
G. Natale, R. van Bijnen, A. Patscheider, D. Petter, M. J. Mark, L. Chomaz, F. Ferlaino Excitation spectrum of a trapped dipolar supersolid and its experimental evidence,
Phys. Rev. Lett. 123 50402 (20190801),
http://dx.doi.org/10.1103/PhysRevLett.123.050402 doi:10.1103/PhysRevLett.123.050402 (ID: 720313)
Toggle Abstract
We study the spectrum of elementary excitations of a trapped dipolar Bose gas across the superfluidsupersolid phase transition. Our calculations, accounting for the experimentally relevant case of confined systems, show that, when entering the supersolid phase, two distinct excitation branches appear, respectively connected to crystal or superfluid orders. These results confirm infinitesystem predictions, showing that finitesize effects play only a small qualitative role. Experimentally, we probe compressional excitations in an Er quantum gas across the phase diagram. While in the BEC regime the system exhibits an ordinary quadrupole oscillation, in the supersolid regime, we observe a striking twofrequency response of the system, related to the two spontaneously broken symmetries.

C. GonzalezBallestero, P. Maurer, D. Windey, L. Novotny, R. Reimann, O. RomeroIsart Theory for Cavity Cooling of Levitated Nanoparticles via Coherent Scattering: Master Equation Approach,
Phys. Rev. A 100 13805 (20190703),
http://dx.doi.org/10.1103/PhysRevA.100.013805 doi:10.1103/PhysRevA.100.013805 (ID: 720143)

D. Heinrich, M. Guggemos, M. GuevaraBertsch, M. I. Hussain, C. F. Roos, R. Blatt Ultrafast coherent excitation of a Ca+ ion,
New J. Phys. 21 73017 (20190701),
http://dx.doi.org/10.1088/13672630/ab2a7e doi:10.1088/13672630/ab2a7e (ID: 720108)
Toggle Abstract
Trapped ions are a wellstudied and promising system for the realization of a scalable quantum computer. Faster quantum gates would greatly improve the applicability of such a system and allow for greater flexibility in the number of calculation steps. In this paper we present a pulsed laser system, delivering picosecond pulses at a repetition rate of 5 GHz and resonant to the S1/2 to P3/2 transition in Ca+ for coherent population transfer to implement fast phase gate operations. The optical pulse train is derived from a modelocked, stabilized optical frequency comb and inherits its frequency stability. Using a single trapped ion, we implement three different techniques for measuring the ionlaser coupling strength and characterizing the pulse train emitted by the laser, and show how all requirements can be met for an implementation of a fast phase gate operation.

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.

D. Petter, G. Natale, R. van Bijnen, A. Patscheider, M. J. Mark, L. Chomaz, F. Ferlaino Probing the roton excitation spectrum of a stable dipolar Bose gas,
Phys. Rev. Lett. 122 183401 (20190508),
http://dx.doi.org/10.1103/PhysRevLett.122.183401 doi:10.1103/PhysRevLett.122.183401 (ID: 720098)
Toggle Abstract
We measure the excitation spectrum of a stable dipolar BoseEinstein condensate over a wide momentumrange via Bragg spectroscopy. We precisely control the relative strength, εdd, of the dipolar to the contact interactions and observe that the spectrum increasingly deviates from the linear phononic behavior for increasing εdd. Reaching the dipolar dominated regime εdd>1, we observe the emergence of a roton minimum in the spectrum and its softening towards instability. We characterize how the excitation energy and the strength of the densitydensity correlations at the roton momentum vary with εdd. Our findings are in excellent agreement with numerical calculations based on meanfield Bogoliubov theory. When including beyondmeanfield corrections, in the form of a LeeHuangYang potential, we observe a quantitative deviation from the experiment, questioning the validity of such a description in the roton regime.

M. Mamaev, R. Blatt, J. Ye, A. M. Rey Cluster State Generation with SpinOrbit Coupled Fermionic Atoms in Optical Lattices,
Phys. Rev. Lett. 122 160402 (20190427),
http://dx.doi.org/10.1103/PhysRevLett.122.160402 doi:10.1103/PhysRevLett.122.160402 (ID: 720107)
Toggle Abstract
Measurementbased quantum computation, an alternative paradigm for quantum information processing, uses simple measurements on qubits prepared in cluster states, a class of multiparty entangled states with useful properties. Here we propose and analyze a scheme that takes advantage of the interplay between spinorbit coupling and superexchange interactions, in the presence of a coherent drive, to deterministically generate macroscopic arrays of cluster states in fermionic alkaline earth atoms trapped in three dimensional (3D) optical lattices. The scheme dynamically generates cluster states without the need of engineered transport, and is robust in the presence of holes, a typical imperfection in cold atom Mott insulators. The protocol is of particular relevance for the new generation of 3D optical lattice clocks with coherence times >10 s, two orders of magnitude larger than the cluster state generation time. We propose the use of collective measurements and timereversal of the Hamiltonian to benchmark the underlying Ising model dynamics and the generated manybody correlations.

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.

L. Chomaz, D. Petter, P. Ilzhöfer, G. Natale, A. Trautmann, C. Politi, G. Durastante, R. van Bijnen, A. Patscheider, M. Sohmen, M. J. Mark, F. Ferlaino LongLived and Transient Supersolid Behaviors in Dipolar Quantum Gases,
Phys. Rev. X 9 21012 (20190419),
http://dx.doi.org/10.1103/PhysRevX.9.021012 doi:10.1103/PhysRevX.9.021012 (ID: 720203)
Toggle Abstract
By combining theory and experiments, we demonstrate that dipolar quantum gases of both 166Er and 164Dy support a state with supersolid properties, where a spontaneous density modulation and a global phase coherence coexist. This paradoxical state occurs in a well defined parameter range, separating the phases of a regular BoseEinstein condensate and of an insulating droplet array, and is rooted in the roton mode softening, on the one side, and in the stabilization driven by quantum fluctuations, on the other side. Here, we identify the parameter regime for each of the three phases. In the experiment, we rely on a detailed analysis of the interference patterns resulting from the free expansion of the gas, quantifying both its density modulation and its global phase coherence. Reaching the phases via a slow interaction tuning, starting from a stable condensate, we observe that 166Er and 164Dy exhibit a striking difference in the lifetime of the supersolid properties, due to the different atom loss rates in the two systems. Indeed, while in 166Er the supersolid behavior only survives a few tens of milliseconds, we observe coherent density modulations for more than 150ms in 164Dy. Building on this long lifetime, we demonstrate an alternative path to reach the supersolid regime, relying solely on evaporative cooling starting from a thermal gas.

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

B. Huang, I. Fritsche, R. Lous, C. Baroni, J. T. Walraven, E. Kirilov, R. Grimm Breathing Mode of a BEC Repulsively Interacting with a Fermionic Reservoir,
Phys. Rev. A 99 41602 (20190403),
http://dx.doi.org/10.1103/PhysRevA.99.041602 doi:10.1103/PhysRevA.99.041602 (ID: 720106)
Toggle Abstract
We investigate the fundamental breathing mode of a smallsized elongated BoseEinstein condensate coupled to a large Fermi sea of atoms. Our observations show a dramatic shift of the breathing frequency when the mixture undergoes phase separation at strong interspecies repulsion. We find that the maximum frequency shift in the full phaseseparation limit depends essentially on the atom number ratio of the components. We interpret the experimental observations within a model that assumes an adiabatic response of the Fermi sea, or within another model that considers single fermion trajectories for a fully phaseseparated mixture. These two complementary models capture the observed features over the full range of interest.

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.

D. Windey, C. GonzalezBallestero, P. Maurer, L. Novotny, O. RomeroIsart, R. Reimann CavityBased 3D Cooling of a Levitated Nanoparticle via Coherent Scattering,
Phys. Rev. Lett. 122 123601 (20190327),
http://dx.doi.org/10.1103/PhysRevLett.122.123601 doi:10.1103/PhysRevLett.122.123601 (ID: 720109)

R. Grimm Efimov States in an Ultracold Gas: How it Happened in the Laboratory,
FewBody Syst. 60 23 (20190323),
http://dx.doi.org/10.1007/s006010191495y doi:10.1007/s006010191495y (ID: 720253)

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.

K. Kustura, C. C. Rusconi, O. RomeroIsart Quadratic Quantum Hamiltonians: General Canonical Transformation to a Normal Form,
Phys. Rev. A 99 22130 (20190228),
http://dx.doi.org/10.1103/PhysRevA.99.022130 doi:10.1103/PhysRevA.99.022130 (ID: 720073)
Toggle Abstract
A system of linearly coupled quantum harmonic oscillators can be diagonalized when the system is dynamically stable using a Bogoliubov canonical transformation. However, this is just a particular case of more general canonical transformations that can be performed even when the system is dynamically unstable. Specific canonical transformations can transform a quadratic Hamiltonian into a normal form, which greatly helps to elucidate the underlying physics of the system. Here, we provide a selfcontained review of the normal form of a quadratic Hamiltonian as well as stepbystep instructions to construct the corresponding canonical transformation for the most general case. Among other examples, we show how the standard twomode Hamiltonian with a quadratic position coupling presents, in the stability diagram, all the possible normal forms corresponding to different types of dynamical instabilities.

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.

C. Maier, T. Brydges, P. Jurcevic, N. Trautmann, C. Hempel, B. P. Lanyon, P. Hauke, R. Blatt, C. F. Roos Environmentassisted quantum transport in a 10qubit network,
Phys. Rev. Lett. 122 50501 (20190208),
http://dx.doi.org/10.1103/PhysRevLett.122.050501 doi:10.1103/PhysRevLett.122.050501 (ID: 720065)
Toggle Abstract
The way in which energy is transported through an interacting system governs fundamental properties in many areas of physics, chemistry, and biology. Remarkably, environmental noise can enhance the transport, an effect known as environmentassisted quantum transport (ENAQT). In this paper, we study ENAQT in a network of coupled spins subject to engineered static disorder and temporally varying dephasing noise. The interacting spin network is realized in a chain of trapped atomic ions and energy transport is represented by the transfer of electronic excitation between ions. With increasing noise strength, we observe a crossover from coherent dynamics and Anderson localization to ENAQT and finally a suppression of transport due to the quantum Zeno effect. We found that in the regime where ENAQT is most effective the transport is mainly diffusive, displaying coherences only at very short times. Further, we show that dephasing characterized by nonMarkovian noise can maintain coherences longer than white noise dephasing, with a strong influence of the spectral structure on the transport effciency. Our approach represents a controlled and scalable way to investigate quantum transport in manybody networks under static disorder and dynamic noise.

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)

R. Blatt QCVV Trapped Ion Qubits ,
QCPR 2019 (Hotel Annapolis MD , 20190729) (20190801),
(ID: 720339)

O. RomeroIsart Quantum Acoustomechanics with a Micromagnet,
699 WEHeraeusSeminar on Levitated Optomechanics (Bad Honnef (Germany), 20190729) URL (20190730),
(ID: 720344)

R. Blatt Quantum Computation and Quantum Simultion with Strings of Trapped Ca+Ions,
NACTI2019 (Edward St John Learning and Teaching Center, University of Maryland, 20190722) (20190723),
(ID: 720340)

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

R. Blatt Quantum Computation and Quantum Simulation with Strings of Trapped Ca+ Ions ,
ICQT2019 (The Radisson Collection Hotel Moskow, 20190715) (20190716),
(ID: 720338)

C. Kokail Variational Quantum Simulation of Lattice Gauge Theories with SelfVerification,
Workshop on Quantum Simulation: Gauge Fields, Holography, and Topology (Bilbao, 20190710) URL (20190710),
(ID: 720297)

R. Blatt Quantum computation and quantum simulation with strings of trapped Ca+ ions,
MCQST Munich conference on Quantum Science & Technology (Deutsches Museum München, 20190708) (20190708),
(ID: 720322)

C. F. Roos Investigating a manybody quantum system with random measurements,
Seminar talk at Weizmann Institute of Science (Rehovot) (20190626),
(ID: 720292)

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)

R. Blatt Quantum Computation and Quantum Simulation with Trapped Ca+ Ions,
CEWQO2019 (Paderborn University, 20190602) (20190603),
(ID: 720287)

T. Weiß Quantum Motional State Tomography using Quartic Potentials and Neural Networks,
Ringberg Castle Retreat  MPL Theory Division + friends  Physics above the lake (Ringberg Castle, Germany, 20190529) (20190530),
(ID: 720284)

R. Blatt Quantum Computation and Quantum simultion with trapped ions,
2nd Quantum Symposium by PicoQuant, 2019 Berlin (Dorint Adlershof Berlin, 20190522) (20190522),
(ID: 720285)

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)

R. Blatt Quantum Computation and Quantum simulation with trapped Ca+ ions,
Quantum for ever, A Symposium to honor H.Rauch´s 80th birthday and the Designation of the Atominstitut as an EPS Historic site (TU Wien, Atominstitut, 20190521) (20190521),
(ID: 720286)

C. GonzalezBallestero Levitating nanoparticles: toward ground state cooling and beyond,
XVI International Conference on Quantum Optics and Quantum Information (Minsk, Belarus, 20190513) URL (20190515),
(ID: 720278)

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)

C. Kokail SelfVerification of Hubbard Models using StateDependent Optical Lattices,
Workshop on MultiPoint Correlations in Quantum ManyBody Systems (Heidelberg, 20190412) URL (20190412),
(ID: 720296)

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.

C. GonzalezBallestero Levitating nanoparticles: from new tools to fundamental questions,
Quantum Nanophotonics (Benasque (Spain), 20190318) URL (20190321),
(ID: 720255)

R. Blatt Quantum simultation with trapped Ca+Ions,
Solvay Workshop on Quantum Simultation (Brussels, 20190217) (20190218),
(ID: 720201)

R. Grimm Ultracold fermion mixtures tuned into resonance,
SFB Conference FoQuS (Innsbruck, Austria, 20190204) URL (20190205),
(ID: 720178)

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.

R. Blatt Quantum Information Using Trapped IonsStatus and Perspectives ,
Scalable Hardware Platforms for Quantum computing (Bad Honnef, 20190113) (20190114),
(ID: 720154)

O. RomeroIsart Heating in Nanophotonic Traps for Cold Atoms,
7th International Topic Meeting on Nanophotonics and Metamaterials (NANOMETA) (Seefeld, Austria, 20190103) (20190105),
URL (ID: 720121)

J. Hond, R. van Bijnen, S. J. Kokkelmans, R. Spreeuw, H. van Linden van den Heuvell, N. van Druten From coherent collective excitation to Rydberg blockade on an atom chip,
Phys. Rev. A 98 2714 (20181227),
http://dx.doi.org/10.1103/PhysRevA.98.062714 doi:10.1103/PhysRevA.98.062714 (ID: 720124)
Toggle Abstract
Using timeresolved measurements, we demonstrate coherent collective Rydberg excitation crossing over into Rydberg blockade in a dense and ultracold gas trapped at a distance of 100 μm from a roomtemperature atom chip. We perform Ramseytype measurements to characterize the coherence. The experimental data are in good agreement with numerical results from a master equation using a meanfield approximation and with results from a superatombased Hamiltonian. This represents significant progress in exploring a strongly interacting driven Rydberg gas on an atom chip.

C. Ravensbergen, V. Corre, E. Soave, M. Kreyer, E. Kirilov, R. Grimm Production of a degenerate FermiFermi mixture of dysprosium and potassium atoms,
Phys. Rev. A 98 63624 (20181220),
http://dx.doi.org/10.1103/PhysRevA.98.063624 doi:10.1103/PhysRevA.98.063624 (ID: 720087)
Toggle Abstract
We report on the realization of a mixture of fermionic 161^{161}Dy and fermionic 40^{40}K where both species are deep in the quantumdegenerate regime. Both components are spinpolarized in their absolute ground states, and the low temperatures are achieved by means of evaporative cooling of the dipolar dysprosium atoms together with sympathetic cooling of the potassium atoms. We describe the trapping and cooling methods, in particular the final evaporation stage, which leads to Fermi degeneracy of both species. Analyzing crossspecies thermalization we obtain an estimate of the magnitude of the interspecies sswave scattering length at low magnetic field. We demonstrate magnetic levitation of the mixture as a tool to ensure spatial overlap of the two components. The properties of the DyK mixture make it a very promising candidate to explore the physics of strongly interacting massimbalanced FermiFermi mixtures.

A. Kosior, M. Lewenstein, A. Celi Unruh effect for interacting particles with ultracold atoms,
SciPost Phys. 5 (20181211),
http://dx.doi.org/10.21468/SciPostPhys.5.6.061 doi:10.21468/SciPostPhys.5.6.061 (ID: 720168)
Toggle Abstract
The Unruh effect is a quantum relativistic effect where the accelerated observer perceives the vacuum as a thermal state. Here we propose the experimental realization of the Unruh effect for interacting ultracold fermions in optical lattices by a sudden quench resulting in vacuum acceleration with varying interactions strengths in the real temperature background. We observe the inversion of statistics for the low lying excitations in the Wightman function as a result of competition between the spacetime and BCS Bogoliubov transformations. This paper opens up new perspectives for simulators of quantum gravity.

