even for nanoresonators initially well above their ground states and can be utilized for teleportation of states from an atomic ensemble to the mechanical system.

B. CapogrossoSansone, S. Wessel, H. Büchler, P. Zoller, G. Pupillo Phase diagram of onedimensional hardcore bosons with threebody interactions,
Phys. Rev. B 79 020503 (R) (20090109),
http://dx.doi.org/10.1103/PhysRevB.79.020503 doi:10.1103/PhysRevB.79.020503 (ID: 603828)
Toggle Abstract
We determine the phase diagram of a onedimensional system of hardcore lattice bosons interacting via repulsive threebody interactions by analytic methods and extensive quantum Monte Carlo simulations. Such threebody interactions can be derived from a microscopic theory for polar molecules trapped in an optical lattice. Depending on the strength of the interactions and the particle density, we find superfluid and solid phases, the latter appearing at an unconventional filling of the lattice and displaying a coexistence of chargedensity wave and bond orders
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A. J. Daley, M. M. Boyd, J. Ye, P. Zoller Quantum Computing with AlkalineEarthMetal Atoms,
Phys. Rev. Lett. 101 170504 (20081023),
http://dx.doi.org/10.1103/PhysRevLett.101.170504 doi:10.1103/PhysRevLett.101.170504 (ID: 606889)
Toggle Abstract
We present a complete scheme for quantum information processing using the unique features of alkaline earth atoms. We show how two completely independent lattices can be formed for the $^1$S$_0$ and $^3$P$_0$ states, with one used as a storage lattice for qubits encoded on the nuclear spin, and the other as a transport lattice to move qubits and perform gate operations. We discuss how the $^3$P$_2$ level can be used for addressing of individual qubits, and how collisional losses from metastable states can be used to perform gates via a lossy blockade mechanism.
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M. Müller, L. Liang, I. Lesanovsky, P. Zoller Trapped Rydberg Ions: From Spin Chains to Fast Quantum Gates,
New J. Phys. 10 093009 (20080910),
http://dx.doi.org/10.1088/13672630/10/9/093009 doi:10.1088/13672630/10/9/093009 (ID: 520204)
Toggle Abstract
We study the quantum dynamics of trapped ions in Rydberg states, which have large electric dipole moments induced by the trapping fields. In the limit where the localization of the ion core is much smaller than the Rydberg orbit the dynamics of the system is determined by the interplay between large dipoledipole energy shifts and strong tunable internalstatedependent forces due to dipolecharge interactions. This provides a new tool for manipulating and entangling ions by strong interactions and on fast time scales.
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S. Diehl, A. Micheli, A. Kantian, B. Kraus, H. Büchler, P. Zoller Quantum States and Phases in Driven Open Quantum Systems with Cold Atoms,
Nature Phys. 4 883 (20080907),
http://dx.doi.org/10.1038/nphys1073 doi:10.1038/nphys1073 (ID: 576233)
Toggle Abstract
An open quantum system, whose time evolution is governed by a master equation, can be driven into a given pure quantum state by an appropriate design of the systemreservoir coupling. This points out a route towards preparing many body states and nonequilibrium quantum phases by quantum reservoir engineering. Here we discuss in detail the example of a \emph{driven dissipative Bose Einstein Condensate} of bosons and of paired fermions, where atoms in an optical lattice are coupled to a bath of Bogoliubov excitations via the atomic current representing \emph{local dissipation}. In the absence of interactions the lattice gas is driven into a pure state with long range order. Weak interactions lead to a weakly mixed state, which in 3D can be understood as a depletion of the condensate, and in 1D and 2D exhibits properties reminiscent of a Luttinger liquid or a KosterlitzThouless critical phase at finite temperature, with the role of the ``finite temperature'' played by the interactions.

A. V. Gorshkov, P. Rabl, G. Pupillo, A. Micheli, P. Zoller, M. Lukin, H. Büchler Suppression of Inelastic Collisions Between Polar Molecules With a Repulsive Shield,
Phys. Rev. Lett. 101 073201 (20080814),
http://dx.doi.org/10.1103/PhysRevLett.101.073201 doi:10.1103/PhysRevLett.101.073201 (ID: 582039)
Toggle Abstract
We propose and analyze a technique that allows to suppress inelastic collisions and simultaneously enhance elastic interactions between cold polar molecules. The main idea is to cancel the leading dipoledipole interaction with a suitable combination of static electric and microwave fields in such a way that the remaining vanderWaalstype potential forms a threedimensional repulsive shield. We analyze the elastic and inelastic scattering cross sections relevant for evaporative cooling of polar molecules and discuss the prospect for the creation of crystalline structures
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S. Morrison, A. Kantian, A. J. Daley, H. Katzgraber, M. Lewenstein, H. Büchler, P. Zoller Physical replicas and the Boseglass in cold atomic gases,
New J. Phys. 10 073032 (20080716),
http://dx.doi.org/10.1088/13672630/10/7/073032 doi:10.1088/13672630/10/7/073032 (ID: 582497)
Toggle Abstract
We study cold atomic gases in a disorder potential and analyse the correlations between different systems subjected to the same disorder landscape. Such independent copies with the same disorder landscape are known as replicas. While, in general, these are not accessible experimentally in condensed matter systems, they can be realized using standard tools for controlling cold atomic gases in an optical lattice. Of special interest is the overlap function which represents a natural order parameter for disordered systems and is a correlation function between the atoms of two independent replicas with the same disorder. We demonstrate an efficient measurement scheme for the determination of this disorderinduced correlation function. As an application, we focus on the disordered Bose–Hubbard model and determine the overlap function within the perturbation theory and a numerical analysis. We find that the measurement of the overlap function allows for the identification of the Boseglass phase in certain parameter regimes.
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W. Yi, A. J. Daley, G. Pupillo, P. Zoller Statedependent, addressable subwavelength lattices with cold atoms,
New J. Phys. 10 073015 (20080708),
http://dx.doi.org/10.1088/13672630/10/7/073015 doi:10.1088/13672630/10/7/073015 (ID: 582500)
Toggle Abstract
We discuss how adiabatic potentials can be used to create addressable lattices on a subwavelength scale, which can be used as a tool for local operations and readout within a lattice substructure, while taking advantage of the faster timescales and higher energy and temperature scales determined by the shorter lattice spacing. For alkalineearthlike atoms with nonzero nuclear spin, these potentials can be made state dependent, for which we give specific examples with $^{171}$Yb atoms. We discuss in detail the limitations in generating the lattice potentials, in particular nonadiabatic losses, and show that the loss rates can always be made exponentially small by increasing the laser power.
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M. Wallquist, P. Rabl, M. Lukin, P. Zoller Theory of cavityassisted microwave cooling of polar molecules,
New J. Phys. 10 063005 (20080604),
http://dx.doi.org/10.1088/13672630/10/6/063005 doi:10.1088/13672630/10/6/063005 (ID: 567588)
Toggle Abstract
We analyze cavityassisted cooling schemes for polar molecules in the microwave domain, where molecules are excited on a rotational transition and energy is dissipated via strong interactions with a lossy stripline cavity, as recently proposed by André et al 2006 Nat. Phys. 2 636. We identify the dominant cooling and heating mechanisms in this setup and study cooling rates and final temperatures in various parameter regimes. In particular, we analyze the effects of a finite environment temperature on the cooling efficiency, and find minimal temperature and optimized cooling rate in the strong drive regime. Further, we discuss the tradeoff between efficiency of cavity cooling and robustness with respect to ubiquitous imperfections in a realistic experimental setup, such as anharmonicity of the trapping potential.
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B. Kraus, H. Büchler, S. Diehl, A. Kantian, A. Micheli, P. Zoller Preparation of Entangled States by Quantum Markov Processes,
Phys. Rev. A 78 042307 (20080502),
http://dx.doi.org/10.1103/PhysRevA.78.042307 doi:10.1103/PhysRevA.78.042307 (ID: 576234)
Toggle Abstract
We investigate the possibility of using a dissipative process to prepare a quantum system in a desired state. We derive for any multipartite pure state a dissipative process for which this state is the unique stationary state and solve the corresponding master equation analytically. For certain states, such as the cluster states, we use this process to show that the jump operators can be chosen quasilocally, i.e. they act nontrivially only on a few, neighboring qubits. Furthermore, the relaxation time of this dissipative process is independent of the number of subsystems. We demonstrate the general formalism by considering arbitrary matrixproduct states or projected entangled pair states. In particular, we show that the ground state of the AffleckKennedyLiebTasaki model can be prepared employing a quasilocal dissipative process.