M. GuevaraBertsch, A. ChavarríaSibaja, A. GodínezSandí, O. HerreraSancho Energy exchange between atoms with a quartz crystal μbalance,
New J. Phys. 20 123001 (20181205),
http://dx.doi.org/10.1088/13672630/aaf117 doi:10.1088/13672630/aaf117 (ID: 720163)
Toggle Abstract
We propose an experimental method to fully characterize the energy exchange of particles during the physical vapor deposition (PVD) process of thin surface layers. Our approach is based on the careful observation of perturbations of the oscillation frequency of a quartz crystal μbalance induced by the particles' interaction. With this technique it is possible to measure the momentum exchange of the atoms during the evaporation process and determine the ideal evaporation rate for uniform energy distribution. We are able to follow the desorption dynamics of particles' immediately after the first layers have been formed. These results are in close relation to the surface binding energy of the evaporated material and offer a better control to obtain the desired properties of the thin surface layer. The novelty of our work consists of the demonstration that quartz crystal μbalance can be successfully implemented to monitor complex dynamics through the energy interaction of particles during PVD. We applied our technique to investigate the PVD mechanism for diverse elements, usually implemented in the development of film surface layers, such as Cu, W, Au, Gd and In, and confirm that our results are in agreement with measurements done previously with other techniques such as low temperature photoluminescence.

W. Weiss, M. Gerster, D. Jaschke, P. Silvi, S. Montangero KibbleZurek scaling of the onedimensional BoseHubbard model at finite temperatures,
Phys. Rev. A 98 3601 (20181203),
http://dx.doi.org/10.1103/PhysRevA.98.063601 doi:10.1103/PhysRevA.98.063601 (ID: 720125)
Toggle Abstract
We use tensor network methods—matrix product states, tree tensor networks, and locally purified tensor networks—to simulate the onedimensional BoseHubbard model for zero and finite temperatures in experimentally accessible regimes. First we explore the effect of thermal fluctuations on the system ground state by characterizing its Mott and superfluid features. Then we study the behavior of the outofequilibrium dynamics induced by quenches of the hopping parameter. We confirm a KibbleZurek scaling for zero temperature and characterize the finitetemperature behavior, which we explain by means of a simple argument.

L. Sieberer, M. T. Rieder, M. H. Fischer, I. C. Fulga Statistical periodicity in driven quantum systems: General formalism and application to noisy Floquet topological chains,
Phys. Rev. B 98 (20181203),
http://dx.doi.org/10.1103/PhysRevB.98.214301 doi:10.1103/PhysRevB.98.214301 (ID: 720136)
Toggle Abstract
Much recent experimental effort has focused on the realization of exotic quantum states and dynamics predicted to occur in periodically driven systems. But how robust are the soughtafter features, such as Floquet topological surface states, against unavoidable imperfections in the periodic driving? In this paper, we address this question in a broader context and study the dynamics of quantum systems subject to noise with periodically recurring statistics. We show that the stroboscopic time evolution of such systems is described by a noiseaveraged Floquet superoperator. The eigenvectors and values of this superoperator generalize the familiar concepts of Floquet states and quasienergies and allow us to describe decoherence due to noise efficiently. Applying the general formalism to the example of a noisy Floquet topological chain, we rederive and corroborate our recent findings on the noiseinduced decay of topologically protected end states. These results follow directly from an expansion of the end state in eigenvectors of the Floquet superoperator.

A. Grankin, P. Grangier, E. Brion Diagrammatic treatment of fewphoton scattering from a Rydbergblockaded atomic ensemble in a cavity,
Phys. Rev. A 98 (20181130),
http://dx.doi.org/10.1103/PhysRevA.98.053860 doi:10.1103/PhysRevA.98.053860 (ID: 720167)
Toggle Abstract
In a previous Letter [A. Grankin et al., Phys. Rev. Lett. 117, 253602 (2016)] we studied the giant optical nonlinearities of a Rydberg atomic medium within an optical cavity, in the SchwingerKeldysh formalism. In particular, we calculated the nonlinear contributions to the spectrum of the light transmitted through the cavity. In this article we spell out the essential details of this calculation and we show how it can be extended to higher input photon numbers and higherorder correlation functions. As a relevant example, we calculate and discuss the threephoton correlation function of the transmitted light and discuss its physical significance in terms of the polariton energy levels of the Rydberg medium within the optical cavity.

M. J. Mark, F. Meinert, K. Lauber, H. Nägerl MottInsulatorAided Detection of UltraNarrow Feshbach Resonances,
SciPost Phys. 5 55 (20181129),
http://dx.doi.org/10.21468/SciPostPhys.5.5.055 doi:10.21468/SciPostPhys.5.5.055 (ID: 720052)
Toggle Abstract
We report on the detection of extremely narrow Feshbach resonances by employing a Mottinsulating state for cesium atoms in a threedimensional optical lattice. The Mott insulator protects the atomic ensemble from high background threebody losses in a magnetic field region where a broad Efimov resonance otherwise dominates the atom loss in bulk samples. Our technique reveals three ultranarrow and previously unobserved Feshbach resonances in this region with widths below ≈10μG, measured via LandauZenertype molecule formation and confirmed by theoretical predictions. For comparatively broader resonances we find a latticeinduced substructure in the respective atomloss feature due to the interplay of tunneling and strong particle interactions. Our results provide a powerful tool to identify and characterize narrow scattering resonances, particularly in systems with complex Feshbach spectra. The observed ultranarrow Feshbach resonances could be interesting candidates for precision measurements.

T. Langen, M. J. Mark Ultrakalt magnetisiert,
Physik Journal 12/2018 35 (20181122),
URL (ID: 720092)

A. Trautmann, P. Ilzhöfer, G. Durastante, C. Politi, M. Sohmen, M. J. Mark, F. Ferlaino Dipolar Quantum Mixtures of Erbium and Dysprosium Atoms,
Phys. Rev. Lett. 121 213601 (20181121),
http://dx.doi.org/10.1103/PhysRevLett.121.213601 doi:10.1103/PhysRevLett.121.213601 (ID: 720050)
Toggle Abstract
We report on the first realization of heteronuclear dipolar quantum mixtures of highly magnetic erbium and dysprosium atoms. With a versatile experimental setup, we demonstrate binary BoseEinstein condensation in five different ErDy isotope combinations, as well as one ErDy BoseFermi mixture. Finally, we present first studies of the interspecies interaction between the two species for one mixture.

J. PratCamps, P. Maurer, G. Kirchmair, O. RomeroIsart Circumventing Magnetostatic Reciprocity: A Diode for Magnetic Fields,
Phys. Rev. Lett. 121 213903 (20181120),
http://dx.doi.org/10.1103/PhysRevLett.121.213903 doi:10.1103/PhysRevLett.121.213903 (ID: 719978)
Toggle Abstract
Lorentz reciprocity establishes a stringent relation between electromagnetic fields and their sources. For static magnetic fields, a relation between magnetic sources and fields can be drawn in analogy to the Green’s reciprocity principle for electrostatics. So far, the magnetostatic reciprocity principle remains unchallenged and the magnetostatic interaction is assumed to be symmetric (reciprocal). Here, we theoretically and experimentally show that a linear and isotropic electrically conductive material moving with constant velocity is able to circumvent the magnetostatic reciprocity principle and realize a diode for magnetic fields. This result is demonstrated by measuring an extremely asymmetric magnetic coupling between two coils that are located near a moving conductor. The possibility to generate controlled unidirectional magnetic couplings implies that the mutual inductances between magnetic elements or circuits can be made extremely asymmetric. We anticipate that this result will provide novel possibilities for applications and technologies based on magnetically coupled elements and might open fundamentally new avenues in artificial magnetic spin systems.

K. Sinha, B. Venkatesh, P. Meystre Collective Effects in CasimirPolder Forces,
Phys. Rev. Lett. 121 183605 (20181101),
http://dx.doi.org/10.1103/PhysRevLett.121.183605 doi:10.1103/PhysRevLett.121.183605 (ID: 719993)
Toggle Abstract
We study cooperative phenomena in the fluctuationinduced forces between a surface and a system of neutral twolevel quantum emitters prepared in a coherent collective state, showing that the total CasimirPolder force on the emitters can be modified via their mutual correlations. Particularly, we find that a onedimensional chain of emitters prepared in a super or subradiant state experiences an enhanced or suppressed collective vacuuminduced force, respectively. The collective nature of dispersion forces can be understood as resulting from the interference between the different processes contributing to the surfacemodified resonant dipoledipole interaction. Such cooperative fluctuation forces depend singularly on the surface response at the resonance frequency of the emitters, thus being easily maneuverable. Our results demonstrate the potential of collective phenomena as a new tool to selectively tailor vacuum forces.

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.

A. Rubio López, C. GonzalezBallestero, O. RomeroIsart Internal Quantum Dynamics of a Nanoparticle in a Thermal Electromagnetic Field: a Minimal Model,
Phys. Rev. B 98 155405 (20181008),
http://dx.doi.org/10.1103/PhysRevB.98.155405 doi:10.1103/PhysRevB.98.155405 (ID: 720040)

T. Fösel, P. Tighineanu, T. Weiß, F. Marquardt Reinforcement Learning with Neural Networks for Quantum Feedback,
Phys. Rev. X 8 31084 (20180927),
http://dx.doi.org/10.1103/PhysRevX.8.031084 doi:10.1103/PhysRevX.8.031084 (ID: 720025)

A. Ciamei, J. Szczepkowski, A. Bayerle, N. Barberán, L. Reichsöllner, S. Tzanova, C. Chen, B. Pasquiou, A. Grochola, P. Kowalczyk, W. Jastrzebski, F. Schreck The RbSr 2Σ+ ground state investigated via spectroscopy of hot and ultracold molecules,
Phys. Chem. Chem. Phys. 20 (20180921),
http://dx.doi.org/10.1039/C8CP03919D doi:10.1039/C8CP03919D (ID: 720160)
Toggle Abstract
We report on spectroscopic studies of hot and ultracold RbSr molecules, and combine the results in an analysis that allows us to fit a potential energy curve (PEC) for the X(1)2Σ+ ground state bridging the shorttolongrange domains. The ultracold RbSr molecules are created in a μK sample of Rb and Sr atoms and probed by twocolour photoassociation spectroscopy. The data yield the longrange dispersion coefficients C6 and C8, along with the total number of supported bound levels. The hot RbSr molecules are created in a 1000 K gas mixture of Rb and Sr in a heatpipe oven and probed by thermoluminescence and laserinduced fluorescence spectroscopy. We compare the hot molecule data with spectra we simulated using previously published PECs determined by three different ab initio theoretical methods. We identify several band heads corresponding to radiative decay from the B(2)2Σ+ state to the deepest bound levels of X(1)2Σ+. We determine a massscaled highprecision model for X(1)2Σ+ by fitting all data using a single fit procedure. The corresponding PEC is consistent with all data, thus spanning shorttolong internuclear distances and bridging an energy gap of about 75% of the potential well depth, still uncharted by any experiment. We benchmark previous ab initio PECs against our results, and give the PEC fit parameters for both X(1)2Σ+ and B(2)2Σ+ states. As first outcomes of our analysis, we calculate the swave scattering properties for all stable isotopic combinations and corroborate the locations of Fano–Feshbach resonances between alkali Rb and closedshell Sr atoms recently observed [V. Barbé et al., Nat. Phys., 2018, 14, 881]. These results and more generally our strategy should greatly contribute to the generation of ultracold alkali–alkalineearth dimers, whose applications range from quantum simulation to statecontrolled quantum chemistry.

X. Huang, E. Zeuthen, D. Vasilyev, Q. He, K. Hammerer, E. Polzik Unconditional SteadyState Entanglement in Macroscopic Hybrid Systems by Coherent Noise Cancellation,
Phys. Rev. Lett. 103602 (20180905),
http://dx.doi.org/10.1103/PhysRevLett.121.103602 doi:10.1103/PhysRevLett.121.103602 (ID: 720079)
Toggle Abstract
The generation of entanglement between disparate physical objects is a key ingredient in the field of quantum technologies, since they can have different functionalities in a quantum network. Here we propose and analyze a generic approach to steadystate entanglement generation between two oscillators with different temperatures and decoherence properties coupled in cascade to a common unidirectional light field. The scheme is based on a combination of coherent noise cancellation and dynamical cooling techniques for two oscillators with effective masses of opposite signs, such as quasispin and motional degrees of freedom, respectively. The interference effect provided by the cascaded setup can be tuned to implement additional noise cancellation leading to improved entanglement even in the presence of a hot thermal environment. The unconditional entanglement generation is advantageous since it provides a readytouse quantum resource. Remarkably, by comparing to the conditional entanglement achievable in the dynamically stable regime, we find our unconditional scheme to deliver a virtually identical performance when operated optimally.

L. Postler, A. Rivas, P. Schindler, A. Erhardt, R. Stricker, D. Nigg, T. Monz, R. Blatt, M. Müller Experimental quantification of spatial correlations in quantum dynamics,
Quantum 2 90 (20180903),
http://dx.doi.org/10.22331/q2018090390 doi:10.22331/q2018090390 (ID: 720164)
Toggle Abstract
Correlations between different partitions of quantum systems play a central role in a variety of manybody quantum systems, and they have been studied exhaustively in experimental and theoretical research. Here, we investigate dynamical correlations in the time evolution of multiple parts of a composite quantum system. A rigorous measure to quantify correlations in quantum dynamics based on a full tomographic reconstruction of the quantum process has been introduced recently [Á. Rivas et al., New Journal of Physics, 17(6) 062001 (2015).]. In this work, we derive a lower bound for this correlation measure, which does not require full knowledge of the quantum dynamics. Furthermore we also extend the correlation measure to multipartite systems. We directly apply the developed methods to a trapped ion quantum information processor to experimentally characterize the correlations in quantum dynamics for two and fourqubit systems. The method proposed and demonstrated in this work is scalable, platformindependent and applicable to other composite quantum systems and quantum information processing architectures. We apply the method to estimate spatial correlations in environmental noise processes, which are crucial for the performance of quantum error correction procedures.

S. Baier, D. Petter, J. H. Becher, A. Patscheider, G. Natale, L. Chomaz, M. J. Mark, F. Ferlaino Realization of a Strongly Interacting Fermi Gas of Dipolar Atoms,
Phys. Rev. Lett. 121 93602 (20180829),
http://dx.doi.org/10.1103/PhysRevLett.121.093602 doi:10.1103/PhysRevLett.121.093602 (ID: 720004)
Toggle Abstract
We realize a twocomponent dipolar Fermi gas with tunable interactions, using erbium atoms. Employing a latticeprotection technique, we selectively prepare deeply degenerate mixtures of the two lowest spin states and perform highresolution Feshbach spectroscopy in an optical dipole trap. We identify a comparatively broad Feshbach resonance and map the interspin scattering length in its vicinity. The Fermi mixture shows a remarkable collisional stability in the strongly interacting regime, providing a first step towards studies of superfluid pairing, crossing from Cooper pairs to bound molecules, in presence of dipoledipole 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.

L. Sieberer, E. Altman Topological Defects in Anisotropic Driven Open Systems,
Phys. Rev. Lett. 121 (20180824),
http://dx.doi.org/10.1103/PhysRevLett.121.085704 doi:10.1103/PhysRevLett.121.085704 (ID: 720137)
Toggle Abstract
We study the dynamics and unbinding transition of vortices in the compact anisotropic KardarParisiZhang equation. The combination of nonequilibrium conditions and strong spatial anisotropy drastically affects the structure of vortices and amplifies their mutual binding forces, thus stabilizing the ordered phase. We find novel universal critical behavior in the vortexunbinding crossover in finitesize systems. These results are relevant for a wide variety of physical systems, ranging from strongly coupled lightmatter quantum systems to dissipative time crystals.

C. Hempel, C. Maier, J. Romero, J. McClean, T. Monz, H. Shen, P. Jurcevic, B. P. Lanyon, P. Love, R. Babbush, A. AspuruGuzik, R. Blatt, C. F. Roos Quantum chemistry calculations on a trappedion quantum simulator,
Phys. Rev. X 8 31022 (20180724),
http://dx.doi.org/10.1103/PhysRevX.8.031022 doi:10.1103/PhysRevX.8.031022 (ID: 720003)
Toggle Abstract
Quantumclassical hybrid algorithms are emerging as promising candidates for nearterm practical applications of quantum information processors in a wide variety of fields ranging from chemistry to physics and materials science. We report on the experimental implementation of such an algorithm to solve a quantum chemistry problem, using a digital quantum simulator based on trapped ions. Specifically, we implement the variational quantum eigensolver algorithm to calculate the molecular ground state energies of two simple molecules and experimentally demonstrate and compare different encoding methods using up to four qubits. Furthermore, we discuss the impact of measurement noise as well as mitigation strategies and indicate the potential for adaptive implementations focused on reaching chemical accuracy, which may serve as a crossplatform benchmark for multiqubit quantum simulators.