L. Jiang, G. Brennen, A. V. Gorshkov, K. Hammerer, M. Hafezi, E. Demler, M. Lukin, P. Zoller Anyonic interferometry and protected memories in atomic spin lattices,
Nature Phys. 4 482 (20080420),
http://dx.doi.org/10.1038/nphys943 doi:10.1038/nphys943 (ID: 538104)
Toggle Abstract
Strongly correlated quantum systems can exhibit exotic behavior called topological order which is characterized by nonlocal correlations that depend on the system topology. Such systems can exhibit remarkable phenomena such as quasiparticles with anyonic statistics and have been proposed as candidates for naturally faulttolerant quantum computation. Despite these remarkable properties, anyons have never been observed in nature directly. Here we describe how to unambiguously detect and characterize such states in recently proposed spin lattice realizations using ultracold atoms or molecules trapped in an optical lattice. We propose an experimentally feasible technique to access nonlocal degrees of freedom by performing global operations on trapped spins mediated by an optical cavity mode. We show how to reliably read and write topologically protected quantum memory using an atomic or photonic qubit. Furthermore, our technique can be used to probe statistics and dynamics of anyonic excitations.
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A. J. Daley, P. Zoller, B. Trauzettel Andreevlike reflections with cold atoms,
Phys. Rev. Lett. 100 110404 (20080320),
http://dx.doi.org/10.1103/PhysRevLett.100.110404 doi:10.1103/PhysRevLett.100.110404 (ID: 530306)
Toggle Abstract
We propose a setup in which Andreevlike reflections predicted for 1D transport systems could be observed timedependently using cold atoms in a 1D optical lattice. Using timedependent Density Matrix Renormalisation Group methods we analyse the wavepacket dynamics as a density excitation propagates across a boundary in the interaction strength. These phenomena exhibit good correspondence with predictions from Luttinger liquid models and could be observed in current experiments in the context of the BoseHubbard model.
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A. V. Gorshkov, L. Jiang, M. Greiner, P. Zoller, M. Lukin Coherent Quantum Optical Control with Subwavelength Resolution,
Phys. Rev. Lett. 100 093005 (20080307),
http://dx.doi.org/10.1103/PhysRevLett.100.093005 doi:10.1103/PhysRevLett.100.093005 (ID: 519563)
Toggle Abstract
We suggest a new method for quantum optical control with nanoscale resolution. Our method allows for coherent farfield manipulation of individual quantum systems with spatial selectivity that is not limited by the wavelength of radiation and can, in principle, approach a few nanometers. The selectivity is enabled by the nonlinear atomic response, under the conditions of Electromagnetically Induced Transparency, to a control beam with intensity vanishing at a certain location. Practical performance of this technique and its potential applications to quantum information science with cold atoms, ions, and solidstate qubits are discussed.
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G. Pupillo, A. Griessner, A. Micheli, M. Ortner, Wang, DawWei, P. Zoller Cold Atoms and Molecules in SelfAssembled Dipolar Lattices,
Phys. Rev. Lett. 100 050402 (20080205),
http://dx.doi.org/10.1103/PhysRevLett.100.050402 doi:10.1103/PhysRevLett.100.050402 (ID: 519056)
Toggle Abstract
We study the realization of lattice models, where cold atoms and molecules
move as extra particles in a dipolar crystal of trapped polar molecules. The
crystal is a selfassembled floating mesoscopic lattice structure with quantum
dynamics given by phonons. We show that within an experimentally accessible
parameter regime extended Hubbard models with tunable longrange
phononmediated interactions describe the effective dynamics of dressed
particles.
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T. Calarco, P. Grangier, A. Wallraff, P. Zoller Quantum leaps in small steps,
Nature Phys. 4 3 (20080101),
http://dx.doi.org/10.1038/nphys818 doi:10.1038/nphys818 (ID: 602500)
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A. Kantian, A. J. Daley, P. Törmä, P. Zoller Atomic lattice excitons: from condensates to crystals,
New J. Phys. 9 407 (20071113),
http://dx.doi.org/10.1088/13672630/9/11/407 doi:10.1088/13672630/9/11/407 (ID: 514263)
Toggle Abstract
We discuss atomic lattice excitons (ALEs), bound particlehole pairs formed by fermionic atoms in two bands of an optical lattice. Such a system provides a clean setup to study fundamental properties of excitons, ranging from condensation to exciton crystals (which appear for a large effective mass ratio between particles and holes). Using both meanfield treatments and 1D numerical computation, we discuss the properities of ALEs under varying conditions, and discuss in particular their preparation and measurement.
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P. Rabl, P. Zoller Molecular Dipolar Crystals as High Fidelity Quantum Memory for Hybrid Quantum Computing,
Phys. Rev. A 76 042308 (20071004),
http://dx.doi.org/10.1103/PhysRevA.76.042308 doi:10.1103/PhysRevA.76.042308 (ID: 494007)
Toggle Abstract
We study collective excitations of rotational and spin states of an ensemble of polar molecules, which are prepared in a dipolar crystalline phase, as a candidate for a high fidelity quantum memory. While dipolar crystals are formed in the high density limit of cold clouds of polar molecules under 1D and 2D trapping conditions, the crystalline structure protects the molecular qubits from detrimental effects of short range collisions. We calculate the lifetime of the quantum memory by identifying the dominant decoherence mechanisms, and estimate their effects on gate operations, when a molecular ensemble qubit is transferred to a superconducting strip line cavity (circuit QED). In the case rotational excitations coupled by dipoledipole interactions we identify phonons as the main limitation of the life time of qubits. We study specific setups and conditions, where the coupling to the phonon modes is minimized. Detailed results are presented for a 1D dipolar chain.

A. Micheli, G. Pupillo, H. Büchler, P. Zoller Cold polar molecules in 2D traps: Tailoring interactions with external fields for novel quantum phases,
Phys. Rev. A 76 043604 (20071003),
http://dx.doi.org/10.1103/PhysRevA.76.043604 doi:10.1103/PhysRevA.76.043604 (ID: 460560)
Toggle Abstract
We discuss techniques to engineer effective longrange interactions between polar molecules using external static electric and microwave fields. We consider a setup where molecules are trapped in a twodimensional pancake geometry by a faroffresonance optical trap, which ensures the stability of the dipolar collisions. We detail how to modify the shape and the strength of the longrange part of interaction potentials, which can be utilized to realize interesting quantum phases in the context of cold molecular gases.
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Z. Idziaszek, T. Calarco, P. Zoller Controlled collisions of a single atom and ion guided by movable trapping potentials,
Phys. Rev. A 76 033409 (20070919),
http://dx.doi.org/10.1103/PhysRevA.76.033409 doi:10.1103/PhysRevA.76.033409 (ID: 470101)
Toggle Abstract
We consider a system composed of a trapped atom and a trapped ion. The ion charge induces in the atom an electric dipole moment, which attracts it with an r^{4} dependence at large distances. In the regime considered here, the characteristic range of the atomion interaction is comparable or larger than the characteristic size of the trapping potential, which excludes the application of the contact pseudopotential. The shortrange part of the interaction is described in the framework of quantumdefect theory, by introducing some shortrange parameters, which can be related to the swave scattering length. When the separation between traps is changed we observe trapinduced shape resonances between molecular bound states and vibrational states of the external trapping potential. Our analysis is extended to quasionedimensional geometries, when the scattering exhibit confinementinduced resonances, similar to the ones studied before for shortrange interactions. For quasionedimensional systems we investigate the effects of coupling between the center of mass and relative motion, which occurs for different trapping frequencies of atom and ion traps. Finally, we show how the two types of resonances can be employed for quantum state control and spectroscopy of atomion molecules.
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H. Büchler, A. Micheli, P. Zoller Threebody interactions with cold polar molecules,
Nature Phys. 3 731 (20070722),
http://dx.doi.org/10.1038/nphys678 doi:10.1038/nphys678 (ID: 462909)
Toggle Abstract
We show that polar molecules driven by microwave fields give naturally rise to strong threebody interactions, while the twoparticle interaction can be independently controlled and even switched off. The derivation of these effective interaction potentials is based on a microscopic understanding of the underlying molecular physics, and follows from a well controlled and systematic expansion into manybody interaction terms. For molecules trapped in an optical lattice, we show that these interaction potentials give rise to Hubbard models with strong nearestneighbor twobody and threebody interaction. As an illustration, we study the onedimensional BoseHubbard model with dominant threebody interaction and derive its phase diagram.
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G. Brennen, A. Micheli, P. Zoller Designing spin1 lattice models using polar molecules,
New J. Phys. 9 138 (20070518),
http://dx.doi.org/10.1088/13672630/9/5/138 doi:10.1088/13672630/9/5/138 (ID: 430363)
Toggle Abstract
We describe how to design a large class of always on spin1 interactions between polar molecules trapped in an optical lattice. The spin degrees of freedom correspond to the hyperfine levels of a rovibrational ground state molecule. Interactions are induced using a microwave field to mix ground states in one hyperfine manifold with the spin entangled dipoledipole coupled excited states. Using multiple fields anistropic models in one, two, or three dimensions, can be built with tunable spatial range. An illustrative example in one dimension is the generalized Haldane model, which at a specific parameter has a gapped valence bond solid ground state. The interaction strengths are large compared to decoherence rates and should allow for probing the rich phase structure of strongly correlated systems, including dimerized and gapped phases.
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H. Büchler, E. Demler, M. Lukin, A. Micheli, N. Prokofev, G. Pupillo, P. Zoller Strongly correlated 2D quantum phases with cold polar molecules: controlling the shape of the interaction potential,
Phys. Rev. Lett. 98 060404 (20070208),
http://dx.doi.org/10.1103/PhysRevLett.98.060404 doi:10.1103/PhysRevLett.98.060404 (ID: 376275)
Toggle Abstract
We discuss techniques to tune and shape the longrange part of the interaction potentials in quantum gases of bosonic polar molecules by dressing rotational excitations with static and microwave fields. This provides a novel tool towards engineering strongly correlated quantum phases in combination with lowdimensional trapping geometries. As an illustration, we discuss the 2D superfluidcrystal quantum phase transition for polar molecules interacting via an electricfieldinduced dipoledipole potential.
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S. Tewari, S. D. Sarma, C. Nayak, C. Zhang, P. Zoller Quantum Computation using Vortices and Majorana Zero Modes of a px+ipy Superfluid of Fermionic Cold Atoms,
Phys. Rev. Lett. 98 010506 (20070105),
http://dx.doi.org/10.1103/PhysRevLett.98.010506 doi:10.1103/PhysRevLett.98.010506 (ID: 436036)
Toggle Abstract
We propose to use the recently predicted twodimensional “weakpairing” px+ipy superfluid state of fermionic cold atoms as a platform for topological quantum computation. In the core of a vortex, this state supports a zeroenergy Majorana mode, which moves to finite energy in the corresponding topologically trivial “strongpairing” state. By braiding vortices in the “weakpairing” state, unitary quantum gates can be applied to the Hilbert space of Majorana zero modes. For readout of the topological qubits, we propose realistic schemes suitable for atomic superfluids.
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A. Griessner, A. J. Daley, S. Clark, D. Jaksch, P. Zoller Dissipative dynamics of atomic Hubbard models coupled to a phonon bath: Dark state cooling of atoms within a Bloch band of an optical lattice,
New J. Phys. 9 44 (2007),
http://dx.doi.org/10.1088/13672630/9/2/044 doi:10.1088/13672630/9/2/044 (ID: 428626)
Toggle Abstract
We analyse a laser assisted sympathetic cooling scheme for atoms within the lowest Bloch band of an optical lattice. This scheme borrows ideas from subrecoil laser cooling, implementing them in a new context in which the atoms in the lattice are coupled to a Bose–Einstein condensate (BEC) reservoir. In this scheme, excitation of atoms between Bloch bands replaces the internal structure of atoms in normal laser cooling, and spontaneous emission of photons is replaced by creation of excitations in the BEC reservoir. We analyse the cooling process for many bosons and fermions, and obtain possible temperatures corresponding to a small fraction of the Bloch band width within our model. This system can be seen as a novel realisation of a manybody open quantum system.
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A. Griessner, A. J. Daley, S. Clark, D. Jaksch, P. Zoller Dark state cooling of atoms by superfluid immersion,
Phys. Rev. Lett. 97 220403 (20061204),
http://dx.doi.org/10.1103/PhysRevLett.97.220403 doi:10.1103/PhysRevLett.97.220403 (ID: 375452)
Toggle Abstract
We propose and analyze a scheme to cool atoms in an optical lattice to ultralow temperatures within a Bloch band and away from commensurate filling. The protocol is inspired by ideas from darkstate laser cooling but replaces electronic states with motional levels and spontaneous emission of photons by emission of phonons into a BoseEinstein condensate, in which the lattice is immersed. In our model, achievable temperatures correspond to a small fraction of the Bloch bandwidth and are much lower than the reservoir temperature. This is also a novel realization of an open quantum optical system, where known tools are combined with new ideas involving cooling via a reservoir.
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André, A, D. DeMille, J. Doyle, M. Lukin, S. E. Maxwell, P. Rabl, R. J. Schoelkopf, P. Zoller A coherent allelectrical interface between polar molecules and mesoscopic superconducting resonators,
Nature Phys. 2 636 (20060827),
http://dx.doi.org/10.1038/nphys386 doi:10.1038/nphys386 (ID: 363758)
Toggle Abstract
Building a scalable quantum processor requires coherent control and preservation of quantum coherence in a largescale quantum system. Mesoscopic solidstate systems such as Josephson junctions and quantum dots feature robust control techniques using local electrical signals and selfevident scaling; however, in general the quantum states decohere rapidly. In contrast, quantum optical systems based on trapped ions and neutral atoms exhibit much better coherence properties, but their miniaturization and integration with electrical circuits remains a challenge. Here we describe methods for the integration of a singleparticle system—an isolated polar molecule—with mesoscopic solidstate devices in a way that produces robust, coherent, quantumlevel control. Our setup provides a scalable cavityQEDtype quantum computer architecture, where entanglement of distant qubits stored in longlived rotational molecular states is achieved via exchange of microwave photons.
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M. Cozzini, T. Calarco, A. Recati, P. Zoller Fast Rydberg gates without dipole blockade via quantum control,
Opt. Com. 264 375 (20060815),
arXiv:quantph/0511118 arXiv:quantph/0511118 (ID: 313117)
Toggle Abstract
We propose a scheme for controlling interactions between Rydbergexcited neutral atoms in order to perform a fast highfidelity quantum gate. Unlike dipoleblockade mechanisms already found in the literature, we drive resonantly the atoms with a statedependent excitation to Rydberg levels, and we exploit the resulting dipole–dipole interaction to induce a controlled atomic motion in the trap, in a similar way as discussed in recent iontrap quantum computing proposals. This leads atoms to gain the required gate phase, which turns out to be a combination of a dynamic and a geometrical contribution. The fidelity of this scheme is studied including small anharmonicity and temperature effects, with promising results for reasonably achievable experimental parameters.
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P. Rabl, D. DeMille, J. Doyle, M. Lukin, R. J. Schoelkopf, P. Zoller Hybrid Quantum Processors: molecular ensembles as quantum memory for solid state circuits,
Phys. Rev. Lett. 97 033003 (20060721),
http://dx.doi.org/10.1103/PhysRevLett.97.033003 doi:10.1103/PhysRevLett.97.033003 (ID: 353814)
Toggle Abstract
We investigate a hybrid quantum circuit where ensembles of cold polar molecules serve as longlived quantum memories and optical interfaces for solid state quantum processors. The quantum memory realized by collective spin states (ensemble qubit) is coupled to a highQ stripline cavity via microwave Raman processes. We show that, for convenient trapsurface distances of a few µm, strong coupling between the cavity and ensemble qubit can be achieved. We discuss basic quantum information protocols, including a swap from the cavity photon bus to the molecular quantum memory, and a deterministic two qubit gate. Finally, we investigate coherence properties of molecular ensemble quantum bits.
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S. Trebst, U. Schollwöck, M. Troyer, P. Zoller dWave Resonating Valence Bond States of Fermionic Atoms in Optical Lattices,
Phys. Rev. Lett. 96 250402 (20060630),
http://dx.doi.org/10.1103/PhysRevLett.96.250402 doi:10.1103/PhysRevLett.96.250402 (ID: 372594)
Toggle Abstract
We study controlled generation and measurement of superfluid dwave resonating valence bond (RVB) states of fermionic atoms in 2D optical lattices. Starting from loading spatial and spin patterns of atoms in optical superlattices as pure quantum states from a Fermi gas, we adiabatically transform this state to an RVB state by a change of the lattice parameters. Results of exact timedependent numerical studies for ladders systems are presented, suggesting generation of RVB states on a time scale smaller than typical experimental decoherence times.
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K. Winkler, G. Thalhammer, F. Lang, R. Grimm, J. Hecker Denschlag, A. J. Daley, A. Kantian, H. Büchler, P. Zoller Repulsively bound atom pairs in an optical lattice,
Nature 441 853 (20060615),
http://dx.doi.org/10.1038/nature04918 doi:10.1038/nature04918 (ID: 371157)
Toggle Abstract
Throughout physics, stable composite objects are usually formed by way of attractive forces, which allow the constituents to lower their energy by binding together. Repulsive forces separate particles in free space. However, in a structured environment such as a periodic potential and in the absence of dissipation, stable composite objects can exist even for repulsive interactions. Here we report the observation of such an exotic bound state, which comprises a pair of ultracold rubidium atoms in an optical lattice. Consistent with our theoretical analysis, these repulsively bound pairs exhibit long lifetimes, even under conditions when they collide with one another. Signatures of the pairs are also recognized in the characteristic momentum distribution and through spectroscopic measurements. There is no analogue in traditional condensed matter systems of such repulsively bound pairs, owing to the presence of strong decay channels. Our results exemplify the strong correspondence between the optical lattice physics of ultracold bosonic atoms and the Bose–Hubbard model—a link that is vital for future applications of these systems to the study of strongly correlated condensed matter and to quantum information.
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A. Micheli, G. Brennen, P. Zoller A toolbox for lattice spin models with polar molecules,
Nature Phys. 2 341 (20060500),
http://dx.doi.org/10.1038/nphys287 doi:10.1038/nphys287 (ID: 313100)
Toggle Abstract
There is growing interest in states of matter with topological order. These are characterized by highly stable ground states robust to perturbations that preserve the topology, and which support excitations with socalled anyonic statistics. Topologically ordered states can arise in twodimensional latticespin models, which were proposed as the basis for a new class of quantum computation. Here, we show that the relevant hamiltonians for such spin lattice models can be systematically engineered with polar molecules stored in optical lattices, where the spin is represented by a singlevalence electron of a heteronuclear molecule. The combination of microwave excitation with dipole–dipole interactions and spin–rotation couplings enables building a complete toolbox for effective twospin interactions with designable range, spatial anisotropy and coupling strengths significantly larger than relevant decoherence rates. Finally, we illustrate two models: one with an energy gap providing for errorresilient qubit encoding, and another leading to topologically protected quantum memory.
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P. Bushev, D. Rotter, A. Wilson, F. Dubin, C. Becher, J. Eschner, R. Blatt, V. Steixner, P. Rabl, P. Zoller Feedback cooling of a single trapped ion,
Phys. Rev. Lett. 96 043003 (20060203),
http://dx.doi.org/10.1103/PhysRevLett.96.043003 doi:10.1103/PhysRevLett.96.043003 (ID: 332288)
Toggle Abstract
Based on a realtime measurement of the motion of a single ion in a Paul trap, we demonstrate its electromechanical cooling below the Doppler limit by homodyne feedback control (cold damping). The feedback cooling results are well described by a model based on a quantum mechanical master equation.