L. Lepori, A. Celi, A. Trombettoni, M. Mannarelli Synthesis of Majorana mass terms in lowenergy quantum systems,
New J. Phys. 20 (20180620),
http://dx.doi.org/10.1088/13672630/aac91e doi:10.1088/13672630/aac91e (ID: 720169)

A. Bergschneider, V. M. Klinkhamer, J. H. Becher, R. Klemt, G. Zürn, P. M. Preiss, S. Jochim Spinresolved singleatom imaging of Li6 in free space,
Phys. Rev. A 97 63613 (20180615),
http://dx.doi.org/10.1103/PhysRevA.97.063613 doi:10.1103/PhysRevA.97.063613 (ID: 720159)
Toggle Abstract
We present a versatile imaging scheme for fermionic Li6 atoms with singleparticle sensitivity. Our method works for freely propagating particles and completely eliminates the need for confining potentials during the imaging process. We illuminate individual atoms in free space with resonant light and collect their fluorescence on an electronmultiplying CCD camera using a highnumericalaperture imaging system. We detect approximately 20 photons per atom during an exposure of 20μs and identify individual atoms with a fidelity of (99.4±0.3)%. By addressing different optical transitions during two exposures in rapid succession, we additionally resolve the hyperfine spin state of each particle. The position uncertainty of the imaging scheme is 4.0μm, given by the diffusive motion of the particles during the imaging pulse. The absence of confining potentials enables readout procedures such as the measurement of singleparticle momenta in time of flight, which we demonstrate here. Our imaging scheme is technically simple and easily adapted to other atomic species.

V. Veljic, A. R. Lima, L. Chomaz, S. Baier, M. J. Mark, F. Ferlaino, A. Pelster, A. Balaz Ground state of an ultracold Fermi gas of tilted dipoles,
New J. Phys. 20 93016 (20180614),
http://dx.doi.org/10.1088/13672630/aade24 doi:10.1088/13672630/aade24 (ID: 720051)
Toggle Abstract
Manybody dipolar effects in Fermi gases are quite subtle as they energetically compete with the large kinetic energy at and below the Fermi surface (FS). Recently it was experimentally observed that its FS is deformed from a sphere to an ellipsoid due to the presence of the anisotropic and longrange dipoledipole interaction. Moreover, it was suggested that, when the dipoles are rotated by means of an external field, the Fermi surface follows their rotation, thereby keeping the major axis of the momentumspace ellipsoid parallel to the dipoles. Here we generalise a previous HartreeFock meanfield theory to systems confined in an elongated triaxial trap with an arbitrary orientation of the dipoles relative to the trap. With this we study for the first time the effects of the dipoles' arbitrary orientation on the groundstate properties of the system. Furthermore, taking into account the geometry of the system, we show how the ellipsoidal FS deformation can be reconstructed, assuming ballistic expansion, from the experimentally measurable realspace aspect ratio after a free expansion. We compare our theoretical results with new experimental data measured with erbium Fermi gas for various trap parameters and dipole orientations. The observed remarkable agreement demonstrates the ability of our model to capture the full angular dependence of the FS deformation. Moreover, for systems with even higher dipole moment, our theory predicts an additional unexpected effect: the FS does not simply follow rigidly the orientation of the dipoles but softens showing a change in the aspect ratio depending on the dipoles' orientation relative to the trap geometry, as well as on the trap anisotropy itself. Our theory provides the basis for understanding and interpreting phenomena in which the investigated physics depends on the underlying structure of the FS, such as fermionic pairing and superfluidity.

L. Sieberer, W. Lechner Programmable superpositions of Ising congurations,
Phys. Rev. A 97 (20180529),
http://dx.doi.org/10.1103/PhysRevA.97.052329 doi:10.1103/PhysRevA.97.052329 (ID: 720138)
Toggle Abstract
We present a framework to prepare superpositions of bit strings, i.e., manybody spin configurations, with deterministic programmable probabilities. The spin configurations are encoded in the degenerate ground states of the latticegauge representation of an alltoall connected Ising spin glass. The groundstate manifold is invariant under variations of the gauge degrees of freedom, which take the form of fourbody parity constraints. Our framework makes use of these degrees of freedom by individually tuning them to dynamically prepare programmable superpositions. The dynamics combines an adiabatic protocol with controlled diabatic transitions. We derive an effective model that allows one to determine the control parameters efficiently even for large system sizes.

M. T. Rieder, L. Sieberer, M. H. Fischer, I. C. Fulga Localization Counteracts Decoherence in Noisy Floquet Topological Chains,
Phys. Rev. Lett. 120 (20180525),
http://dx.doi.org/10.1103/PhysRevLett.120.216801 doi:10.1103/PhysRevLett.120.216801 (ID: 720139)
Toggle Abstract
The topological phases of periodically driven, or Floquet systems, rely on a perfectly periodic modulation of system parameters in time. Even the smallest deviation from periodicity leads to decoherence, causing the boundary (end) states to leak into the system’s bulk. Here, we show that in one dimension this decay of topologically protected end states depends fundamentally on the nature of the bulk states: a dispersive bulk results in an exponential decay, while a localized bulk slows the decay down to a diffusive process. The localization can be due to disorder, which remarkably counteracts decoherence even when it breaks the symmetry responsible for the topological protection. We derive this result analytically, using a novel, discretetime FloquetLindblad formalism and confirm our findings with the help of numerical simulations. Our results are particularly relevant for experiments, where disorder can be tailored to protect Floquet topological phases from decoherence.

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.

G. Araneda, D. Higginbottom, L. Slodicka, Y. Colombe, R. Blatt Interference of Single Photons Emitted by Entangled Atoms in Free Space,
Phys. Rev. Lett. 120 193603 (20180511),
http://dx.doi.org/10.1103/PhysRevLett.120.193603 doi:10.1103/PhysRevLett.120.193603 (ID: 720162)
Toggle Abstract
The generation and manipulation of entanglement between isolated particles has precipitated rapid progress in quantum information processing. Entanglement is also known to play an essential role in the optical properties of atomic ensembles, but fundamental effects in the controlled emission and absorption from small, welldefined numbers of entangled emitters in free space have remained unobserved. Here we present the control of the emission rate of a single photon from a pair of distant, entangled atoms into a freespace optical mode. Changing the length of the optical path connecting the atoms modulates the singlephoton emission rate in the selected mode with a visibility V=0.27±0.03 determined by the degree of entanglement shared between the atoms, corresponding directly to the concurrence Cρ=0.31±0.10 of the prepared state. This scheme, together with population measurements, provides a fully optical determination of the amount of entanglement. Furthermore, large sensitivity of the interference phase evolution points to applications of the presented scheme in highprecision gradient sensing.

A. Rubio López, P. M. Poggi, F. C. Lombardo, V. Giannini Landauer's formula breakdown for radiative heat transfer and nonequilibrium Casimir forces,
Phys. Rev. A 97 42508 (20180430),
http://dx.doi.org/10.1103/PhysRevA.97.042508 doi:10.1103/PhysRevA.97.042508 (ID: 719890)
Toggle Abstract
In this work we analyze the incidence of the plates' thickness on the Casimir force and radiative heat transfer for a configuration of parallel plates in a nonequilibrium scenario, relating to Lifshitz's and Landauer's formulas. From a firstprinciples canonical quantization scheme for the study of the matterfield interaction, we give closedform expressions for the nonequilibrium Casimir force and the heat transfer between plates of thickness $d_{\rm L},d_{\rm R}$. We distinguish three different contributions to the Casimir force and to the heat transfer in the general nonequilibrium situation: two associated to each of the plates, and one to the initial state of the field. We analyze the dependence of the Casimir force and heat transfer with the plate thickness (setting $d_{\rm L}=d_{\rm R}\equiv d$), showing the scale at which each magnitude converges to the value of infinite thickness ($d\rightarrow+\infty$) and how to correctly reproduce the nonequilibrium Lifshitz's formula. For the heat transfer, we show that Landauer's formula does not apply to every case (where the three contributions are present), but it is correct for some specific situations. We also analyze the interplay of the different contributions for realistic experimental and nanotechnological conditions, showing the impact of the thickness in the measurements. For small thickness (compared to the separation distance), the plates act to decrease the background blackbody flux, while for large thickness the heat is given by the baths' contribution only. The combination of these behaviors allows for the possibility of having a tunable minimum in the heat transfer that is experimentally attainable and observable for metals, and also of having vanishing heat flux in the gap when those difference are of opposite signs (thermal shielding). These features turns out to be relevant for nanotechnological applications.

L. Lepori, A. Maraga, A. Celi, L. Dell'Anna, A. Trombettoni Effective Control of Chemical Potentials by Rabi Coupling with RFFields in Ultracold Mixtures,
Condensed Matter 3 (20180417),
http://dx.doi.org/10.3390/condmat3020014 doi:10.3390/condmat3020014 (ID: 720171)
Toggle Abstract
We show that a linear term coupling the atoms of an ultracold binary mixture provides a simple method to induce an effective and tunable population imbalance between them. This term is easily realized by Rabi coupling between different hyperfine levels of the same atomic species. The resulting effective imbalance holds for oneparticle states dressed by the Rabi coupling and obtained by diagonalizing the mixing matrix of the Rabi term. This way of controlling the chemical potentials applies to both bosonic and fermionic atoms and it also allows for spatially and temporallydependent imbalances. As a first application, we show that, in the case of two attractive fermionic hyperfine levels with equal chemical potentials coupled by the Rabi pulse, the same superfluid properties of an imbalanced binary mixture are recovered. We finally discuss the properties of mspecies mixtures in the presence of SU(m)invariant interactions.

N. Friis, O. Marty, C. Maier, C. Hempel, M. Holzapfel, P. Jurcevic, M. Plenio, M. Huber, C. F. Roos, R. Blatt, B. P. Lanyon Observation of Entangled States of a Fully Controlled 20Qubit System,
Phys. Rev. X 8 21012 (20180410),
http://dx.doi.org/10.1103/PhysRevX.8.021012 doi:10.1103/PhysRevX.8.021012 (ID: 720009)
Toggle Abstract
We generate and characterize entangled states of a register of 20 individually controlled qubits, where each qubit is encoded into the electronic state of a trapped atomic ion. Entanglement is generated amongst the qubits during the outofequilibrium dynamics of an Isingtype Hamiltonian, engineered via laser fields. Since the qubitqubit interactions decay with distance, entanglement is generated at early times predominantly between neighboring groups of qubits. We characterize entanglement between these groups by designing and applying witnesses for genuine multipartite entanglement. Our results show that, during the dynamical evolution, all neighboring qubit pairs, triplets, most quadruplets, and some quintuplets simultaneously develop genuine multipartite entanglement. Witnessing genuine multipartite entanglement in larger groups of qubits in our system remains an open challenge.

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.

K. Sinha Repulsive vacuuminduced forces on a magnetic particle,
Phys. Rev. A 97 32513 (20180319),
http://dx.doi.org/10.1103/PhysRevA.97.032513 doi:10.1103/PhysRevA.97.032513 (ID: 719918)
Toggle Abstract
We study the possibility of obtaining a repulsive vacuuminduced force for a magnetic point particle near a surface. Considering the toy model of a particle with an electricdipole transition and a large magnetic spin, we analyze the interplay between the repulsive magneticdipole and the attractive electricdipole contributions to the total CasimirPolder force. Particularly noting that the magneticdipole interaction is longerranged than the electricdipole due to the difference in their respective characteristic transition frequencies, we find a regime where the repulsive magnetic contribution to the total force can potentially exceed the attractive electric part in magnitude for a sufficiently large spin. We analyze ways to further enhance the magnitude of the repulsive magnetic CasimirPolder force for an excited particle, such as by preparing it in a "superradiant" magnetic sublevel, and designing surface resonances close to the magnetic transition frequency.

L. Chomaz, R. van Bijnen, D. Petter, G. Faraoni, S. Baier, M. J. Mark, F. Wächtler, L. Santos, F. Ferlaino Observation of roton mode population in a dipolar quantum gas,
Nature Phys. 14 446 (20180305),
http://dx.doi.org/10.1038/s4156701800547 doi:10.1038/s4156701800547 (ID: 719813)
Toggle Abstract
The concept of a roton, a special kind of elementary excitation, forming a minimum of energy at finite momentum, has been essential to understand the properties of superfluid 4He. In quantum liquids, rotons arise from strong interparticle interactions, whose microscopic description remains debated. In the realm of highlycontrollable quantum gases, a roton mode has been predicted to emerge due to dipolar interparticle interactions despite of their weaklyinteracting character. Yet it has remained elusive to observations. Here we report momentumdistribution measurements in dipolar quantum gases of highlymagnetic erbium atoms, revealing the existence of the longsought roton. We observe the appearance of peculiar peaks at welldefined momentum matching the inverse of the tight confinement length as expected for dipolar rotons. Our combined theoretical and experimental work demonstrates unambiguously the roton softening of the excitation spectrum and provides a further step in the quest towards supersolidity.

P. Ilzhöfer, G. Durastante, A. Patscheider, A. Trautmann, M. J. Mark, F. Ferlaino Twospecies fivebeam magnetooptical trap for erbium and dysprosium,
Phys. Rev. A 97 23633 (20180226),
http://dx.doi.org/10.1103/PhysRevA.97.023633 doi:10.1103/PhysRevA.97.023633 (ID: 719919)
Toggle Abstract
We report on the first realization of a twospecies magnetooptical trap (MOT) for erbium and dysprosium. The MOT operates on an intercombination line for the respective species. Owing to the narrowline character of such a cooling transition and the action of gravity, we demonstrate a novel trap geometry employing only five beams in orthogonal configuration. We observe that the mixture is cooled and trapped very efficiently, with up to \num{5e8} Er atoms and \num{e9} Dy atoms at temperatures of about 10μK. Our results offer an ideal starting condition for the creation of a dipolar quantum mixture of highly magnetic atoms.

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.
(local copy)

R. Lous, I. Fritsche, M. Jag, F. Lehmann, E. Kirilov, B. Huang, R. Grimm Probing the Interface of a PhaseSeparated State in a Repulsive BoseFermi Mixture,
Phys. Rev. Lett. 120 243403 (20180206),
http://dx.doi.org/10.1103/PhysRevLett.120.243403 doi:10.1103/PhysRevLett.120.243403 (ID: 719979)
Toggle Abstract
We probe the interface between a phaseseparated BoseFermi mixture consisting of a small BEC of 41K residing in a large Fermi sea of 6Li. We quantify the residual spatial overlap between the two components by measuring threebody recombination losses for variable strength of the interspecies repulsion. A comparison with a numerical meanfield model highlights the importance of the kinetic energy term for the condensed bosons in maintaining the thin interface far into the phaseseparated regime. Our results demonstrate a corresponding smoothing of the phase transition in a system of finite size.

N. Trautmann, P. Hauke Trappedion quantum simulation of excitation transport: Disordered, noisy, and longrange connected quantum networks,
Phys. Rev. A 97 (20180205),
http://dx.doi.org/10.1103/PhysRevA.97.023606 doi:10.1103/PhysRevA.97.023606 (ID: 720170)
Toggle Abstract
The transport of excitations governs fundamental properties of matter. Particularly rich physics emerges in the interplay between disorder and environmental noise, even in small systems such as photosynthetic biomolecules. Counterintuitively, noise can enhance coherent quantum transport, which has been proposed as a mechanism behind the high transport efficiencies observed in photosynthetic complexes. This effect has been called “environmentassisted quantum transport”. Here, we propose a quantum simulation of the excitation transport in an open quantum network, taking advantage of the high controllability of current trappedion experiments. Our scheme allows for the controlled study of various different aspects of the excitation transfer, ranging from the influence of static disorder and interaction range, over the effect of Markovian and nonMarkovian dephasing, to the impact of a continuous insertion of excitations. Our paper discusses experimental error sources and realistic parameters, showing that it can be implemented in stateoftheart ionchain experiments.

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.
(local copy)

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.
(local copy)

P. R. Muppalla, O. Gargiulo, S. Mirzaei, B. Venkatesh, M. L. Juan, L. Grünhaupt, I. M. Pop, G. Kirchmair Bistability in a Mesoscopic Josephson Junction Array Resonator,
Phys. Rev. B 97 24518 (20180130),
http://dx.doi.org/10.1103/PhysRevB.97.024518 doi:10.1103/PhysRevB.97.024518 (ID: 719817)
Toggle Abstract
We present an experimental investigation of stochastic switching of a bistable Josephson junctions array resonator with a resonance frequency in the GHz range. As the device is in the regime where the anharmonicity is on the order of the linewidth, the bistability appears for a pump strength of only a few photons. We measure the dynamics of the bistability by continuously observing the jumps between the two metastable states, which occur with a rate ranging from a few Hz down to a few mHz. The switching rate strongly depends on the pump strength, readout strength and the temperature, following Kramer's law. The interplay between nonlinearity and coupling, in this little explored regime, could provide a new resource for nondemolition measurements, single photon switches or even elements for autonomous quantum error correction.

H. Pino, J. PratCamps, K. Sinha, B. Venkatesh, O. RomeroIsart Onchip quantum interference of a superconducting microsphere,
Quantum Sci. Technol. 3 25001 (20180125),
http://dx.doi.org/10.1088/20589565/aa9d15 doi:10.1088/20589565/aa9d15 (ID: 719522)
Toggle Abstract
We propose and analyze an allmagnetic scheme to perform a Young's double slit experiment with a micronsized superconducting sphere of mass $\gtrsim {10}^{13}$ amu. We show that its center of mass could be prepared in a spatial quantum superposition state with an extent of the order of half a micrometer. The scheme is based on magnetically levitating the sphere above a superconducting chip and letting it skate through a static magnetic potential landscape where it interacts for short intervals with quantum circuits. In this way, a protocol for fast quantum interferometry using quantum magnetomechanics is passively implemented. Such a tabletop earthbased quantum experiment would operate in a parameter regime where gravitational energy scales become relevant. In particular, we show that the faint parameterfree gravitationallyinduced decoherence collapse model, proposed by Diósi and Penrose, could be unambiguously falsified.