P. Zoller, T. Beth, D. Binosi, R. Blatt, H. J. Briegel, D. Bruss, T. Calarco, J. I. Cirac, D. Deutsch, J. Eisert, A. Ekert, C. Fabre, N. Gisin, P. Grangiere, M. Grassl, S. Haroche, A. Imamoglu, A. Karlson, J. Kempe, L. Louwenhofen, S. Kröll, G. Leuchs, M. Quantum information processing and communication,
Eur. Phys. J. D 36/2 203  228 (20051101),
http://dx.doi.org/10.1140/epjd/e2005002511 doi:10.1140/epjd/e2005002511 (ID: 375863)
Toggle Abstract
We present an excerpt of the document “Quantum Information Processing and Communication: Strategic report on current status, visions and goals for research in Europe”, which has been recently published in electronic form at the website of FET (the Future and Emerging Technologies Unit of the Directorate General Information Society of the European Commission, http://www.cordis.lu/ist/fet/qipcsr.htm). This document has been elaborated, following a former suggestion by FET, by a committee of QIPC scientists to provide input towards the European Commission for the preparation of the Seventh Framework Program. Besides being a document addressed to policy makers and funding agencies (both at the European and national level), the document contains a detailed scientific assessment of the stateoftheart, main research goals, challenges, strengths, weaknesses, visions and perspectives of all the most relevant QIPC subfields, that we report here. Dedicated to the memory of Prof. Th. Beth, one of the pioneers of QIPC, whose contributions have had a significant scientific impact on the development as well as on the visibility of a field that he enthusiastically helped to shape since its early days.