M. Garttner, P. Hauke, A. M. Rey Relating OutofTimeOrder Correlations to Entanglement via MultipleQuantum Coherences,
Phys. Rev. Lett. 120 (20180124),
http://dx.doi.org/10.1103/PhysRevLett.120.040402 doi:10.1103/PhysRevLett.120.040402 (ID: 720166)
Toggle Abstract
Abstract
Outoftimeorder correlations (OTOCs) characterize the scrambling, or delocalization, of quantum information over all the degrees of freedom of a system and thus have been proposed as a proxy for chaos in quantum systems. Recent experimental progress in measuring OTOCs calls for a more thorough understanding of how these quantities characterize complex quantum systems, most importantly in terms of the buildup of entanglement. Although a connection between OTOCs and entanglement entropy has been derived, the latter only quantifies entanglement in pure systems and is hard to access experimentally. In this work, we formally demonstrate that the multiplequantum coherence spectra, a specific family of OTOCs well known in NMR, can be used as an entanglement witness and as a direct probe of multiparticle entanglement. Our results open a path to experimentally testing the fascinating idea that entanglement is the underlying glue that links thermodynamics, statistical mechanics, and quantum gravity.

B. Venkatesh, M. L. Juan, O. RomeroIsart Cooperative Effects in Closely Packed Quantum Emitters with Collective Dephasing,
Phys. Rev. Lett. 120 33602 (20180119),
http://dx.doi.org/10.1103/PhysRevLett.120.033602 doi:10.1103/PhysRevLett.120.033602 (ID: 719800)
Toggle Abstract
In a closely packed ensemble of quantum emitters, cooperative effects are typically suppressed due to the dephasing induced by the dipoledipole interactions. Here, we show that by adding sufficiently strong collective dephasing, cooperative effects can be restored. Specifically, we show that the dipole force on a closely packed ensemble of strongly driven twolevel quantum emitters, which collectively dephase, is enhanced in comparison to the dipole force on an independent noninteracting ensemble. Our results are relevant to solidstate systems with embedded quantum emitters such as color centers in diamond and superconducting qubits in microwave cavities and waveguides.

J. H. Becher, S. Baier, K. Aikawa, M. Lepers, J. Wyart, O. Dulieu, F. Ferlaino Anisotropic polarizability of erbium atoms,
Phys. Rev. A 97 12509 (20180119),
http://dx.doi.org/10.1103/PhysRevA.97.012509 doi:10.1103/PhysRevA.97.012509 (ID: 719925)
Toggle Abstract
We report on the determination of the dynamical polarizability of ultracold erbium atoms in the ground and in one excited state at three different wavelengths, which are particularly relevant for optical trapping. Our study combines experimental measurements of the light shift and theoretical calculations. In particular, our experimental approach allows us to isolate the different contributions to the polarizability, namely the isotropic scalar and anisotropic tensor part. For the latter contribution, we observe a clear dependence of the atomic polarizability on the angle between the laserfieldpolarization axis and the quantization axis, set by the external magnetic field. Such an angledependence is particularly pronounced in the excitedstate polarizability. We compare our experimental findings with the theoretical values, based on semiempirical electronicstructure calculations and we observe a very good overall agreement. Our results pave the way to exploit the anisotropy of the tensor polarizability for spinselective preparation and manipulation.

C. Ravensbergen, V. Corre, E. Soave, M. Kreyer, S. Tzanova, E. Kirilov, R. Grimm Accurate Determination of the Dynamical Polarizability of Dysprosium,
Phys. Rev. Lett. 120 223001 (20180118),
http://dx.doi.org/10.1103/PhysRevLett.120.223001 doi:10.1103/PhysRevLett.120.223001 (ID: 719964)
Toggle Abstract
We report a measurement of the dynamical polarizability of dysprosium atoms in their electronic ground state at the optical wavelength of 1064 nm, which is of particular interest for laser trapping experiments. Our method is based on collective oscillations in an optical dipole trap, and reaches unprecedented accuracy and precision by comparison with an alkali atom (potassium) as a reference species. We obtain values of 184.4(2.4) a.u. and 1.7(6) a.u. for the scalar and tensor polarizability, respectively. Our experiments have reached a level that permits meaningful tests of current theo retical descriptions and provides valuable information for future experiments utilizing the intriguing properties of heavy lanthanide atoms.

R. Schmidt, M. Knap, D. Ivanov, J. You, M. Cetina, E. Demler Universal manybody response of heavy impurities coupled to a Fermi sea: a review of recent progress,
Rep. Prog. Phys. 81 24401 (20180105),
http://dx.doi.org/10.1088/13616633/aa9593 doi:10.1088/13616633/aa9593 (ID: 720161)
Toggle Abstract
Reports on Progress in Physics
Report on Progress
Universal manybody response of heavy impurities coupled to a Fermi sea: a review of recent progress
Richard Schmidt1,2,3
, Michael Knap4, Dmitri A Ivanov5,6
, JhihShih You1, Marko Cetina7,8,9 and Eugene Demler1
Published 5 January 2018 • © 2018 IOP Publishing Ltd
Reports on Progress in Physics, Volume 81, Number 2
Download Article PDF
Figures
References
Download PDF
775 Total downloads
12
12 total citations on Dimensions.
Article has an altmetric score of 1
Turn on MathJax
Get permission to reuse this article
Share this article
Share this content via email
Share on Facebook
Share on Twitter
Share on Google+
Share on CiteULike
Share on Mendeley
Article information
Corresponding Editor Professor Maciej Lewenstein
Abstract
In this report we discuss the dynamical response of heavy quantum impurities immersed in a Fermi gas at zero and at finite temperature. Studying both the frequency and the time domain allows one to identify interaction regimes that are characterized by distinct manybody dynamics. From this theoretical study a picture emerges in which impurity dynamics is universal on essentially all time scales, and where the highfrequency fewbody response is related to the longtime dynamics of the Anderson orthogonality catastrophe by Tan relations. Our theoretical description relies on different and complementary approaches: functional determinants give an exact numerical solution for time and frequencyresolved responses, bosonization provides accurate analytical expressions at low temperatures, and the theory of Toeplitz determinants allows one to analytically predict response up to high temperatures. Using these approaches we predict the thermal decoherence rate of the fermionic system and prove that within the considered model the fastest rate of longtime decoherence is given by $\gamma=\pi k_{\rm B}T/4$ . We show that Feshbach resonances in cold atomic systems give access to new interaction regimes where quantum effects can prevail even in the thermal regime of manybody dynamics. The key signature of this phenomenon is a crossover between different exponential decay rates of the realtime Ramsey signal. It is shown that the physics of the orthogonality catastrophe is experimentally observable up to temperatures $T/T_{\rm F}\lesssim 0.2$ where it leaves its fingerprint in a powerlaw temperature dependence of thermal spectral weight and we review how this phenomenon is related to the physics of heavy ions in liquid ${\hspace{0pt}}^3$ He and the formation of Fermi polarons. The presented results are in excellent agreement with recent experiments on LiK mixtures, and we predict several new phenomena that can be tested using currently available experimental technology.

N. Fläschner, D. Vogel, M. Tarnowski, B. Rem, D. Lühmann, M. Heyl, J. Budich, L. Mathey, K. Sengstock, C. Weitenberg Observation of dynamical vortices after quenches in a system with topology,
Nature Phys. 14 268 (20180101),
http://dx.doi.org/10.1038/s4156701700138 doi:10.1038/s4156701700138 (ID: 720165)
Toggle Abstract
Topological phases constitute an exotic form of matter characterized by nonlocal properties rather than local order parameters1. The paradigmatic Haldane model on a hexagonal lattice features such topological phases distinguished by an integer topological invariant known as the first Chern number2. Recently, the identification of nonequilibrium signatures of topology in the dynamics of such systems has attracted particular attention3,4,5,6. Here, we experimentally study the dynamical evolution of the wavefunction using time and momentumresolved full state tomography for spinpolarized fermionic atoms in driven optical lattices7. We observe the appearance, movement and annihilation of dynamical vortices in momentum space after sudden quenches close to the topological phase transition. These dynamical vortices can be interpreted as dynamical Fisher zeros of the Loschmidt amplitude8, which signal a socalled dynamical phase transition9,10. Our results pave the way to a deeper understanding of the connection between topological phases and nonequilibrium dynamics.

R. Blatt Quantencomputer Rechenkunst mit Quantenphysik,
43 Forum Akademie, TU Graz (TU Graz, 20181129) (20181129),
(ID: 720102)

C. F. Roos Variational quantum simulation with a twentyion string,
5th European Conference on Trapped Ions (Rehovot, 20181117) (20181119),
(ID: 720091)

R. Blatt Quantum Information with trapped Ions: Verification and Validation,
ECTI 2018 (The David Lopatie Conference Centre, Weizmann Institute of Science, 20181117) (20181118),
(ID: 720095)

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.

G. Kirchmair Quantum Information Processing with Superconducting Circuits,
Exner Lecture (Vienna, 20181023) URL (20181023),
(ID: 720115)
Toggle Abstract
Superconducting quantum circuits are one of the most promising platforms for realizing a quantum computer. In this talk I will present some of the research activities of the Superconducting Quantum Circuits group in Innsbruck. I will give a short introduction to circuit quantum electrodynamics and highlight the progress of superconducting qubits. I will then show, how this architecture can be used to realize a platform for quantum computation and for simulating interacting quantum manybody systems

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].

R. Blatt Quantum computations and quantum Simulations with Trapped Ca+ Ions,
Opportunities for Quantum Technologies 2018: A Dialog (Vancouver Convention Center , 20181003) (20181003),
(ID: 720082)

R. Blatt The Quantum Way of Doing Computations,
Public Talk (University of British Columbia, PITP, 20181002) (20181002),
(ID: 720081)

R. Blatt Quantum Information using trapped ionsstatus and perspectives,
TCP 2018 (Park Place Hotel Traverse City , 20180929) (20181001),
(ID: 720083)

O. RomeroIsart Hot or Cold Nanospheres? Maybe none of the above,
Quantum Engineering of Levitated Systems (Benasque, Spain, 20180916) URL (20180920),
(ID: 720066)

R. Blatt Quantencomputer Rechenkunst mit Quantenphysik ,
130.GDNÄ Versammlung (Universität Saarland Saarbrücken , 20180914) (20180915),
(ID: 720074)

O. RomeroIsart Levitated Nanoparticles in the Quantum Regime: Challenges and Opportunities,
French Research Network on Quantum OptoMechanics and Nanomechanics (GdR MecaQ), (Université Paris Diderot, Paris (France), 20180912) URL (20180913),
(ID: 720059)

R. Blatt Quantum Computera New Technology for the Information AG,
Eurosensor 2018 (Universität Graz, 20180910) (20180911),
(ID: 720075)

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.

R. Grimm Massimbalanced fermion mixtures,
AIAS Associate Event: Joint CQOM & ITAMP Workshop (Aarhus, Denmark, 20180813) URL (20180815),
(ID: 720177)

G. Kirchmair Superconducting Qubits for Analog Quantum Simulation,
Superconducting quantum technologies (Moskow, 20180730) URL (20180731),
(ID: 720116)
Toggle Abstract
In this talk I want to present the research activities of the Superconducting Quantum Circuits group at the Institute for Quantum Optics and Quantum Information in Innsbruck.
I will give an introduction to circuit quantum electrodynamics and our 3D circuit QED architecture. I will show how we want to use this architecture to realize a platform for quantum many body simulations of dipolar XY models on 2D lattices using state of the art circuit QED technology. The central idea is to exploit the naturally occurring dipolar interactions between 3D superconducting qubits to simulate models of interacting quantum spins. The ability to arrange the qubits on essentially arbitrary geometries allows us to design spin models with more than nearestneighbor interaction in various geometries.
Combining these ideas with our waveguide architecture, will allow us to study open system dynamics with interacting spin systems. The platform will allow us to investigate the interplay between short range direct interactions, long range photon mediated interaction via the waveguide and the dissipative coupling to an open system.

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

R. Blatt QuantencomputerRechenkunst mit Quantenphysik ,
Hector Fellow Academy, "Die zweite Quantenrevolution" (Literaturhaus München , 20180719) (20180719),
(ID: 720046)

R. Grimm How it finally (35 years later) happened in the lab,
22nd International Conference on FewBody Problems in Physics (Caen, France, 20180709) URL (20180711),
(ID: 720175)

R. Blatt Quantum Tehnologies made in Tyrol,
WIRE2018 (Innsbruck, 20180704) (20180706),
(ID: 720039)

R. Grimm Ultracold atoms in optical dipole traps,
50th anniversary celebration, Institute of Spectroscopy, Russian Academy of Sciences (Troitsk/Moscow, 20190306) URL (20180620),
(ID: 720174)

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

C. GonzalezBallestero Fewphoton quantum protocols and devices based on chiral waveguideemitter couplings.,
First Workshop on Waveguide QED (Mazara del Vallo, Italy, 20180604) URL (20180607),
(ID: 720279)

R. Blatt Quantum simulations with cold trapped ions,
Quantum Technology Workshop (The Hebrew University of Jerusalem, 20180605) (20180605),
(ID: 720036)

R. Blatt The Quantum Way of doing Computations  New Technologies for the Information Age,
Racah Lecture (University of Jerusalem, 20180604) (20180604),
(ID: 720035)

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.

C. F. Roos Entanglement creation and characterization in a trappedion quantum simulator,
Seminar talk (Collège de France, Paris, 20180516) (20180516),
(ID: 720026)

R. Blatt Quantum computations and quantum simulations with trapped ions,
SPIE Photonics Europe (Strasbourg, 20180422) (20180426),
(ID: 720032)

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.

R. Blatt Quantum Computations and Quantum simulations with trapped ions,
Beyond Digital Computing (Heidelberg University , 20180319) (20180320),
(ID: 720000)

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

O. RomeroIsart UltraWeak Force Sensing,
Gordon Research Conference on Mechanical Systems in the Quantum Regime (Ventura, California (USA), 20180225) (20180228),
URL (ID: 719998)

M. Baranov Subwavelength ‘Dark State’ Optical Potentials for Cold Atoms and its Applications,
Quantum Optics 2018 (Obergurgl, 20180225) URL (20180226),
(ID: 720111)

R. Blatt Quantum Computations and quantum simulations with trapped ions ,
Quantum Optics Obergurgl 2018 (Universitätszentrum Obergurgl, 20180225) (20180225),
(ID: 719988)

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)

R. Blatt Opportunities of Excellence Strategies ,
RESOLV Neujahrsempfang " Exzellenzinitiative Chance für die Hochschule" (TU Dortmund, 20180219) (20180219),
(ID: 719985)

Analog Quantum Simulation with Superconducting Qubits,
FCQO2018 (IST Klosterneuburg, 20180212) URL (20180214),
(ID: 720120)
Toggle Abstract
In this talk I want to present the research activities of the Superconducting Quantum Circuits group at the Institute for Quantum Optics and Quantum Information in Innsbruck.
In the first part I will give an introduction to circuit quantum electrodynamics and our 3D circuit QED architecture. I will show how we want to use this architecture to realize a platform for quantum many body simulations of dipolar XY models on 2D lattices using state of the art circuit QED technology. The central idea is to exploit the naturally occurring dipolar interactions between 3D superconducting qubits to simulate models of interacting quantum spins. The ability to arrange the qubits on essentially arbitrary geometries allows us to design spin models with more than nearestneighbor interaction in various geometries.
Combining these ideas with our waveguide architecture, will allow us to study open system dynamics with interacting spin systems. The platform will allow us to investigate the interplay between short range direct interactions, long range photon mediated interaction via the waveguide and the dissipative coupling to an open system. Especially interesting in this context are dissipative state preparation protocols and engineering the spectrum of our bath in the spirit of band engineering in photonic crystals.

G. Kirchmair Analog Quantum Simulation with Superconducting Qubits,
FCQO2018 (IST Klosterneuburg, 20180212) URL (20180214),
(ID: 720127)

R. Blatt Quantum computation and quantum simulation with trapped ca+ ions,
Workshop Quantum Simulation & Computation (University of the Basque Country, Paraninfo, Science and Tech Faculty Bilbao, 20180211) (20180212),
(ID: 719984)

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)

C. F. Roos Creation and detection of entanglement in trappedion experiments,
UQUAM Workshop on Entanglement (IQOQI Innsbruck, 20180123) (20180124),
(ID: 719969)

“Advanced Quantum Technologies with Trapped Atoms and Ions",
Quantum technology labinauguration ceremony (BenGurion University Beer Sheva, 20180122) (20180122),
(ID: 719975)

R. Blatt Quantum Computations and Quantum Simulations with Trapped Ions",
Inauguration Ceremony MPHQ (Munich, 20180111) (20180112),
(ID: 719970)

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

A. Bermudez, X. Xu, R. Nigmatullin, J. O’Gorman, V. Negnevitsky, P. Schindler, T. Monz, U. Poschinger, C. Hempel, J. Home, F. SchmidtKaler, M. Biercuk, R. Blatt, S. Benjamin, M. Müller Assessing the Progress of TrappedIon Processors Towards FaultTolerant Quantum Computation,
Phys. Rev. X 7 41061 (20171217),
http://dx.doi.org/10.1103/PhysRevX.7.041061 doi:10.1103/PhysRevX.7.041061 (ID: 719935)
Toggle Abstract
A quantitative assessment of the progress of small prototype quantum processors towards faulttolerant quantum computation is a problem of current interest in experimental and theoretical quantum information science. We introduce a necessary and fair criterion for quantum error correction (QEC), which must be achieved in the development of these quantum processors before their sizes are sufficiently big to consider the wellknown QEC threshold. We apply this criterion to benchmark the ongoing effort in implementing QEC with topological color codes using trappedion quantum processors and, more importantly, to guide the future hardware developments that will be required in order to demonstrate beneficial QEC with small topological quantum codes. In doing so, we present a thorough description of a realistic trappedion toolbox for QEC and a physically motivated error model that goes beyond standard simplifications in the QEC literature. We focus on laserbased quantum gates realized in twospecies trappedion crystals in highoptical aperture segmented traps. Our largescale numerical analysis shows that, with the foreseen technological improvements described here, this platform is a very promising candidate for faulttolerant quantum computation.