A. J. Daley, S. Clark, D. Jaksch, P. Zoller Numerical analysis of coherent manybody currents in a single atom transistor,
Phys. Rev. A 72 043618 (20051028),
http://dx.doi.org/10.1103/PhysRevA.72.043618 doi:10.1103/PhysRevA.72.043618 (ID: 308211)
Toggle Abstract
We study the dynamics of many atoms in the recently proposed singleatomtransistor setup [A. Micheli, A. J. Daley, D. Jaksch, and P. Zoller, Phys. Rev. Lett. 93, 140408 (2004)] using recently developed numerical methods. In this setup, a localized spin1/2 impurity is used to switch the transport of atoms in a onedimensional optical lattice: in one state the impurity is transparent to probe atoms, but in the other acts as a singleatom mirror. We calculate timedependent currents for bosons passing the impurity atom, and find interesting manybody effects. These include substantially different transport properties for bosons in the strongly interacting (Tonks) regime when compared with fermions, and an unexpected decrease in the current when weakly interacting probe atoms are initially accelerated to a nonzero mean momentum. We also provide more insight into the application of our numerical methods to this system, and discuss open questions about the currents approached by the system on long time scales.
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P. Rabl, V. Steixner, P. Zoller Quantumlimited velocity readout and quantum feedback cooling of a trapped ion via electromagnetically induced transparency,
Phys. Rev. A 72 043823 (20051027),
http://dx.doi.org/10.1103/PhysRevA.72.043823 doi:10.1103/PhysRevA.72.043823 (ID: 308209)
Toggle Abstract
We discuss continuous observation of the momentum of a single atom by employing the high velocity sensitivity of the index of refraction in a driven Lambdasystem based on electromagnetically induced transparency. In the ideal limit of unit collection efficiency this provides a quantumlimited measurement with minimal backaction on the atomic motion. A feedback loop, which drives the atom with a force proportional to measured signal, provides a cooling mechanism for the atomic motion. We derive the master equation which describes the feedback cooling and show that in the LambDicke limit the steady state energies are close to the ground state, limited only by the photon collection efficiency. Outside of the LambDicke regime the predicted temperatures are well below the Doppler limit.
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J. I. Cirac, P. Zoller Qubits, Gatter und Register,
Physik Journal 11 31 (20051000),
URL (ID: 332271)

A. Griessner, A. J. Daley, D. Jaksch, P. Zoller Faulttolerant dissipative preparation of atomic quantum registers with fermions,
Phys. Rev. A 72 032332 (20050928),
http://dx.doi.org/10.1103/PhysRevA.72.032332 doi:10.1103/PhysRevA.72.032332 (ID: 308207)
Toggle Abstract
We propose a faulttolerant loading scheme to produce an array of fermions in an optical lattice of the high fidelity required for applications in quantuminformation processing and the modeling of strongly correlated systems. A cold reservoir of fermions plays a dual role as a source of atoms to be loaded into the lattice via a Raman process and as a heat bath for sympathetic cooling of lattice atoms. Atoms are initially transferred into an excited motional state in each lattice site and then decay to the motional ground state, creating particlehole pairs in the reservoir. Atoms transferred into the ground motional level are no longer coupled back to the reservoir, and doubly occupied sites in the motional ground state are prevented by Pauli blocking. This scheme has strong conceptual connections with optical pumping and can be extended to load highfidelity patterns of atoms.
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U. Dorner, T. Calarco, P. Zoller, A. Browaeys, P. Grangiere Quantum logic via optimal control in holographic dipole traps,
J. Opt. B: Quantum Semiclass. Opt. 7 341346 (20050921),
http://dx.doi.org/10.1088/14644266/7/10/020 doi:10.1088/14644266/7/10/020 (ID: 308221)
Toggle Abstract
We propose a scheme for quantum logic with neutral atoms stored in an array of holographic dipole traps where the positions of the atoms can be rearranged by using holographic optical tweezers. In particular, this allows for the transport of two atoms to the same well where an external control field is used to perform gate operations via the molecular interaction between the atoms. We show that optimal control techniques allow for the fast implementation of the gates with high fidelity.
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W. H. Zurek, U. Dorner, P. Zoller Dynamics of a Quantum Phase Transition,
Phys. Rev. Lett. 95 105701 (20050901),
http://dx.doi.org/10.1103/PhysRevLett.95.105701 doi:10.1103/PhysRevLett.95.105701 (ID: 325892)
Toggle Abstract
We present two approaches to the dynamics of a quenchinduced phase transition in the quantum Ising model. One follows the standard treatment of thermodynamic second order phase transitions but applies it to the quantum phase transitions. The other approach is quantum, and uses LandauZener formula for transition probabilities in avoided level crossings. We show that predictions of the two approaches of how the density of defects scales with the quench rate are compatible, and discuss the ensuing insights into the dynamics of quantum phase transitions.
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V. Steixner, P. Rabl, P. Zoller Quantum feedback cooling of a single trapped ion in front of a mirror,
Phys. Rev. A 72 043826 (20050831),
http://dx.doi.org/10.1103/PhysRevA.72.043826 doi:10.1103/PhysRevA.72.043826 (ID: 308208)
Toggle Abstract
We develop a theory of quantum feedback cooling of a single ion trapped in front of a mirror. By monitoring the motional sidebands of the light emitted into the mirror mode we infer the position of the ion, and act back with an appropriate force to cool the ion. We derive a feedback master equation along the lines of the quantum feedback theory developed by Wiseman and Milburn, which provides us with cooling times and final temperatures as a function of feedback gain and various system parameters.
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K. Osterloh, M. Baig, L. Santos, P. Zoller, M. Lewenstein Cold Atoms in NonAbelian Gauge Potentials: From the Hofstadter "Moth" to Lattice Gauge Theory,
Phys. Rev. Lett. 95 010403 (20050628),
http://dx.doi.org/10.1103/PhysRevLett.95.010403 doi:10.1103/PhysRevLett.95.010403 (ID: 308223)
Toggle Abstract
We demonstrate how to create artificial external nonAbelian gauge potentials acting on cold atoms in optical lattices. The method employs atoms with k internal states, and laser assisted state sensitive tunneling, described by unitary k×k matrices. The singleparticle dynamics in the case of intense U(2) vector potentials lead to a generalized Hofstadter butterfly spectrum which shows a complex mothlike structure. We discuss the possibility to realize nonAbelian interferometry (AharonovBohm effect) and to study manybody dynamics of ultracold matter in external lattice gauge fields.

J. Taylor, W. Dür, P. Zoller, A. Yacoby, C. M. Marcus, M. Lukin SolidState Circuit for Spin Entanglement Generation and Purification,
Phys. Rev. Lett. 94 236803 (20050615),
http://dx.doi.org/10.1103/PhysRevLett.94.236803 doi:10.1103/PhysRevLett.94.236803 (ID: 308222)
Toggle Abstract
We show how realistic charge manipulation and measurement techniques, combined with the exchange interaction, allow for the robust generation and purification of fourparticle spin entangled states in electrically controlled semiconductor quantum dots. The generated states are immunized to the dominant sources of noise via a dynamical decoherencefree subspace; all additional errors are corrected by a purification protocol. This approach may find application in quantum computation, communication, and metrology.
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J. J. GarcíaRipoll, P. Zoller, J. I. Cirac Coherent control of trapped ions using offresonant lasers,
Phys. Rev. A 71 062309 (20050609),
http://dx.doi.org/10.1103/PhysRevA.71.062309 doi:10.1103/PhysRevA.71.062309 (ID: 308224)
Toggle Abstract
In this paper we develop a unified framework to study the coherent control of trapped ions subject to statedependent forces. Taking different limits in our theory, we can reproduce previous designs of quantum gates and propose a different design of fast gates based on continuous laser beams. We demonstrate how to simulate Ising Hamiltonians in a many ions setup, and how to create highly entangled states and induce squeezing. Finally, in a detailed analysis we identify the physical limits of this technique and study the dependence of errors on the temperature.
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J. J. GarcíaRipoll, P. Zoller, J. I. Cirac Quantum information processing with cold atoms and trapped ions,
J. Phys. B: At. Mol. Opt. Phys. 38 567578 (20050425),
http://dx.doi.org/10.1088/09534075/38/9/008 doi:10.1088/09534075/38/9/008 (ID: 308228)
Toggle Abstract
This paper summarizes some important achievements of quantum information processing with trapped ions or neutral atoms. In particular, we describe the storage of information and realization of twoqubit gates with ions, as well as the creation of entanglement and quantum simulation with cold atoms in optical lattices.
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A. Micheli, P. Zoller A Single Atom Mirror for 1D Atomic Lattice Gases,
Phys. Rev. A 73 043613 (20050420),
http://dx.doi.org/10.1103/PhysRevA.73.043613 doi:10.1103/PhysRevA.73.043613 (ID: 313116)
Toggle Abstract
We propose a scheme utilizing quantum interference to control the transport of atoms in a 1D optical lattice by a single impurity atom. The two internal state of the impurity represent a spin1/2 (qubit), which in one spin state is perfectly transparent to the lattice gas, and in the other spin state acts as a single atom mirror, confining the lattice gas. This allows to ``amplify'' the state of the qubit, and provides a singleshot quantum nondemolition measurement of the state of the qubit. We derive exact analytical expression for the scattering of a single atom by the impurity, and give approximate expressions for the dynamics a gas of many interacting bosonic of fermionic atoms.
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A. Recati, P. O. Fedichev, W. Zwerger, J. von Delft, P. Zoller Atomic Quantum Dots Coupled to a Reservoir of a Superfluid BoseEinstein Condensate,
Phys. Rev. Lett. 94 040404 (20050202),
http://dx.doi.org/10.1103/PhysRevLett.94.040404 doi:10.1103/PhysRevLett.94.040404 (ID: 308229)
Toggle Abstract
We study the dynamics of an atomic quantum dot, i.e., a single atom in a tight optical trap which is coupled to a superfluid reservoir via laser transitions. Quantum interference between the collisional interactions and the laser induced coupling results in a tunable dotbath coupling, allowing an essentially complete decoupling from the environment. Quantum dots embedded in a 1D Luttinger liquid of cold bosonic atoms realize a spinboson model with Ohmic coupling, which exhibits a dissipative phase transition and allows us to directly measure atomic Luttinger parameters.

J. Taylor, H. Engel, W. Dür, A. Yacoby, C. M. Marcus, P. Zoller, M. Lukin Faulttolerant architecture for quantum computation using electrically controlled semiconductor spins,
Nature Phys. 1 177183 (20050101),
http://dx.doi.org/10.1038/nphys174 doi:10.1038/nphys174 (ID: 308201)
Toggle Abstract
Information processing using quantum systems provides new paradigms for computation and communication and may yield insights into our understanding of the limits of quantum mechanics. However, realistic systems are never perfectly isolated from their environment, hence all quantum operations are subject to errors. Realization of a physical system for processing of quantum information that is tolerant of errors is a fundamental problem in quantum science and engineering. Here, we develop an architecture for quantum computation using electrically controlled semiconductor spins by extending the Loss–DiVincenzo scheme and by combining actively protected quantum memory and longdistance coupling mechanisms. Our approach is based on a demonstrated encoding of qubits in longlived twoelectron states, which immunizes qubits against the dominant error from hyperfine interactions. We develop a universal set of quantum gates compatible with active error suppression for these encoded qubits and an effective longrange interaction between the qubits by controlled electron transport. This approach yields a scalable architecture with favourable error thresholds for faulttolerant operation, consistent with present experimental parameters.
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H. Büchler, M. Hermele, S. D. Huber, M. P. Fisher, P. Zoller Atomic quantum simulator for lattice gauge theories and ring exchange models,
Phys. Rev. Lett. 95 040402 (2005),
arXiv:condmat/0503254 arXiv:condmat/0503254 (ID: 302730)
Toggle Abstract
We present the design of a ring exchange interaction in cold atomic gases subjected to an optical lattice using well understood tools for manipulating and controlling such gases. The strength of this interaction can be tuned independently and describes the correlated hopping of two bosons. We discuss a setup where this coupling term may allows for the realization and observation of exotic quantum phases, including a deconfined insulator described by the Coulomb phase of a threedimensional U(1) lattice gauge theory.
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D. Jaksch, P. Zoller The cold atom Hubbard toolbox,
Ann. Phys. 315 5279 (2005),
http://dx.doi.org/10.1016/j.aop.2004.09.010 doi:10.1016/j.aop.2004.09.010 (ID: 308231)
Toggle Abstract
We review recent theoretical advances in cold atom physics concentrating on strongly correlated cold atoms in optical lattices. We discuss recently developed quantum optical tools for manipulating atoms and show how they can be used to realize a wide range of many body Hamiltonians. Then, we describe connections and differences to condensed matter physics and present applications in the fields of quantum computing and quantum simulations. Finally, we explain how defects and atomic quantum dots can be introduced in a controlled way in optical lattice systems.
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L. Tian, R. Blatt, P. Zoller Scalable ion trap quantum computing without moving ions,
Eur. Phys. J. D 32 201208 (2005),
http://dx.doi.org/10.1140/epjd/e2004001725 doi:10.1140/epjd/e2004001725 (ID: 308239)
Toggle Abstract
A hybrid quantum computing scheme is studied where the hybrid qubit is made of an ion trap qubit serving as the information storage and a solidstate charge qubit serving as the quantum processor, connected by a superconducting cavity. In this paper, we extend our previous work [1] and study the decoherence, coupling and scalability of the hybrid system. We present our calculations of the decoherence of the coupled ioncharge system due to the charge fluctuations in the solidstate system and the dissipation of the superconducting cavity under laser radiation. A gate scheme that exploits rapid state flips of the charge qubit to reduce decoherence by the charge noise is designed. We also study a superconducting switch that is inserted between the cavity and the charge qubit and provides tunable coupling between the qubits. The scalability of the hybrid scheme is discussed together with several potential experimental obstacles in realizing this scheme.

S. Morrison, H. Büchler, A. J. Daley, H. Katzgraber, M. Lewenstein, P. Zoller Replicas in Disordered Cold Atomic Gases,
Theory of Cold Gases Conference, Cortona (Cortona, Italy, 20051029) (20051029),
(ID: 332790)

H. Büchler, M. Hermele, S. D. Huber, M. P. Fisher, P. Zoller Atomic Quantum Simulation of Lattice Gauge Theories and Ring Exchange,
BoseEinstein Condensation (San Feliu de Guixols, Spain, 20050910) URL (20050910),
(ID: 332062)

V. Steixner, P. Rabl, P. Zoller Feedback cooling of trapped ions,
Obergurgl Quantum Optics Meeting 2005 URL (20050303),
(ID: 326842)

K. Jähne, P. Rabl, I. WilsonRae, P. Zoller Cavity QED with superconducting qubits and quantum state transfer in such a system,
Obergurgl Quantum Optics Meeting URL (20050300),
(ID: 326857)
Toggle Abstract
A Cooper pair box is a promising candidate for the realization of a qubit in a solid state environment. Recently, it has been demonstrated that such a qubit can be strongly coupled to the radiation field in a microwave cavity [1].
The system, consisting of a Cooper box that is located at the center of a transmission line, forms one node of a network, whilst different nodes are connected by other transmission lines. Motivated by the proposal of ref. [2], we want to investigate, whether ideal quantum state transfer can be achieved for this system.
[1] A. Wallraff, D. I. Schuster, A. Blais, L. Frunzio, R.S. Huang, J. Majer, S.
Kumar, S. M. Girvin, and R. J. Schoelkopf, Nature 431, 162 (2004)
[2] J. I. Cirac, P. Zoller, H. J. Kimble, and H. Mabuchi, Phys. Rev. Lett. 78,
3221 (1996).