J. Zeiher, J. Choi, A. Rubio Abadal, T. Pohl, R. van Bijnen, I. Bloch, C. Gross Coherent manybody spin dynamics in a longrange interacting Ising chain,
Phys. Rev. X 7 41063 (20171214),
http://dx.doi.org/10.1103/PhysRevX.7.041063 doi:10.1103/PhysRevX.7.041063 (ID: 719831)
Toggle Abstract
Coherent manybody quantum dynamics lies at the heart of quantum simulation and quantum computation. Both require coherent evolution in the exponentially large Hilbert space of an interacting manybody system. To date, trapped ions have defined the state of the art in terms of achievable coherence times in interacting spin chains. Here, we establish an alternative platform by reporting on the observation of coherent, fully interactiondriven quantum revivals of the magnetization in Rydbergdressed Ising spin chains of atoms trapped in an optical lattice. We identify partial manybody revivals at up to about ten times the characteristic time scale set by the interactions. At the same time, singlesiteresolved correlation measurements link the magnetization dynamics with interspin correlations appearing at different distances during the evolution. These results mark an enabling step towards the implementation of Rydberg atom based quantum annealers, quantum simulations of higher dimensional complex magnetic Hamiltonians, and itinerant longrange interacting quantum matter.
(local copy)

O. RomeroIsart Coherent Inflation for Large Quantum Superpositions of Microspheres,
New J. Phys. 19 719711 (20171212),
http://dx.doi.org/10.1088/13672630/aa99bf doi:10.1088/13672630/aa99bf (ID: 719711)
Toggle Abstract
We show that coherent inflation, namely quantum dynamics generated by inverted conservative potentials acting on the center of mass of a massive object, is an enabling tool to prepare large spatial quantum superpositions in a doubleslit experiment. Combined with cryogenic, extreme high vacuum, and lowvibration environments, we argue that it is experimentally feasible to exploit coherent inflation to prepare the center of mass of a micrometersized object in a spatial quantum superposition comparable to its size. In such a hitherto unexplored parameter regime gravitationallyinduced decoherence could be unambiguously falsified. We present a protocol to implement coherent inflation in a doubleslit experiment by letting a levitated microsphere traverse a static potential landscape. Such a protocol could be experimentally implemented with an allmagnetic scheme using superconducting microspheres.

B. P. Lanyon, C. Maier, M. Holzapfel, T. Baumgratz, C. Hempel, P. Jurcevic, I. Dhand, A. S. Buyskik, A. J. Daley, M. Cramer, M. Plenio, R. Blatt, C. F. Roos Efficient tomography of a quantum manybody system,
Nature Phys. 13 1158 (20171205),
http://dx.doi.org/10.1038/nphys4244 doi:10.1038/nphys4244 (ID: 719716)
Toggle Abstract
Quantum state tomography (QST) is the gold standard technique for obtaining an estimate for the state of small quantum systems in the laboratory. Its application to systems with more than a few constituents (e.g. particles) soon becomes impractical as the effort required grows exponentially in the number of constituents. Developing more efficient techniques is particularly pressing as preciselycontrollable quantum systems that are well beyond the reach of QST are emerging in laboratories. Motivated by this, there is a considerable ongoing effort to develop new characterisation tools for quantum manybody systems. Here we demonstrate Matrix Product State (MPS) tomography, which is theoretically proven to allow the states of a broad class of quantum systems to be accurately estimated with an effort that increases efficiently with constituent number. We first prove that this broad class includes the outofequilbrium states produced by 1D systems with finiterange interactions, up to any fixed point in time. We then use the technique to reconstruct the dynamical state of a trappedion quantum simulator comprising up to 14 entangled spins (qubits): a size far beyond the reach of QST. Our results reveal the dynamical growth of entanglement and description complexity as correlations spread out during a quench: a necessary condition for future beyondclassical performance. MPS tomography should find widespread use to study large quantum manybody systems and to benchmark and verify quantum simulators and computers.

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.
(local copy)

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.
(local copy)

A. Bermudez, P. Schindler, T. Monz, R. Blatt, M. Müller Micromotionenabled improvement of quantum logic gates with trapped ions,
New J. Phys. 19 113038 (20171124),
http://dx.doi.org/10.1088/13672630/aa86eb doi:10.1088/13672630/aa86eb (ID: 719937)
Toggle Abstract
The micromotion of ion crystals confined in Paul traps is usually considered an inconvenient nuisance, and is thus typically minimized in highprecision experiments such as highfidelity quantum gates for quantum information processing (QIP). In this work, we introduce a particular scheme where this behavior can be reversed, making micromotion beneficial for QIP. We show that using laserdriven micromotion sidebands, it is possible to engineer statedependent dipole forces with a reduced effect of offresonant couplings to the carrier transition. This allows one, in a certain parameter regime, to devise entangling gate schemes based on geometric phase gates with both a higher speed and a lower error, which is attractive in light of current efforts towards faulttolerant QIP. We discuss the prospects of reaching the parameters required to observe this micromotionenabled improvement in experiments with current and future trap designs.

M. Gerster, M. Rizzi, P. Silvi, M. Dalmonte, S. Montangero Fractional quantum Hall effect in the interacting Hofstadter model via tensor networks,
Phys. Rev. B 96 195123 (20171113),
http://dx.doi.org/10.1103/PhysRevB.96.195123 doi:10.1103/PhysRevB.96.195123 (ID: 719821)
Toggle Abstract
We show via tensor network methods that the HarperHofstadter Hamiltonian for hardcore bosons on a square geometry supports a topological phase realizing the ν=1/2 fractional quantum Hall effect on the lattice. We address the robustness of the ground state degeneracy and of the energy gap, measure the manybody Chern number, and characterize the system using Green functions, showing that they decay algebraically at the edges of open geometries, indicating the presence of gapless edge modes. Moreover, we estimate the topological entanglement entropy, which is compatible with the expected value γ=1/2 . Finally, we describe the phase transition from a FQH state to density wave states as a function of a staggered chemical potential. Our results provide extensive evidence that FQH states are within reach of stateoftheart cold atom experiments.
(local copy)

M. Łącki, B. Damski Spatial KibbleZurek mechanism through susceptibilities: the inhomogeneous quantum Ising model case,
J. Stat. Mech. (20171024),
http://dx.doi.org/10.1088/17425468/aa8c20/meta doi:10.1088/17425468/aa8c20/meta (ID: 719901)
(local copy)

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.

C. C. Rusconi, V. Pöchhacker, K. Kustura, J. I. Cirac, O. RomeroIsart Quantum Spin Stabilized Magnetic Levitation,
Phys. Rev. Lett. 119 167202 (20171019),
http://dx.doi.org/10.1103/PhysRevLett.119.167202 doi:10.1103/PhysRevLett.119.167202 (ID: 719772)
Toggle Abstract
We theoretically show that, despite Earnshaw's theorem, a nonrotating single magnetic domain nanoparticle can be stably levitated in an external static magnetic field. The stabilization relies on the quantum spin origin of magnetization, namely the gyromagnetic effect. We predict the existence of two stable phases related to the Einsteinde Haas effect and the Larmor precession. At a stable point, we derive a quadratic Hamiltonian that describes the quantum fluctuations of the degrees of freedom of the system. We show that in the absence of thermal fluctuations, the quantum state of the nanomagnet at the equilibrium point contains entanglement and squeezing.

C. C. Rusconi, V. Pöchhacker, J. I. Cirac, O. RomeroIsart Linear Stability Analysis of a Levitated Nanomagnet in a Static Magnetic Field: Quantum Spin Stabilized Magnetic Levitation,
Phys. Rev. B 96 134419 (20171018),
http://dx.doi.org/10.1103/PhysRevB.96.134419 doi:10.1103/PhysRevB.96.134419 (ID: 719731)
Toggle Abstract
We theoretically study the levitation of a single magnetic domain nanosphere in an external static magnetic field. We show that apart from the stability provided by the mechanical rotation of the nanomagnet (as in the classical Levitron), the quantum spin origin of its magnetization provides two additional mechanisms to stably levitate the system. Despite of the Earnshaw theorem, such stable phases are present even in the absence of mechanical rotation. For large magnetic fields, the Larmor precession of the quantum magnetic moment stabilizes the system in full analogy with magnetic trapping of a neutral atom. For low magnetic fields, the magnetic anisotropy stabilizes the system via the Einsteinde Haas effect. These results are obtained with a linear stability analysis of a single magnetic domain rigid nanosphere with uniaxial anisotropy in a IoffePritchard magnetic field.

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.
(local copy)

V. Krutyanskiy, M. R. Meraner, J. Schupp, B. P. Lanyon Polarisationpreserving photon frequency conversion from a trappedioncompatible wavelength to the telecom Cband,
Appl. Phys. B Las. Opt. 123 228 (20170930),
http://dx.doi.org/10.1007/s0034001768068 doi:10.1007/s0034001768068 (ID: 719939)
Toggle Abstract
We demonstrate polarisationpreserving frequency conversion of singlephotonlevel light at 854 nm, resonant with a trappedion transition and qubit, to the 1550nm telecom C band. A total photon in / fibercoupled photon out efficiency of ∼30% is achieved, for a freerunning photon noise rate of ∼60 Hz. This performance would enable telecom conversion of 854 nm polarisation qubits, produced in existing trappedion systems, with a signaltonoise ratio greater than 1. In combination with nearfuture trappedion systems, our converter would enable the observation of entanglement between an ion and a photon that has travelled more than 100 km in optical fiber: three orders of magnitude further than the stateoftheart.

C. Navarrete, T. Weiß, S. Walter, G. J. de Valcárcel General linearized theory of quantum fluctuations around arbitrary limit cycles,
Phys. Rev. Lett. 119 133601 (20170926),
http://dx.doi.org/10.1103/PhysRevLett.119.133601 doi:10.1103/PhysRevLett.119.133601 (ID: 720024)

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.
(local copy)

R. MachBatlle, A. Parra, J. PratCamps, S. Laut, C. Navau, À. Sánchez Negative permeability in magnetostatics and its experimental demonstration,
Phys. Rev. B 96 94422 (20170919),
http://dx.doi.org/10.1103/PhysRevB.96.094422 doi:10.1103/PhysRevB.96.094422 (ID: 719880)
Toggle Abstract
The control of magnetic fields, essential for our science and technology, is currently achieved by magnetic materials with positive permeability, including ferromagnetic, paramagnetic, and diamagnetic types. Here we introduce materials with negative static permeability as a new paradigm for manipulating magnetic fields. As a first step, we extend the solutions of Maxwell magnetostatic equations to include negativepermeability values. The understanding of these new solutions allow us to devise a negativepermeability material as a suitably tailored set of currents arranged in space, overcoming the fact that passive materials with negative permeability do no exist in magnetostatics. We confirm the theory by experimentally creating a spherical shell that emulates a negativepermeability material in a uniform magnetic field. Our results open new possibilities for creating and manipulating magnetic fields, which can be useful for practical applications.

C. GonzalezBallestero, F. A. Y. N. Schröder, A. Chin Uncovering nonperturbative dynamics of the biased subOhmic spinboson model with variational matrix product states,
Phys. Rev. B 96 115427 (20170915),
http://dx.doi.org/10.1103/PhysRevB.96.115427 doi:10.1103/PhysRevB.96.115427 (ID: 719833)
Toggle Abstract
We study the dynamics of the biased subOhmic spinboson model by means of a timedependent variational matrix product state (TDVMPS) algorithm. The evolution of both the system and the environment is obtained in the weak and the strongcoupling regimes, respectively characterized by damped spin oscillations and by a nonequilibrium process where the spin freezes near its initial state, which are explicitly shown to arise from a variety of reactive environmental quantum dynamics. We also explore the rich phenomenology of the intermediatecoupling case, a nonperturbative regime where the system shows a complex dynamical behavior, combining features of both the weakly and the strongly coupled case in a sequential, timeretarded fashion. Our work demonstrates the potential of TDVMPS methods for exploring otherwise elusive, nonperturbative regimes of complex open quantum systems, and points to the possibilities of exploiting the qualitative, realtime modification of quantum properties induced by nonequilibrium bath dynamics in ultrafast transient processes.

B. Vogell, B. Vermersch, T. E. Northup, B. P. Lanyon, C. A. Muschik Deterministic quantum state transfer between remote qubits in cavities,
Quantum Sci. Technol. 2 45003 (20170908),
http://dx.doi.org/10.1088/20589565/aa868b doi:10.1088/20589565/aa868b (ID: 719794)
Toggle Abstract
Performing a faithful transfer of an unknown quantum state is a key challenge for enabling quantum networks. The realization of networks with a small number of quantum links is now actively pursued, which calls for an assessment of different state transfer methods to guide future design decisions. Here, we theoretically investigate quantum state transfer between two distant qubits, each in a cavity, connected by a waveguide, e.g., an optical fiber. We evaluate the achievable state transfer fidelities for two different protocols: standard wave packet shaping and adiabatic passage. The main loss sources are transmission losses in the waveguide and absorption losses in the cavities. While special cases studied in the literature indicate that adiabatic passages may be beneficial in this context, it remained an open question under which conditions this is the case and whether their use will be advantageous in practice. We answer these questions by providing a full analysis, showing that state transfer by adiabatic passage  in contrast to wave packet shaping  can mitigate the effects of undesired cavity losses, far beyond the regime of coupling to a single waveguide mode and the regime of lossless waveguides, as was proposed so far. We also clarify that neither protocol can avoid losses in the waveguide and discuss how the cavity parameters should be chosen.
(local copy)

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.
(local copy)

J. PratCamps, C. Teo, C. C. Rusconi, W. Wieczorek, O. RomeroIsart Ultrasensitive Inertial and Force Sensors with Diamagnetically Levitated Magnets,
Phys. Rev. Applied 8 34002 (20170907),
http://dx.doi.org/10.1103/PhysRevApplied.8.034002 doi:10.1103/PhysRevApplied.8.034002 (ID: 719764)
Toggle Abstract
We theoretically show that a magnet can be stably levitated on top of a punctured superconductor sheet in the Meissner state without applying any external field. The trapping potential created by such inducedonly superconducting currents is characterized for magnetic spheres ranging from tens of nanometers to tens of millimeters. Such a diamagnetically levitated magnet is predicted to be extremely well isolated from the environment. We therefore propose to use it as an ultrasensitive force and inertial sensor. A magnetomechanical readout of its displacement can be performed by using superconducting quantum interference devices. An analysis using current technology shows that force and acceleration sensitivities on the order of 10−23N/Hz‾‾‾√ (for a 100 nm magnet) and 10−14g/Hz‾‾‾√ (for a 10 mm magnet) might be within reach in a cryogenic environment. Such unprecedented sensitivities can be used for a variety of purposes, from designing ultrasensitive inertial sensors for technological applications (i.e. gravimetry, avionics, and space industry), to scientific investigations on measuring Casimir forces of magnetic origin and gravitational physics.