A. J. Daley, A. Griessner, D. Jaksch, P. Zoller Phonons as a new Element of the Optical Lattices Toolbox: Nondestructive Qubit Cooling and Register Initialisation,
ESFJSPS Frontier Science Conference Seies for Young Researchers: Quantum Information and Quantum Physics (Shonan Village Center, Japan, 20050300) (20050300),
(ID: 332782)

A. J. Daley, A. Micheli, D. Jaksch, P. Zoller The Single Atom Transistor, Exact Time Dependent Studies of 1D Systems, and Spectroscopy of Many Body Wavefunctions,
Obergurgl Quantum Optics Meeting (Obergurgl, Austria, 20050227) (20050227),
(ID: 332783)

S. Morrison, H. Büchler, A. J. Daley, H. Katzgraber, M. Lewenstein, P. Zoller Replicas in Disordered Cold Atomic Gases,
Quantum Optics Obergurgl meeting (Obergurgl, Austria, 20050227) (20050227),
(ID: 332791)

K. Jähne, P. Rabl, I. WilsonRae, P. Zoller Cavity QED with superconducting qubits and quantum state transfer in such a system,
11th Young Atom Opticians (YAO) Conference URL (20050200),
(ID: 326872)
Toggle Abstract
A Cooper pair box is a promising candidate for the realization of a qubit in a solid state environment. Recently, it has been demonstrated that such a qubit can be strongly coupled to the radiation field in a microwave cavity [1].
The system, consisting of a Cooper box that is located at the center of a transmission line, forms one node of a network, whilst different nodes are connected by other transmission lines. Motivated by the proposal of ref. [2], we want to investigate, whether ideal quantum state transfer can be achieved for this system.
[1] A. Wallraff, D. I. Schuster, A. Blais, L. Frunzio, R.S. Huang, J. Majer, S.
Kumar, S. M. Girvin, and R. J. Schoelkopf, Nature 431, 162 (2004)
[2] J. I. Cirac, P. Zoller, H. J. Kimble, and H. Mabuchi, Phys. Rev. Lett. 78,
3221 (1996)

A. Micheli, P. Zoller Protected Quantum Memory and Quantum Communication with polar heteronuclear molecules in twodimensional Optical Lattices,
2nd TermReview of the SFB (Innsbruck, Austria, 20050100) (2005),
(ID: 332442)

A. Micheli, P. Zoller Protected Quantum Memory and Quantum Communication with polar heteronuclear molecules in twodimensional Optical Lattices,
Obergurgl Quantum Optics Meeting 2005 (Obergurgl, Austria, 20050227) URL (2005),
(ID: 332444)

J. I. Cirac, P. Zoller New Frontiers in Quantum Information With Atoms and Ions,,
Physics Today 3844 (20040300),
URL (ID: 324583)
Toggle Abstract
Both the precision control of trappedion systems and very large samples of cold neutral atoms are opening important new possibilities for quantum computation and simulation
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P. O. Fedichev, M. J. Bijlsma, P. Zoller Extended Molecules and Geometric Scattering Resonances in Optical Lattices,
Phys. Rev. Lett. 92 080401 (20040200),
http://dx.doi.org/10.1103/PhysRevLett.92.080401 doi:10.1103/PhysRevLett.92.080401 (ID: 324597)
Toggle Abstract
We develop a theory describing neutral atom scattering at low energies in an optical lattice. We show that, for a repulsive interaction, as the microscopic scattering length increases the effective scattering amplitude approaches a limiting value which depends only on the lattice parameters. In the case of attractive interaction a geometric resonance occurs before reaching this limit. Close to the resonance, the effective interaction becomes repulsive and supports a weakly bound state, which can extend over several lattice sites.

A. J. Daley, P. O. Fedichev, P. Zoller Singleatom cooling by superfluid immersion: A nondestructive method for qubits,
Phys. Rev. A 69 022306 (20040200),
http://dx.doi.org/10.1103/PhysRevA.69.022306 doi:10.1103/PhysRevA.69.022306 (ID: 324602)
Toggle Abstract
We present a scheme to cool the motional state of neutral atoms confined in sites of an optical lattice by immersing the system in a superfluid. The motion of the atoms is damped by the generation of excitations in the superfluid, and under appropriate conditions the internal state of the atom remains unchanged. This scheme can thus be used to cool atoms used to encode a series of entangled qubits nondestructively. Within realizable parameter ranges, the rate of cooling to the ground state is found to be sufficiently large to be useful in experiments.

B. Paredes, P. Zoller, J. I. Cirac, J. J. GarcíaRipoll Strong correlation effects and quantum information theory of low dimensional atomic gases,
J. Phys. IV 116 135168 (2004),
http://dx.doi.org/10.1051/jp4:2004116005 doi:10.1051/jp4:2004116005 (ID: 314540)
Toggle Abstract
These lecture notes present an introduction to the strongly correlated regime of low dimensional atomic gases. The discussion is concentrated on situations in which the strongly correlated limit is achieved by creating degeneracies in the oneparticle motional states. Three different schemes of experimental relevance are analyzed: bosonic atoms in a two dimensional rapidly rotating trap, bosonic atoms in a one dimensional optical lattice, and bosonic atoms with frozen motional degrees of freedom and two internal states. The corresponding entangled multiparticle states (Laughlin liquids, Mott phases, squeezed states), and the different strongly correlated phenomena that appear (fermionization, fractional statistics) are studied. Emphasis is given to the possibility of observing novel strongly correlated phenomena as well as to the possible implementations for quantum computation and quantum information.

P. Zoller, L. Tian Coupled IonNanomechanical Systems,
Phys. Rev. Lett. 93 266403 (2004),
http://dx.doi.org/10.1103/PhysRevLett.93.266403 doi:10.1103/PhysRevLett.93.266403 (ID: 314555)
Toggle Abstract
We study ions in a nanotrap, where the electrodes are nanomechanical resonators. The ions play the role of a quantum optical system that acts as a probe and control, and allows entanglement with or between nanomechanical resonators.
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A. Micheli, A. J. Daley, D. Jaksch, P. Zoller Single Atom Transistor in a 1D Optical Lattice,
Phys. Rev. Lett. 93 140408 (2004),
http://dx.doi.org/10.1103/PhysRevLett.93.140408 doi:10.1103/PhysRevLett.93.140408 (ID: 314620)
Toggle Abstract
We propose a scheme utilizing a quantum interference phenomenon to switch the transport of atoms in a 1D optical lattice through a site containing an impurity atom. The impurity represents a qubit which in one spin state is transparent to the probe atoms, but in the other acts as a single atom mirror. This allows a singleshot quantum nondemolition measurement of the qubit spin.
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W. V. Liu, F. Wilczek, P. Zoller Spindependent Hubbard model and a quantum phase transition in cold atoms,
Phys. Rev. A 70 033603 (2004),
http://dx.doi.org/10.1103/PhysRevA.70.033603 doi:10.1103/PhysRevA.70.033603 (ID: 314622)
Toggle Abstract
We describe an experimental protocol for introducing spindependent lattice structure in a cold atomic Fermi gas using lasers. It can be used to realize Hubbard models whose hopping parameters depend on spin and whose interaction strength can be controlled with an external magnetic field. We suggest that exotic superfluidities will arise in this framework. An especially interesting possibility is a class of states that support coexisting superfluid and normal components, even at zero temperature. The quantity of normal component varies with external parameters. We discuss some aspects of the quantum phase transition that arises at the point where it vanishes.