F. Schrodi, P. Silvi, F. Tschirsich, R. Fazio, S. Montangero Densityofstates of manybody quantum systems from tensor networks,
Phys. Rev. B 96 94303 (20170906),
http://dx.doi.org/10.1103/PhysRevB.96.094303 doi:10.1103/PhysRevB.96.094303 (ID: 719819)
Toggle Abstract
We present a technique to compute the microcanonical thermodynamical properties of a manybody quantum system using tensor networks. The Density Of States (DOS), and more general spectral properties, are evaluated by means of a HubbardStratonovich transformation performed on top of a realtime evolution, which is carried out via numerical methods based on tensor networks. As a consequence, the free energy and thermal averages can be also calculated. We test this approach on the onedimensional Ising and FermiHubbard models. Using matrix product states, we show that the thermodynamical quantities as a function of temperature are in very good agreement with the exact results. This approach can be extended to higherdimensional system by properly employing other types of tensor networks.
(local copy)

A. Mazloom Shahraki, B. Vermersch, M. Baranov, M. Dalmonte Adiabatic state preparation of stripe phases with strongly magnetic atoms,
Phys. Rev. A 96 33602 (20170901),
http://dx.doi.org/10.1103/PhysRevA.96.033602 doi:10.1103/PhysRevA.96.033602 (ID: 719755)
Toggle Abstract
We propose a protocol for realizing the stripe phase in two spin models on a twodimensional square lattice, which can be implemented with strongly magnetic atoms (Cr, Dy, Er, etc.) in optical lattices by encoding spin states into Zeeman sublevels of the ground state manifold. The protocol is tested with clustermeanfield timedependent variational ans\"atze, validated by comparison with exact results for small systems, which enable us to simulate the dynamics of systems with up to 64 sites during the statepreparation protocol. This allows, in particular, to estimate the time required for preparation of the stripe phase with high fidelity under real experimental conditions.
(local copy)

D. Zöpfl, C. Schneider, P. R. Muppalla, S. Kasemann, S. Partel, G. Kirchmair Characterization of low loss microstrip resonators as a building block for circuit QED in a 3D waveguide,
AIP ADV 7 (20170828),
http://dx.doi.org/10.1063/1.4992070 doi:10.1063/1.4992070 (ID: 719818)
Toggle Abstract
Here we present the microwave characterization of microstrip resonators made from aluminum and niobium inside a 3D microwave waveguide. In the low temperature, low power limit internal quality factors of up to one million were reached. We found a good agreement to models predicting conductive losses and losses to two level systems for increasing temperature. The setup presented here is appealing for testing materials and structures, as it is free of wire bonds and offers a well controlled microwave environment. In combination with transmon qubits, these resonators serve as a building block for a novel circuit QED architecture inside a rectangular waveguide.

P. Jurcevic, H. Shen, P. Hauke, C. Maier, T. Brydges, C. Hempel, B. P. Lanyon, M. Heyl, R. Blatt, C. F. Roos Direct observation of dynamical quantum phase transitions in an interacting manybody system,
Phys. Rev. Lett. 119 80501 (20170821),
http://dx.doi.org/10.1103/PhysRevLett.119.080501 doi:10.1103/PhysRevLett.119.080501 (ID: 719714)
Toggle Abstract
Dynamical quantum phase transitions (DQPTs) extend the concept of phase transitions and thus universality
to the nonequilibrium regime. In this letter, we investigate DQPTs in a string of ions simulating interacting transversefield Ising models. We observe nonequilibrium dynamics induced by a quantum quench and show for strings of up to 10 ions the direct detection of DQPTs by measuring a quantity that becomes nonanalytic in time in the thermodynamic limit. Moreover, we provide a link between DQPTs and the dynamics of other relevant quantities such as the magnetization, and we establish a connection between DQPTs and entanglement production.
(local copy)

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.
(local copy)

M. Zwerger, B. P. Lanyon, T. E. Northup, C. A. Muschik, W. Dür, N. Sangouard Quantum repeaters based on trapped ions with decoherencefree subspace encoding,
Quantum Sci. Technol. 2 44001 (20170804),
http://dx.doi.org/10.1088/20589565/aa7983 doi:10.1088/20589565/aa7983 (ID: 719938)
Toggle Abstract
Quantum repeaters provide an efficient solution to distribute Bell pairs over arbitrarily long distances. While scalable architectures are demanding regarding the number of qubits that need to be controlled, here we present a quantum repeater scheme aiming to extend the range of present day quantum communications that could be implemented in the near future with trapped ions in cavities. We focus on an architecture where ionphoton entangled states are created locally and subsequently processed with linear optics to create elementary links of ionion entangled states. These links are then used to distribute entangled pairs over long distances using successive entanglement swapping operations performed using deterministic ionion gates. We show how this architecture can be implemented while encoding the qubits in a decoherencefree subspace to protect them against collective dephasing. This results in a protocol that can be used to violate a Bell inequality over distances of about 800 km assuming stateoftheart parameters. We discuss how this could be improved to several thousand kilometres in future setups.

J. Cui, R. van Bijnen, T. Pohl, S. Montangero, T. Calarco Optimal control of Rydberg lattice gases,
Quantum Sci. Technol. 2 35006 (20170802),
http://dx.doi.org/10.1088/20589565/aa7daf doi:10.1088/20589565/aa7daf (ID: 719830)
Toggle Abstract
We present optimal control protocols to prepare different manybody quantum states of Rydberg atoms in optical lattices. Specifically, we show how to prepare highly ordered manybody ground states, GHZ states as well as some superposition of symmetric excitation number Fock states, that inherit the translational symmetry from the Hamiltonian, within sufficiently short excitation times minimizing detrimental decoherence effects. For the GHZ states, we propose a twostep detection protocol to experimentally verify the optimal preparation of the target state based only on standard measurement techniques. Realistic experimental constraints and imperfections are taken into account by our optimization procedure making it applicable to ongoing experiments.
(local copy)

P. Maurer, J. PratCamps, J. I. Cirac, T. W. Hänsch, O. RomeroIsart Ultrafocused Electromagnetic Field Pulses with a Hollow Cylindrical Waveguide,
Phys. Rev. Lett. 119 43904 (20170726),
http://dx.doi.org/10.1103/PhysRevLett.119.043904 doi:10.1103/PhysRevLett.119.043904 (ID: 719792)
Toggle Abstract
We theoretically show that a dipole externally driven by a pulse with a lowerbounded temporal width, and placed inside a cylindrical hollow waveguide, can generate a train of arbitrarily short and focused electromagnetic pulses. The waveguide encloses vacuum with perfect electric conducting walls. A dipole driven by a single short pulse, which is properly engineered to exploit the linear spectral filtering of the cylindrical hollow waveguide, excites longitudinal waveguide modes that are coherently refocused at some particular instances of time, thereby producing arbitrarily short and focused electromagnetic pulses. We numerically show that such ultrafocused pulses persist outside the cylindrical waveguide at distances comparable to its radius.

G. Higgins, W. Liu, F. Pokorny, C. Zhang, F. Kress, C. Maier, J. Haag, Q. Bodart, I. Lesanovsky, M. Hennrich Single Strontium Rydberg Ion Confined in a Paul Trap,
Phys. Rev. X 7 21038 (20170726),
(ID: 720044)

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.
(local copy)

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.
(local copy)

F. Cosco, M. Borrelli, P. Silvi, S. Maniscalco, G. De Chiara Nonequilibrium quantum thermodynamics in Coulomb crystals,
Phys. Rev. A 95 63615 (20170620),
http://dx.doi.org/10.1103/PhysRevA.95.063615 doi:10.1103/PhysRevA.95.063615 (ID: 719820)
Toggle Abstract
We present an indepth study of the nonequilibrium statistics of the irreversible work produced during sudden
quenches in proximity to the structural linearzigzag transition of ion Coulomb crystals in 1+1 dimensions. By
employing both an analytical approach based on a harmonic expansion and numerical simulations, we show the
divergence of the average irreversible work in proximity to the transition.We show that the nonanalytic behavior
of the work fluctuations can be characterized in terms of the critical exponents of the quantum Ising chain. Due
to the technological advancements in trappedion experiments, our results can be readily verified.
(local copy)

C. F. Roos, A. Alberti, D. Meschede, P. Hauke, H. Häffner Revealing quantum statistics with a pair of distant atoms,
Phys. Rev. Lett. 119 160401 (20170615),
http://dx.doi.org/10.1103/PhysRevLett.119.160401 doi:10.1103/PhysRevLett.119.160401 (ID: 719812)
Toggle Abstract
Quantum statistics have a profound impact on the properties of systems composed of identical particles. In this Letter, we demonstrate that the quantum statistics of a pair of identical massive particles can be probed by a direct measurement of the exchange symmetry of their wave function even in conditions where the particles always remain spatially well separated and thus the exchange contribution to their interaction energy is negligible. We present two protocols revealing the bosonic or fermionic nature of a pair of particles and discuss possible implementations with a pair of trapped atoms or ions.

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.
(local copy)

C. A. Riofrio, D. Gross, S. Flammia, T. Monz, D. Nigg, R. Blatt, J. Eisert Experimental quantum compressed sensing for a sevenqubit system,
Nat. Commun. 8 15305 (20170517),
http://dx.doi.org/10.1038/ncomms1530 doi:10.1038/ncomms1530 (ID: 719934)
Toggle Abstract
Wellcontrolled quantum devices with their increasing system size face a new roadblock hindering further development of quantum technologies. The effort of quantum tomography—the reconstruction of states and processes of a quantum device—scales unfavourably: stateoftheart systems can no longer be characterized. Quantum compressed sensing mitigates this problem by reconstructing states from incomplete data. Here we present an experimental implementation of compressed tomography of a sevenqubit system—a topological colour code prepared in a trapped ion architecture. We are in the highly incomplete—127 Pauli basis measurement settings—and highly noisy—100 repetitions each—regime. Originally, compressed sensing was advocated for states with few nonzero eigenvalues. We argue that lowrank estimates are appropriate in general since statistical noise enables reliable reconstruction of only the leading eigenvectors. The remaining eigenvectors behave consistently with a randommatrix model that carries no information about the true state.

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.
(local copy)

P. Silvi, E. Rico Ortega, M. Dalmonte, F. Tschirsich, S. Montangero Finitedensity phase diagram of a (1+1)d nonabelian lattice gauge theory with tensor networks,
Quantum 1 9 (20170425),
http://dx.doi.org/10.22331/q201704259 doi:10.22331/q201704259 (ID: 719589)
Toggle Abstract
We investigate the finitedensity phase diagram of a nonabelian SU(2) lattice gauge theory, encoding YangMills microscopical dynamics, in (1+1)dimensions using tensor network methods. We numerically characterise the phase diagram as a function of the filling and of the matterfield coupling, individuating different phases, some of them appearing only at finite densities. At unit filling, we find a meson BCS liquid phase, which at strong coupling undergoes a phase transition to a charge density wave of singlesite (spin0) mesons via spontaneous chiral symmetry breaking. At finite densities, the chiral symmetry is restored almost everywhere, and the meson BCS liquid becomes a simple liquid at strong couplings, with the exception of filling twothirds, where a charge density wave of mesons spreading over neighbouring sites appears. Finally, we individuate two tricritical points between the chiral and the two liquid phases which are compatible with a SU(2) 2 WessZuminoNovikovWitten model.
(local copy)

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.
(local copy)

T. Weiß, S. Walter, F. Marquardt Quantumcoherent phase oscillations in synchronization,
Phys. Rev. A 95 41802 (20170411),
http://dx.doi.org/10.1103/PhysRevA.95.041802 doi:10.1103/PhysRevA.95.041802 (ID: 720023)

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.
(local copy)

C. Navau, R. MachBatlle, A. Parra, J. PratCamps, S. Laut, N. DelValle, À. Sánchez Enhancing the sensitivity of magnetic sensors by 3D metamaterial shells,
Sci. Rep. 7 44762 (20170317),
http://dx.doi.org/10.1038/srep44762 doi:10.1038/srep44762 (ID: 719778)
Toggle Abstract
Magnetic sensors are key elements in our interconnected smart society. Their sensitivity becomes essential for many applications in fields such as biomedicine, computer memories, geophysics, or space exploration. Here we present a universal way of increasing the sensitivity of magnetic sensors by surrounding them with a spherical metamaterial shell with specially designed anisotropic magnetic properties. We analytically demonstrate that the magnetic field in the sensing area is enhanced by our metamaterial shell by a known factor that depends on the shell radii ratio. When the applied field is nonuniform, as for dipolar magnetic field sources, field gradient is increased as well. A proofofconcept experimental realization confirms the theoretical predictions. The metamaterial shell is also shown to concentrate timedependent magnetic fields upto frequencies of 100 kHz.

S. Whitlock, A. Glätzle, P. Hannaford Simulating Quantum Spin Models using RydbergExcited Atomic Ensembles in Magnetic Microtrap Arrays,
J. Phys. B: At. Mol. Opt. Phys. 74001 (20170310),
http://dx.doi.org/10.1088/13616455/aa6149 doi:10.1088/13616455/aa6149 (ID: 719633)
Toggle Abstract
We propose a scheme to simulate lattice spin models based on strong and longrange interacting Rydberg atoms stored in a largespacing array of magnetic microtraps. Each spin is encoded in a collective spin state involving a single nP Rydberg atom excited from an ensemble of groundstate alkali atoms prepared via Rydberg blockade. After the excitation laser is switched off the Rydberg spin states on neighbouring lattice sites interact via general isotropic or anisotropic spinspin interactions. To read out the collective spin states we propose a single Rydberg atom triggered avalanche scheme in which the presence of a single Rydberg atom conditionally transfers a large number of groundstate atoms in the trap to an untrapped state which can be readily detected by siteresolved absorption imaging. Such a quantum simulator should allow the study of quantum spin systems in almost arbitrary twodimensional configurations. This paves the way towards engineering exotic spin models, such as spin models based on triangularsymmetry lattices which can give rise to frustratedspin magnetism.

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.
(local copy)

L. Reichsöllner, A. Schindewolf, T. Takekoshi, R. Grimm, H. Nägerl Quantum engineering of a lowentropy gas of heteronuclear bosonic molecules in an optical lattice,
Phys. Rev. Lett. 118 73201 (20170217),
http://dx.doi.org/10.1103/PhysRevLett.118.073201 doi:10.1103/PhysRevLett.118.073201 (ID: 719625)
Toggle Abstract
We demonstrate a generally applicable technique for mixing twospecies quantum degenerate bosonic samples in the presence of an optical lattice, and we employ it to produce lowentropy samples of ultracold Rb87Cs133 Feshbach molecules with a lattice filling fraction exceeding 30%. Starting from two spatially separated BoseEinstein condensates of Rb and Cs atoms, RbCs atom pairs are efficiently produced by using the superfluidtoMott insulator quantum phase transition twice, first for the Cs sample, then for the Rb sample, after nulling the RbCs interaction at a Feshbach resonance’s zero crossing. We form molecules out of atom pairs and characterize the mixing process in terms of sample overlap and mixing speed. The dense and ultracold sample of more than 5000 RbCs molecules is an ideal starting point for experiments in the context of quantum manybody physics with longrange dipolar interactions.

R. Lous, I. Fritsche, M. Jag, B. Huang, R. Grimm Thermometry of a deeply degenerate Fermi gas with a BoseEinstein condensate,
Phys. Rev. A 95 53627 (20170209),
http://dx.doi.org/10.1103/PhysRevA.95.053627 doi:10.1103/PhysRevA.95.053627 (ID: 719754)
Toggle Abstract
We measure the temperature of a deeply degenerate Fermi gas, by using a weakly interacting sample of heavier bosonic atoms as a probe. This thermometry method relies on the thermalization between the two species and on the determination of the condensate fraction of the bosons. In our experimental implementation, a small sample of 41K atoms serves as the thermometer for a 6Li Fermi sea. We investigate the evaporative cooling of a 6Li spin mixture in a singlebeam optical dipole trap and observe how the condensate fraction of the thermometry atoms depends on the final trap depth. From the condensate fraction, the temperature can be readily extracted. We show that the lowest temperature of 5.9(5)% of the Fermi temperature is obtained, when the decreasing trap depth closely approaches the Fermi energy. To understand the systematic effects that may in uence the results, we carefully investigate the role of the number of bosons and the thermalization dynamics between the two species. Our thermometry approach provides a conceptually simple, accurate, and general way to measure the temperature of deeply degenerate Fermi gases. Since the method is independent of the specific interaction conditions within the Fermi gas, it applies to both weakly and strongly interacting Fermi gases.

R. DiazMendez, F. Mezzacapo, W. Lechner, F. Cinti, E. Babaev, G. Pupillo Glass Transitions in Monodisperse ClusterForming Ensembles: Vortex Matter in Type1.5 Superconductors,
Phys. Rev. Lett. 118 67001 (20170208),
http://dx.doi.org/10.1103/PhysRevLett.118.067001 doi:10.1103/PhysRevLett.118.067001 (ID: 719757)
Toggle Abstract
At low enough temperatures and high densities, the equilibrium configuration of an ensemble of ultrasoft particles is a selfassembled, ordered, cluster crystal. In the present Letter, we explore the outofequilibrium dynamics for a twodimensional realization, which is relevant to superconducting materials with multiscale intervortex forces. We find that, for small temperatures following a quench, the suppression of the thermally activated particle hopping hinders the ordering. This results in a glass transition for a monodispersed ensemble, for which we derive a microscopic explanation in terms of an “effective polydispersity” induced by multiscale interactions. This demonstrates that a vortex glass can form in clean systems of thin films of “type1.5” superconductors. An additional setup to study this physics can be layered superconducting systems, where the shape of the effective vortexvortex interactions can be engineered.

G. Watanabe, B. Venkatesh, P. Talkner, A. del Campo Quantum Performance of Thermal Machines over Many Cycles,
Phys. Rev. Lett. 118 50601 (20170202),
http://dx.doi.org/10.1103/PhysRevLett.118.050601 doi:10.1103/PhysRevLett.118.050601 (ID: 719722)
Toggle Abstract
The performance of quantum heat engines is generally based on the analysis of a single cycle. We challenge this approach by showing that the total work performed by a quantum engine need not be proportional to the number of cycles. Furthermore, optimizing the engine over multiple cycles leads to the identification of scenarios with a quantum enhancement. We demonstrate our findings with a quantum Otto engine based on a twolevel system as the working substance that supplies power to an external oscillator.

J. L. Ville, T. Bienaime, R. SaintJalm, L. Corman, M. Aidelsburger, L. Chomaz, K. Kleinlein, D. Perconte, S. Nascimbene, J. Dalibard, J. Beugnon Loading and compression of a single twodimensional Bose gas in an optical accordion,
Phys. Rev. A 95 13632 (20170127),
http://dx.doi.org/10.1103/PhysRevA.95.013632 doi:10.1103/PhysRevA.95.013632 (ID: 719968)
Toggle Abstract
The experimental realization of twodimensional (2D) Bose gases with a tunable interaction strength is an important challenge for the study of ultracold quantum matter. Here we report on the realization of an optical accordion creating a lattice potential with a spacing that can be dynamically tuned between 11 and 2 μm. We show that we can load ultracold 87Rb atoms into a single node of this optical lattice in the large spacing configuration and then decrease nearly adiabatically the spacing to reach a strong harmonic confinement with frequencies larger than ωz/2π=10 kHz. Atoms are trapped in an additional flatbottom inplane potential that is shaped with a high resolution. By combining these tools we create customshaped uniform 2D Bose gases with tunable confinement along the transverse direction and hence with a tunable interaction strength.

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.
(local copy)

M. Jag, M. Cetina, R. Lous, R. Grimm, J. Levinsen, D. Petrov Lifetime of Feshbach dimers in a FermiFermi mixture of 6Li and 40K,
Phys. Rev. A 94 62706 (20161227),
http://dx.doi.org/10.1103/PhysRevA.94.062706 doi:10.1103/PhysRevA.94.062706 (ID: 719645)
Toggle Abstract
We present a joint experimental and theoretical investigation of the lifetime of weakly bound dimers formed near narrow interspecies Feshbach resonances in massimbalanced FermiFermi systems, considering the specific example of a mixture of Li6 and K40 atoms. Our work addresses the central question of the increase in the stability of the dimers resulting from Pauli suppression of collisional losses, which is a wellknown effect in massbalanced fermionic systems near broad resonances. We present measurements of the spontaneous dissociation of dimers in dilute samples, and of the collisional losses in dense samples arising from both dimerdimer processes and from atomdimer processes. We find that all loss processes are suppressed close to the Feshbach resonance. Our general theoretical approach for fermionic mixtures near narrow Feshbach resonances provides predictions for the suppression of collisional decay as a function of the detuning from resonance, and we find excellent agreement with the experimental benchmarks provided by our Li6−K40 system. We finally present model calculations for other Feshbachresonant FermiFermi systems, which are of interest for experiments in the near future.

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.

C. GonzalezBallestero, E. Moreno, F. J. GarcíaVidal, A. GonzalezTudela Nonreciprocal fewphoton routing schemes based on chiral waveguideemitter couplings,
Phys. Rev. A 94 63817 (20161206),
http://dx.doi.org/10.1103/PhysRevA.94.063817 doi:10.1103/PhysRevA.94.063817 (ID: 719899)
Toggle Abstract
We demonstrate the possibility of designing efficient, nonreciprocal fewphoton devices by exploiting the chiral coupling between two waveguide modes and a single quantum emitter. We show how this system can show nonreciprocal photon transport at the singlephoton level by exploiting a singlephoton routing mechanism. Afterwards, we also demonstrate how the fundamentally different twophoton response makes the system show a transistorlike behavior, where a first photon can open a transmission channel for a second incoming photon. The efficiency in both cases is shown to be large for feasible experimental implementations. Our results illustrate the potential of chiral waveguideemitter couplings for applications in quantum circuitry.

M. Łącki, B. Damski, J. Zakrzewski Locating the quantum critical point of the BoseHubbard model through singularities of simple observables,
Sci. Rep. 6 38340 (20161202),
http://dx.doi.org/10.1038/srep38340 doi:10.1038/srep38340 (ID: 719701)
Toggle Abstract
We show that the critical point of the twodimensional BoseHubbard model can be easily found through studies of either onsite atom number fluctuations or the nearestneighbor twopoint correlation function (the expectation value of the tunnelling operator). Our strategy to locate the critical point is based on the observation that the derivatives of these observables with respect to the parameter that drives the superfluidMott insulator transition are singular at the critical point in the thermodynamic limit. Performing the quantum Monte Carlo simulations of the twodimensional BoseHubbard model, we show that this technique leads to the accurate determination of the position of its critical point. Our results can be easily extended to the threedimensional BoseHubbard model and different Hubbardlike models. They provide a simple experimentallyrelevant way of locating critical points in various cold atomic lattice systems.
(local copy)

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.

L. Chen, K. Qu, H. Shen, W. Zhang, K. Chou, Q. Liu, T. Yan, B. Wang, S. Wang Inline polarization rotator based on the quantumoptical analogy,
Opt. Lett. 41 2113 (20161126),
http://dx.doi.org/10.1364/OL.41.002113 doi:10.1364/OL.41.002113 (ID: 719736)

L. Chomaz, S. Baier, D. Petter, M. J. Mark, F. Wächtler, L. Santos, F. Ferlaino QuantumFluctuationDriven Crossover from a Dilute BoseEinstein Condensate to a Macrodroplet in a Dipolar Quantum Fluid,
Phys. Rev. X 6 41039 (20161122),
http://dx.doi.org/10.1103/PhysRevX.6.041039 doi:10.1103/PhysRevX.6.041039 (ID: 719687)
Toggle Abstract
In a joint experimental and theoretical effort, we report on the formation of a macrodroplet state in an ultracold bosonic gas of erbium atoms with strong dipolar interactions. By precise tuning of the s wave scattering length below the socalled dipolar length, we observe a smooth crossover of the ground state from a dilute BoseEinstein condensate to a dense macrodroplet state of more than 2×10 4 atoms . Based on the study of collective excitations and loss features, we prove that quantum fluctuations stabilize the ultracold gas far beyond the instability threshold imposed by meanfield interactions. Finally, we perform expansion measurements, showing that although selfbound solutions are prevented by losses, the interplay between quantum stabilization and losses results in a minimal timeofflight expansion velocity at a finite scattering length.

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.

A. Wade, M. Mattioli, K. Moelmer Singleatom singlephoton coupling facilitated by atomic ensemble dark state mechanisms,
Phys. Rev. A 53830 (20161116),
http://dx.doi.org/10.1103/PhysRevA.94.053830 doi:10.1103/PhysRevA.94.053830 (ID: 719564)
Toggle Abstract
We propose to couple single atomic qubits to photons incident on a cavity containing an atomic ensemble of a different species that mediates the coupling via Rydberg interactions. Subject to a classical field and the cavity field, the ensemble forms a collective dark state which is resonant with the input photon, while excitation of a qubit atom leads to a secondary "dark" state that splits the cavity resonance. The two different dark state mechanisms yield zero and π reflection phase shifts and can be used to implement quantum gates between atomic and optical qubits.

J. Wallnöfer, M. Zwerger, C. A. Muschik, N. Sangouard, W. Dür Twodimensional quantum repeaters,
Phys. Rev. A 94 52307 (20161107),
http://dx.doi.org/10.1103/PhysRevA.94.052307 doi:10.1103/PhysRevA.94.052307 (ID: 719561)
Toggle Abstract
The endeavour to develop quantum networks gave rise to a rapidly developing field with far reaching applications such as secure communication and the realisation of distributed computing tasks. This ultimately calls for the creation of flexible multiuser structures that allow for quantum communication between arbitrary pairs of parties in the network and facilitate also multiuser applications. To address this challenge, we propose a 2D quantum repeater architecture to establish longdistance entanglement shared between multiple communication partners in the presence of channel noise and imperfect local control operations. The scheme is based on the creation of selfsimilar multiqubit entanglement structures at growing scale, where variants of entanglement swapping and multiparty entanglement purification are combined to create high fidelity entangled states. We show how such networks can be implemented using trapped ions in cavities.

A. Bayerle, S. Tzanova, P. Vlaar, B. Pasquiou, F. Schreck Tapered amplifier laser with frequencyshifted feedback,
SciPost Phys. 1 2 (20161022),
http://dx.doi.org/10.21468/SciPostPhys.1.1.002 doi:10.21468/SciPostPhys.1.1.002 (ID: 719581)
Toggle Abstract
We present a frequencyshifted feedback (FSF) laser based on a tapered amplifier. The laser
operates as a coherent broadband source with up to 370 GHz spectral width and 2.3 mus coherence time. If the FSF laser is seeded by a continuouswave laser a frequency comb spanning the output spectrum appears in addition to the broadband emission. The laser has an output power of 280 mW and a center wavelength of 780 nm. The ease and exibility of use of tapered ampli ers makes our FSF laser attractive for a wide range of applications, especially in metrology.

M. Cetina, M. Jag, R. Lous, I. Fritsche, J. T. Walraven, R. Grimm, J. Levinsen, M. Parish, R. Schmidt, M. Knap, E. Demler Ultrafast manybody interferometry of impurities coupled to a Fermi sea,
Science 354 99 (20161007),
http://dx.doi.org/10.1126/science.aaf5134 doi:10.1126/science.aaf5134 (ID: 719567)
Toggle Abstract
The fastest possible collective response of a quantum manybody system is related to its excitations at the highest possible energy. In condensed matter systems, the time scale for such “ultrafast” processes is typically set by the Fermi energy. Taking advantage of fast and precise control of interactions between ultracold atoms, we observed nonequilibrium dynamics of impurities coupled to an atomic Fermi sea. Our interferometric measurements track the nonperturbative quantum evolution of a fermionic manybody system, revealing in real time the formation dynamics of quasiparticles and the quantum interference between attractive and repulsive states throughout the full depth of the Fermi sea. Ultrafast timedomain methods applied to strongly interacting quantum gases enable the study of the dynamics of quantum matter under extreme nonequilibrium conditions.

C. GonzalezBallestero, J. Feist, E. GonzaloBadia, E. Moreno, F. J. GarcíaVidal Uncoupled Dark States Can Inherit Polaritonic Properties,
Phys. Rev. Lett. 117 156402 (20161007),
http://dx.doi.org/10.1103/PhysRevLett.117.156402 doi:10.1103/PhysRevLett.117.156402 (ID: 719898)
Toggle Abstract
When a collection of quantum emitters interacts with an electromagnetic field, the whole system can enter into the collective strong coupling regime in which hybrid lightmatter states, i.e., polaritons can be created. Only a small portion of excitations in the emitters are coupled to the light field, and there are many dark states that, in principle, retain their pure excitonic nature. Here we theoretically demonstrate that these dark states can have a delocalized character, which is inherent to polaritons, despite the fact that they do not have a photonic component. This unexpected behavior only appears when the electromagnetic field displays a discrete spectrum. In this case, when the main loss mechanism in the hybrid system stems from the radiative losses of the light field, dark states are even more efficient than polaritons in transferring excitations across the structure.

H. Keßle, J. Klinder, B. Venkatesh, C. Georges, A. Hemmerich In situ observation of optomechanical Bloch oscillations in an optical cavity,
New J. Phys. 18 102001 (20161003),
http://dx.doi.org/10.1088/13672630/18/10/102001 doi:10.1088/13672630/18/10/102001 (ID: 719662)
Toggle Abstract
It is shown experimentally that a Bose–Einstein condensate inside an optical cavity, operating in the regime of strong cooperative coupling, responds to an external force by an optomechanical Bloch oscillation, which can be directly observed in the light leaking out of the cavity. Previous theoretical work predicts that the frequency of this oscillation matches with that of conventional Bloch oscillations such that its in situ monitoring may help to increase the data acquisition speed in precision force measurements.

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.
(local copy)

Y. Lin, J. P. Gaebler, F. Reiter, T. R. Tan, R. Bowler, Y. Wang, A. Keith, E. Knill, S. Glancy, K. Coakely, A. Sorensen, D. Leibfried, D. Wineland Preparation of entangled states through Hilbert space engineering,
Phys. Rev. Lett. 140502 (20160916),
http://dx.doi.org/10.1103/PhysRevLett.117.140502 doi:10.1103/PhysRevLett.117.140502 (ID: 719533)

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.
(local copy)

P. Maurer, J. I. Cirac, O. RomeroIsart Ultrashort Pulses for FarField Nanoscopy,
Phys. Rev. Lett. 117 103602 (20160829),
http://dx.doi.org/10.1103/PhysRevLett.117.103602 doi:10.1103/PhysRevLett.117.103602 (ID: 719476)
Toggle Abstract
The Abbe diffraction limit prevents focusing monochromatic light in the farfield beyond a spot size half its wavelength. For microscopy purposes at the nanoscale, namely nanoscopy, such limit can be circumvented by either using nearfields, which are not diffractionlimited, or, in fluorescence nanoscopy, by manipulating bright and dark states of the fluorescent markers. Here we propose and analyze an alternative approach for farfield nanoscopy based on using coherent polychromatic light, that is, ultrashort pulses. Such pulses have spectral bandwidths comparable, and even larger in the attosecond regime, than a carrier optical frequency. We show that a train of ultrashort pulses can be used to excite markers with nanoscale resolution. In particular, we show that they can be focused to a spot size given by the wavelength associated to its spectral bandwidth and that they can excite a twolevel marker with an optical transition. The excitation mechanism is nonconventional for twolevel systems, as it relies on the existence of processes where an excitation is created together with the emission of a photon. The detection of the light emitted after fluorescence, or any other method used to detect the excitation, would thus lead to farfield nanoscopy. In this sense, our results open the door to design fluorescence nanoscopes that circumvent the Abbe's barrier without manipulating the states of the markers but using coherent polychromatic light.

M. L. Juan, G. MolinaTerriza, T. Volz, O. RomeroIsart Nearfield levitated quantum optomechanics with nanodiamonds ,
Phys. Rev. A 94 23841 (20160824),
http://dx.doi.org/10.1103/PhysRevA.94.023841 doi:10.1103/PhysRevA.94.023841 (ID: 719247)
Toggle Abstract
We theoretically show that the dipole force of an ensemble of quantum emitters embedded in a dielectric nanosphere can be exploited to achieve nearfield optical levitation. The key ingredient is that the polarizability from the ensemble of embedded quantum emitters can be larger than the bulk polarizability of the sphere, thereby enabling the use of repulsive optical potentials and consequently the levitation using optical near fields. In levitated cavity quantum optomechanics, this could be used to boost the singlephoton coupling by combining larger polarizability to mass ratio, larger field gradients, and smaller cavity volumes while remaining in the resolved sideband regime and at room temperature. A case study is done with a nanodiamond containing a high density of siliconvacancy color centers that is optically levitated in the evanescent field of a tapered nanofiber and coupled to a highfinesse microsphere cavity.

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.

T. Secker, R. Gerritsma, A. Glätzle, A. Negretti Controlled longrange interactions between Rydberg atoms and ions,
Phys. Rev. A 94 13420 (20160727),
http://dx.doi.org/10.1103/PhysRevA.94.013420 doi:10.1103/PhysRevA.94.013420 (ID: 719618)
Toggle Abstract
We theoretically investigate trapped ions interacting with atoms that are coupled to Rydberg states. The strong polarizabilities of the Rydberg levels increase the interaction strength between atoms and ions by many orders of magnitude, as compared to the case of groundstate atoms, and may be mediated over micrometers. We calculate that such interactions can be used to generate entanglement between an atom and the motion or internal state of an ion. Furthermore, the ion could be used as a bus for mediating spinspin interactions between atomic spins in analogy to much employed techniques in iontrap quantum simulation. The proposed scheme comes with attractive features as it maps the benefits of the trappedion quantum system onto the atomic one without obviously impeding its intrinsic scalability. No groundstate cooling of the ion or atom is required and the setup allows for full dynamical control. Moreover, the scheme is to a large extent immune to the micromotion of the ion. Our findings are of interest for developing hybrid quantum information platforms and for implementing quantum simulations of solidstate physics.

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.
(local copy)

R. Dasgupta, B. Venkatesh, G. Watanabe Attractioninduced dynamical stability of a BoseEinstein condensate in a nonlinear lattice,
Phys. Rev. A 93 63618 (20160615),
http://dx.doi.org/10.1103/PhysRevA.93.063618 doi:10.1103/PhysRevA.93.063618 (ID: 719540)
Toggle Abstract
We study multipleperiod Bloch states of a BoseEinstein condensate with spatially periodic interatomic interaction. Solving the GrossPitaevskii equation for the continuum model, and also using a simplified discrete version of it, we investigate the energyband structures and the corresponding stability properties. We observe an “attractioninduced dynamical stability” mechanism caused by the localization of the density distribution in the attractive domains of the system and the isolation of these higherdensity regions. This makes the superfluid stable near the zone boundary and also enhances the stability of higherperiodic states if the nonlinear interaction strength is sufficiently high.

D. Hoang, B. Venkatesh, S. Han, J. Jo, G. Watanabe, M. Choi Scaling Law for Irreversible Entropy Production in Critical Systems,
Sci. Rep. 6 27603 (20160609),
http://dx.doi.org/10.1038/srep27603 doi:10.1038/srep27603 (ID: 719588)
Toggle Abstract
We examine the Jarzynski equality for a quenching process across the critical point of secondorder phase transitions, where absolute irreversibility and the effect of finitesampling of the initial equilibrium distribution arise in a single setup with equal significance. We consider the Ising model as a prototypical example for spontaneous symmetry breaking and take into account the finite sampling issue by introducing a tolerance parameter. The initially ordered spins become disordered by quenching the ferromagnetic coupling constant. For a sudden quench, the deviation from the Jarzynski equality evaluated from the ideal ensemble average could, in principle, depend on the reduced coupling constant ε0 of the initial state and the system size L. We find that, instead of depending on ε0 and L separately, this deviation exhibits a scaling behavior through a universal combination of ε0 and L for a given tolerance parameter, inherited from the critical scaling laws of secondorder phase transitions. A similar scaling law can be obtained for the finitespeed quench as well within the KibbleZurek mechanism.