H. Büchler, P. Zoller, W. Zwerger Spectroscopy of Superfluid Pairing in Atomic Fermi Gases,
Phys. Rev. Lett. 93 080401 (2004),
http://dx.doi.org/10.1103/PhysRevLett.93.080401 doi:10.1103/PhysRevLett.93.080401 (ID: 314630)
Toggle Abstract
We study the dynamic structure factor for density and spin within the crossover from BCS superfluidity of atomic fermions to the BoseEinstein condensation of molecules. Both structure factors are experimentally accessible via Bragg spectroscopy and allow for the identification of the pairing mechanism: the spin structure factor allows for the determination of the two particle gap, while the collective sound mode in the density structure reveals the superfluid state.
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T. Calarco, U. Dorner, P. S. Julienne, C. Williams, P. Zoller Quantum computations with atoms in optical lattices: Marker qubits and molecular interactions,
Phys. Rev. A 70 012306 (2004),
http://dx.doi.org/10.1103/PhysRevA.70.012306 doi:10.1103/PhysRevA.70.012306 (ID: 314632)
Toggle Abstract
We develop a scheme for quantum computation with neutral atoms, based on the concept of "marker" atoms, i.e., auxiliary atoms that can be efficiently transported in stateindependent periodic external traps to operate quantum gates between physically distant qubits. This allows for relaxing a number of experimental constraints for quantum computation with neutral atoms in microscopic potential, including singleatom laser addressability. We discuss the advantages of this approach in a concrete physical scenario involving molecular interactions.
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L. Tian, P. Rabl, R. Blatt, P. Zoller Interfacing QuantumOptical and SolidState Qubits,
Phys. Rev. Lett. 92 247902 (2004),
http://dx.doi.org/10.1103/PhysRevLett.92.247902 doi:10.1103/PhysRevLett.92.247902 (ID: 314633)
Toggle Abstract
We present a generic model of coupling quantumoptical and solidstate qubits, and the corresponding transfer protocols. The example discussed is a trapped ion coupled to a charge qubit (e.g., Cooper pair box). To enhance the coupling and to achieve compatibility between the different experimental setups we introduce a superconducting cavity as the connecting element.

J. J. GarcíaRipoll, J. I. Cirac, P. Zoller, C. Kollath, U. Schollwöck, J. von Delft Variational ansatz for the superfluid Mottinsulator transition in optical lattices,
Opt. Express 12 4254 (2004),
arXiv:condmat/0306162 arXiv:condmat/0306162 (ID: 314640)
Toggle Abstract
We develop a variational wave function for the ground state of a onedimensional bosonic lattice gas. The variational theory is initially developed for the quantum rotor model and later on extended to the Bose Hubbard model. This theory is compared with quasiexact numerical results obtained by Density Matrix Renormalization Group (DMRG) studies and with results from other analytical approximations. Our approach accurately gives local properties for strong and weak interactions, and it also describes the crossover from the superfluid phase to the Mottinsulator phase.

I. Martin, A. Shnirman, L. Tian, P. Zoller Groundstate cooling of mechanical resonators,
Phys. Rev. B 69 125339 (2004),
http://dx.doi.org/10.1103/PhysRevB.69.125339 doi:10.1103/PhysRevB.69.125339 (ID: 314642)
Toggle Abstract
We propose an application of a single Cooper pair box (Josephson qubit) for active cooling of nanomechanical resonators. Latest experiments with Josephson qubits demonstrated that long coherence time of the order of microsecond can be achieved in special symmetry points. Here we show that this level of coherence is sufficient to perform an analog of the well known in quantum optics "laser" cooling of a nanomechanical resonator capacitively coupled to the qubit. By applying an ac driving to the qubit or the resonator, resonators with frequency of order 100 MHz and quality factors higher than 103 can be efficiently cooled down to their ground state, while lowerfrequency resonators can be cooled down to microKelvin temperatures. We also consider an alternative setup where dcvoltageinduced Josephson oscillations play the role of the ac driving and show that cooling is possible in this case as well.
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E. Pazy, T. Calarco, P. Zoller Spin state readout by quantum jump technique: for the purpose of quantum computing,
IEEE Transactions on Nanotechnology 3 10  16 (2004),
http://dx.doi.org/10.1109/TNANO.2003.820516 doi:10.1109/TNANO.2003.820516 (ID: 314773)
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Utilizing the Pauliblocking mechanism we show that shining circular polarized light on a singlycharged quantum dot induces spin dependent fluorescence. Employing the quantumjump technique we demonstrate that this resonance luminescence, due to a spin dependent optical excitation, serves as an excellent readout mechanism for measuring the spin state of a single electron confined to a quantum dot.

P. Rabl, A. Shnirman, P. Zoller Generation of Squeezed States of Nanomechanical Resonators by Reservoir Engineering,
Phys. Rev. B 70 205304 (2004),
http://dx.doi.org/10.1103/PhysRevB.70.205304 doi:10.1103/PhysRevB.70.205304 (ID: 335015)
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An experimental demonstration of a nonclassical state of a nanomechanical resonator is still an outstanding task. In this paper we show how the resonator can be cooled and driven into a squeezed state by a bichromatic microwave coupling to a charge qubit. The stationary resonator state exhibits a reduced noise in one of the quadrature components by a factor of 0.5–0.2. These values are obtained for a 100 MHz resonator with a Qvalue of 104 to 105 and for support temperatures of T[approximate]25 mK. We show that the coupling to the charge qubit can also be used to detect the squeezed state via measurements of the excited state population. Furthermore, by extending this measurement procedure a complete quantum state tomography of the resonator state can be performed. This provides a universal tool to detect a large variety of different states and to prove the quantum nature of nanomechanical systems.
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A. Griessner, D. Jaksch, P. Zoller Cavityassisted nondestructive laser cooling of atomic qubits,
J. Phys. B: At. Mol. Opt. Phys. 37 1432 (2004),
http://dx.doi.org/10.1088/09534075/37/7/004 doi:10.1088/09534075/37/7/004 (ID: 335134)
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We analyse two configurations for laser cooling of neutral atoms whose internal states store qubits. The atoms are trapped in an optical lattice which is placed inside a cavity. We show that the coupling of the atoms to the damped cavity mode can provide a mechanism which leads to cooling of the motion without destroying the quantum information.
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I. WilsonRae, P. Zoller, A. Imamoglu Laser Cooling of a Nanomechanical Resonator Mode to its Quantum Ground State,
Phys. Rev. Lett. 92 075507 (2004),
http://dx.doi.org/10.1103/PhysRevLett.92.075507 doi:10.1103/PhysRevLett.92.075507 (ID: 336649)
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We show that it is possible to cool a nanomechanical resonator mode to its ground state. The proposed technique is based on resonant laser excitation of a phonon sideband of an embedded quantum dot. The strength of the sideband coupling is determined directly by the difference between the electronphonon couplings of the initial and final states of the quantum dot optical transition. Possible applications of this scheme include generation of nonclassical states of mechanical motion.