A. Amaricci, J. Budich, M. Capone, B. Trauzettel, G. Sangiovanni Strong correlation effects on topological quantum phase transitions in three dimensions,
Phys. Rev. B 93 235112 (20160607),
http://dx.doi.org/10.1103/PhysRevB.93.235112 doi:10.1103/PhysRevB.93.235112 (ID: 719601)
Toggle Abstract
We investigate the role of shortranged electronelectron interactions in a paradigmatic model of threedimensional topological insulators, using dynamical meanfield theory and focusing on nonmagnetically ordered solutions. The noninteracting band structure is controlled by a mass term M , whose value discriminates between three different insulating phases, a trivial band insulator and two distinct topologically nontrivial phases. We characterize the evolution of the transitions between the different phases as a function of the local Coulomb repulsion U and find a remarkable dependence of the U−M phase diagram on the value of the local Hund's exchange coupling J . However, regardless of the value of J , following the evolution of the topological transition line between a trivial band insulator and a topological insulator, we find a critical value of U separating a continuous transition from a firstorder one. When the Hund's coupling is significant, a Mott insulator is stabilized at large U . In proximity of the Mott transition we observe the emergence of an anomalous “Mottlike” strong topological insulator state.

J. Smith, A. Lee, P. Richerme, B. Neyenhuis, P. W. Hess, P. Hauke, M. Heyl, D. A. Huse, C. Monroe Manybody localization in a quantum simulator with programmable random disorder,
Nature Phys. 12 911 (20160606),
http://dx.doi.org/10.1038/NPHYS3783 doi:10.1038/NPHYS3783 (ID: 719341)
Toggle Abstract
When a system thermalizes it loses all local memory of its initial conditions. This is a general feature of open systems and is well described by equilibrium statistical mechanics. Even within a closed (or reversible) quantum system, where unitary time evolution retains all information about its initial state, subsystems can still thermalize using the rest of the system as an effective heat bath. Exceptions to quantum thermalization have been predicted and observed, but typically require inherent symmetries or noninteracting particles in the presence of static disorder. The prediction of manybody localization (MBL), in which disordered quantum systems can fail to thermalize in spite of strong interactions and high excitation energy, was therefore surprising and has attracted considerable theoretical attention. Here we experimentally generate MBL states by applying an Ising Hamiltonian with longrange interactions and programmably random disorder to ten spins initialized far from equilibrium. We observe the essential signatures of MBL: memory retention of the initial state, a Poissonian distribution of energy level spacings, and entanglement growth in the system at long times. Our platform can be scaled to higher numbers of spins, where detailed modeling of MBL becomes impossible due to the complexity of representing such entangled quantum states. Moreover, the high degree of control in our experiment may guide the use of MBL states as potential quantum memories in naturally disordered quantum systems.
(local copy)

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.
(local copy)

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.

E. P. van Loon, M. Katsnelson, L. Chomaz, M. Lemeshko Interactiondriven Lifshitz transition with dipolar fermions in optical lattices,
Phys. Rev. B 93 195145 (20160523),
http://dx.doi.org/10.1103/PhysRevB.93.195145 doi:10.1103/PhysRevB.93.195145 (ID: 719620)
Toggle Abstract
Anisotropic dipoledipole interactions between ultracold dipolar fermions break the symmetry of the Fermi
surface and thereby deform it. Here we demonstrate that such a Fermi surface deformation induces a topological
phase transition—the socalled Lifshitz transition—in the regime accessible to presentday experiments. We
describe the impact of the Lifshitz transition on observable quantities such as the Fermi surface topology, the
densitydensity correlation function, and the excitation spectrum of the system. The Lifshitz transition in ultracold
atoms can be controlled by tuning the dipole orientation and, in contrast to the transition studied in crystalline
solids, is completely interaction driven.

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.
(local copy)

J. PratCamps, C. Navau, À. Sánchez Quasistatic Metamaterials: Magnetic Coupling Enhancement by Effective Space Cancellation,
Adv. Mater. 28 4898 (20160427),
http://dx.doi.org/10.1002/adma.201506376 doi:10.1002/adma.201506376 (ID: 719584)
Toggle Abstract
A novel and broadly applicable way to increase magnetic coupling between distant circuits in the quasistatic regime is introduced. It is shown how the use of magnetic metamaterials enhances the magnetic coupling between emitting and receiving coils. Results are experimentally demonstrated by measuring a boost on the efficiency of the wireless transmission of power between distant circuits.

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.
(local copy)

N. Trautmann, P. Hauke Quantum simulation of the dynamical Casimir effect with trapped ions,
New J. Phys. 18 43029 (20160419),
http://dx.doi.org/10.1088/13672630/18/4/043029 doi:10.1088/13672630/18/4/043029 (ID: 719415)
Toggle Abstract
Quantum vacuum fluctuations are a direct manifestation of Heisenberg's uncertainty principle. The dynamical Casimir effect allows for the observation of these vacuum fluctuations by turning them into real, observable photons. However, the observation of this effect in a cavity QED experiment would require the rapid variation of the length of a cavity with relativistic velocities, a daunting challenge. Here, we propose a quantum simulation of the dynamical Casimir effect using an ion chain confined in a segmented ion trap. We derive a discrete model that enables us to map the dynamics of the multimode radiation field inside a variablelength cavity to radial phonons of the ion crystal. We perform a numerical study comparing the ionchain quantum simulation under realistic experimental parameters to an ideal FabryPerot cavity, demonstrating the viability of the mapping. The proposed quantum simulator, therefore, allows for probing the photon (respectively phonon) production caused by the dynamical Casimir effect on the single photon level.
(local copy)

F. Meinert, M. J. Mark, K. Lauber, A. J. Daley, H. Nägerl Floquet engineering of correlated tunneling in the BoseHubbard model with ultracold atoms,
Phys. Rev. Lett. 116 205301 (20160417),
http://dx.doi.org/10.1103/PhysRevLett.116.205301 doi:10.1103/PhysRevLett.116.205301 (ID: 719499)
Toggle Abstract
We report on the experimental implementation of tunable occupationdependent tunneling in a BoseHubbard system of ultracold atoms via timeperiodic modulation of the onsite interaction energy. The tunneling rate is inferred from a timeresolved measurement of the lattice site occupation after a quantum quench. We demonstrate coherent control of the tunneling dynamics in the correlated manybody system, including full suppression of tunneling as predicted within the framework of Floquet theory. We find that the tunneling rate explicitly depends on the atom number difference in neighboring lattice sites. Our results may open up ways to realize artificial gauge fields that feature density dependence with ultracold atoms.

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.
(local copy)

G. Watanabe, B. Venkatesh, R. Dasgupta Nonlinear Phenomena of Ultracold Atomic Gases in Optical Lattices: Emergence of Novel Features in Extended States,
Entropy 18 118 (20160331),
http://dx.doi.org/10.3390/e18040118 doi:10.3390/e18040118 (ID: 719541)
Toggle Abstract
The system of a cold atomic gas in an optical lattice is governed by two factors: nonlinearity originating from the interparticle interaction, and the periodicity of the system set by the lattice. The high level of controllability associated with such an arrangement allows for the study of the competition and interplay between these two, and gives rise to a whole range of interesting and rich nonlinear effects. This review covers the basic idea and overview of such nonlinear phenomena, especially those corresponding to extended states. This includes “swallowtail” loop structures of the energy band, Bloch states with multiple periodicity, and those in “nonlinear lattices”, i.e., systems with the nonlinear interaction term itself being a periodic function in space.

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.
(local copy)

R. Lechner, C. Maier, C. Hempel, P. Jurcevic, B. P. Lanyon, T. Monz, M. Brownnutt, R. Blatt, C. F. Roos Electromagneticallyinducedtransparency groundstate cooling of long ion strings,
Phys. Rev. A 93 53401 (20160318),
http://dx.doi.org/10.1103/PhysRevA.93.053401 doi:10.1103/PhysRevA.93.053401 (ID: 719536)
Toggle Abstract
Electromagneticallyinducedtransparency (EIT) cooling is a groundstate cooling technique for trapped particles. EIT offers a broader cooling range in frequency space compared to more established methods. In this work, we experimentally investigate EIT cooling in strings of trapped atomic ions. In strings of up to 18 ions, we demonstrate simultaneous groundstate cooling of all radial modes in under 1 ms. This is a particularly important capability in view of emerging quantum simulation experiments with large numbers of trapped ions. Our analysis of the EIT cooling dynamics is based on a technique enabling singleshot measurements of phonon numbers, by rapid adiabatic passage on a vibrational sideband of a narrow transition.

D. Nigg, T. Monz, P. Schindler, E. A. Martínez, M. Hennrich, R. Blatt, M. F. Pusey, T. Rudolph, J. Barrett Can different quantum state vectors correspond to the same physical state? An experimental test,
New J. Phys. 18 13007 (20160304),
http://dx.doi.org/10.1088/13672630/18/1/013007 doi:10.1088/13672630/18/1/013007 (ID: 719518)
Toggle Abstract
A century after the development of quantum theory, the interpretation of a quantum state is still discussed. If a physicist claims to have produced a system with a particular quantum state vector, does this represent directly a physical property of the system, or is the state vector merely a summary of the physicist's information about the system? Assume that a state vector corresponds to a probability distribution over possible values of an unknown physical or 'ontic' state. Then, a recent nogo theorem shows that distinct state vectors with overlapping distributions lead to predictions different from quantum theory. We report an experimental test of these predictions using trapped ions. Within experimental error, the results confirm quantum theory. We analyse which kinds of models are ruled out.

M. Kumph, c. Henkel, P. Rabl, M. Brownnutt, R. Blatt Electricfield noise above a thin dielectric layer on metal electrodes,
New J. Phys. 18 23020 (20160304),
http://dx.doi.org/10.1088/13672630/18/2/023020 doi:10.1088/13672630/18/2/023020 (ID: 719519)
Toggle Abstract
The electricfield noise above a layered structure composed of a planar metal electrode covered by a thin dielectric is evaluated and it is found that the dielectric film considerably increases the noise level, in proportion to its thickness. Importantly, even a thin (mono) layer of a lowloss dielectric can enhance the noise level by several orders of magnitude compared to the noise above a bare metal. Close to this layered surface, the power spectral density of the electric field varies with the inverse fourth power of the distance to the surface, rather than with the inverse square, as it would above a bare metal surface. Furthermore, compared to a clean metal, where the noise spectrum does not vary with frequency (in the radiowave and microwave bands), the dielectric layer can generate electricfield noise which scales in inverse proportion to the frequency. For various realistic scenarios, the noise levels predicted from this model are comparable to those observed in trappedion experiments. Thus, these findings are of particular importance for the understanding and mitigation of unwanted heating and decoherence in miniaturized ion traps.

M. Kumph, P. Holz, K. Langer, M. R. Meraner, M. Niedermayr, M. Brownnutt, R. Blatt Operation of a planarelectrode iontrap array with adjustable RF electrodes,
New J. Phys. 18 23047 (20160304),
http://dx.doi.org/10.1088/13672630/18/2/023047 doi:10.1088/13672630/18/2/023047 (ID: 719520)
Toggle Abstract
One path to realizing systems of trapped atomic ions suitable for largescale quantum computing and simulation is to create a twodimensional (2D) array of ion traps. Interactions between nearestneighbouring ions could then be turned on and off by tuning the ions' relative positions and frequencies. We demonstrate and characterize the operation of a planarelectrode iontrap array. By driving the trap with a network of phaselocked radiofrequency resonators which provide independently variable voltage amplitudes we vary the position and motional frequency of a Ca+ ion in twodimensions within the trap array. Work on fabricating a miniaturised form of this 2D trap array is also described, which could ultimately provide a viable architecture for largescale quantum simulations.

T. Monz, D. Nigg, E. A. Martínez, M. F. Brandl, P. Schindler, R. Rines, S. Wang, I. L. Chuang, R. Blatt Realization of a scalable Shor algorithm,
Science 351 1070 (20160304),
http://dx.doi.org/10.1126/science.aad9480 doi:10.1126/science.aad9480 (ID: 719521)
Toggle Abstract
Certain algorithms for quantum computers are able to outperform their classical counterparts. In 1994, Peter Shor came up with a quantum algorithm that calculates the prime factors of a large number vastly more efficiently than a classical computer. For general scalability of such algorithms, hardware, quantum error correction, and the algorithmic realization itself need to be extensible. Here we present the realization of a scalable Shor algorithm, as proposed by Kitaev. We factor the number 15 by effectively employing and controlling seven qubits and four “cache qubits” and by implementing generalized arithmetic operations, known as modular multipliers. This algorithm has been realized scalably within an iontrap quantum computer and returns the correct factors with a confidence level exceeding 99%.

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.
(local copy)

C. C. Rusconi, O. RomeroIsart Magnetic Rigid Rotor in the Quantum Regime: Theoretical Toolbox,
Phys. Rev. B 93 54427 (20160226),
http://dx.doi.org/10.1103/PhysRevB.93.054427 doi:10.1103/PhysRevB.93.054427 (ID: 719380)
Toggle Abstract
We describe the quantum dynamics of a magnetic rigid rotor in the mesoscopic scale where the EinsteinDe Haas effect is predominant. In particular, we consider a singledomain magnetic nanoparticle with uniaxial anisotropy in a magnetic trap. Starting from the basic Hamiltonian of the system under the macrospin approximation, we derive a bosonized Hamiltonian describing the centerofmass motion, the total angular momentum, and the macrospin degrees of freedom of the particle treated as a rigid body. This bosonized Hamiltonian can be approximated by a simple quadratic Hamiltonian that captures the rich physics of a nanomagnet tightly confined in position, nearly not spinning, and with its macrospin antialigned to the magnetic field. The theoretical tools derived and used here can be applied to other quantum mechanical rigid rotors.

J. Budich, M. Heyl Dynamical Topological Order Parameters far from Equilibrium,
Phys. Rev. B 93 85416 (20160210),
http://dx.doi.org/10.1103/PhysRevB.93.085416 doi:10.1103/PhysRevB.93.085416 (ID: 719244)
Toggle Abstract
Equilibrium phase transitions have a nonequilibrium analog in quantum realtime evolution  coined dynamical quantum phase transitions (DQPTs)  which is driven by progressing time instead of conventional control parameters such as temperature. While the occurrence of DQPTs has recently been reported in various systems, the characterization of the dynamical phases separated by a DQPT in terms of order parameters has remained elusive so far. Here, studying quantum quenches in twobanded Bogoliubov de Gennes models, we identify for the first time a dynamical order parameter that physically distinguishes the involved dynamical phases. More specifically, this novel topological quantum number is a momentum space winding number of the Pancharatnam geometric phase which changes its discrete value only at a DQPT.

M. Dalmonte, S. Montangero Lattice gauge theories simulations in the quantum information era,
Contemp. Phys. 57 388 (20160203),
http://dx.doi.org/10.1080/00107514.2016.1151199 doi:10.1080/00107514.2016.1151199 (ID: 719505)
Toggle Abstract
The manybody problem is ubiquitous in the theoretical description of physical phenomena, ranging from the behavior of elementary particles to the physics of electrons in solids. Most of our understanding of manybody systems comes from analyzing the symmetry properties of Hamiltonian and states: the most striking example are gauge theories such as quantum electrodynamics, where a local symmetry strongly constrains the microscopic dynamics. The physics of such gauge theories is relevant for the understanding of a diverse set of systems, including frustrated quantum magnets and the collective dynamics of elementary particles within the standard model. In the last few years, several approaches have been put forward to tackle the complex dynamics of gauge theories using quantum information concepts. In particular, quantum simulation platforms have been put forward for the realization of synthetic gauge theories, and novel classical simulation algorithms based on quantum information concepts have been formulated. In this review we present an introduction to these approaches, illustrating the basics concepts and highlighting the connections between apparently very different fields, and report the recent developments in this new thriving field of research.

T. Weiß, A. Kronwald, F. Marquardt Noiseinduced transitions in optomechanical synchronization,
New J. Phys. 18 13043 (20160119),
http://dx.doi.org/10.1088/13672630/18/1/013043 doi:10.1088/13672630/18/1/013043 (ID: 720022)

D. Hümmer, E. MartinMartinez, A. Kempf Renormalized UnruhDeWitt particle detector models for boson and fermion fields,
Phys. Rev. D 93 24019 (20160112),
http://dx.doi.org/10.1103/PhysRevD.93.024019 doi:10.1103/PhysRevD.93.024019 (ID: 719462)

J. PratCamps, C. Navau, À. Sánchez, D. Chen Demagnetizing Factors for a Hollow Sphere,
IEEE Magn. Lett. 7 1300104 (20160112),
http://dx.doi.org/10.1109/LMAG.2015.2501281 doi:10.1109/LMAG.2015.2501281 (ID: 719472)

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.
(local copy)