
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)
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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.

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)

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)
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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.

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)
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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.

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)
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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.

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)
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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)
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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.

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)
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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.

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)
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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.

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)
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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)
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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. 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)
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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.

O. RomeroIsart, L. Clemente, C. Navau, A. Sanchez, J. I. Cirac Quantum Magnetomechanics with Levitating Superconducting Microspheres,
Phys. Rev. Lett. 109 147205 (20121005),
http://dx.doi.org/10.1103/PhysRevLett.109.147205 doi:10.1103/PhysRevLett.109.147205 (ID: 718731)
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We show that by magnetically trapping a superconducting microsphere close to a quantum circuit, it is possible to perform groundstate cooling and prepare quantum superpositions of the centerofmass motion of the microsphere. Due to the absence of clamping losses and timedependent electromagnetic fields, the mechanical motion of micrometersized metallic spheres in the Meissner state is predicted to be very well isolated from the environment. Hence, we propose to combine the technology of magnetic microtraps and superconducting qubits to bring relatively large objects to the quantum regime.

R. Kaltenbaek, G. Hechenblaikner, N. Kiesel, O. RomeroIsart, K. C. Schwab, U. Johann, M. Aspelmeyer Macroscopic quantum resonators (MAQRO),
Experimental Astronomy 34 164 (20121001),
http://dx.doi.org/10.1007/s1068601292923 doi:10.1007/s1068601292923 (ID: 718739)
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Quantum physics challenges our understanding of the nature of physical reality and of spacetime and suggests the necessity of radical revisions of their underlying concepts. Experimental tests of quantum phenomena involving massive macroscopic objects would provide novel insights into these fundamental questions. Making use of the unique environment provided by space, MAQRO aims at investigating this largely unexplored realm of macroscopic quantum physics. MAQRO has originally been proposed as a mediumsized fundamentalscience space mission for the 2010 call of Cosmic Vision. MAQRO unites two experiments: DECIDE (DECoherence In DoubleSlit Experiments) and CASE (Comparative Acceleration Sensing Experiment). The main scientific objective of MAQRO, which is addressed by the experiment DECIDE, is to test the predictions of quantum theory for quantum superpositions of macroscopic objects containing more than 108 atoms. Under these conditions, deviations due to various suggested alternative models to quantum theory would become visible. These models have been suggested to harmonize the paradoxical quantum phenomena both with the classical macroscopic world and with our notion of Minkowski spacetime. The second scientific objective of MAQRO, which is addressed by the experiment CASE, is to demonstrate the performance of a novel type of inertial sensor based on optically trapped microspheres. CASE is a technology demonstrator that shows how the modular design of DECIDE allows to easily incorporate it with other missions that have compatible requirements in terms of spacecraft and orbit. CASE can, at the same time, serve as a test bench for the weak equivalence principle, i.e., the universality of free fall with testmasses differing in their mass by 7 orders of magnitude.

A. C. Pflanzer, O. RomeroIsart, J. I. Cirac Masterequation approach to optomechanics with arbitrary dielectrics,
Phys. Rev. A 86 013802 (20120703),
http://dx.doi.org/10.1103/PhysRevA.86.013802 doi:10.1103/PhysRevA.86.013802 (ID: 718733)
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We present a master equation describing the interaction of light with dielectric objects of arbitrary sizes and shapes. The quantum motion of the object, the quantum nature of light, as well as scattering processes to all orders in perturbation theory are taken into account. This formalism extends the standard masterequation approach to the case where interactions among different modes of the environment are considered. It yields a genuine quantum description, including a renormalization of the couplings and decoherence terms. We apply this approach to analyze cavity cooling of the centerofmass mode of large spheres. Furthermore, we derive an expression for the steadystate phonon numbers without relying on resolvedsideband or badcavity approximations.

O. RomeroIsart, M. Rizzi, C. A. Muschik, E. Polzik, M. Lewenstein, A. Sanpera Quantum Memory Assisted Probing of Dynamical Spin Correlations,
Phys. Rev. Lett. 108 065302 (20120210),
http://dx.doi.org/10.1103/PhysRevLett.108.065302 doi:10.1103/PhysRevLett.108.065302 (ID: 718734)
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We propose a method to probe timedependent correlations of nontrivial observables in manybody ultracold lattice gases. The scheme uses a quantum nondemolition matterlight interface, first to map the observable of interest on the manybody system into the light and then to store coherently such information into an external system acting as a quantum memory. Correlations of the observable at two (or more) instances of time are retrieved with a single final measurement that includes the readout of the quantum memory. Such a method brings to reach the study of dynamics of manybody systems in and out of equilibrium by means of quantum memories in the field of quantum simulators.

O. RomeroIsart Quantum superposition of massive objects and collapse models,
Phys. Rev. A 84 052121 (20111128),
http://dx.doi.org/10.1103/PhysRevA.84.052121 doi:10.1103/PhysRevA.84.052121 (ID: 718735)
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We analyze the requirements to test some of the most paradigmatic collapse models with a protocol that prepares quantum superpositions of massive objects. This consists of coherently expanding the wave function of a groundstatecooled mechanical resonator, performing a squared position measurement that acts as a double slit, and observing interference after further evolution. The analysis is performed in a general framework and takes into account only unavoidable sources of decoherence: blackbody radiation and scattering of environmental particles. We also discuss the limitations imposed by the experimental implementation of this protocol using cavity quantum optomechanics with levitating dielectric nanospheres.

O. RomeroIsart, A. C. Pflanzer, F. Blaser, R. Kaltenbaek, N. Kiesel, M. Aspelmeyer, J. I. Cirac Large Quantum Superpositions and Interference of Massive NanometerSized Objects,
Phys. Rev. Lett. 107 020405 (20110707),
http://dx.doi.org/10.1103/PhysRevLett.107.020405 doi:10.1103/PhysRevLett.107.020405 (ID: 718736)
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We propose a method to prepare and verify spatial quantum superpositions of a nanometersized object separated by distances of the order of its size. This method provides unprecedented bounds for objective collapse models of the wave function by merging techniques and insights from cavity quantum optomechanics and matterwave interferometry. An analysis and simulation of the experiment is performed taking into account standard sources of decoherence. We provide an operational parameter regime using presentday and planned technology.

G. De Chiara, O. RomeroIsart, A. Sanpera Probing magnetic order in ultracold lattice gases,
Phys. Rev. A 83 021604 (20110218),
http://dx.doi.org/10.1103/PhysRevA.83.021604 doi:10.1103/PhysRevA.83.021604 (ID: 718737)
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A forthcoming challenge in ultracold lattice gases is the simulation of quantum magnetism. That involves both the preparation of the lattice atomic gas in the desired spin state and the probing of the state. Here we demonstrate how a probing scheme based on atomlight interfaces gives access to the order parameters of nontrivial quantum magnetic phases, allowing us to characterize univocally strongly correlated magnetic systems produced in ultracold gases. This method, which is also nondemolishing, yields spatially resolved spin correlations and can be applied to bosons or fermions. As a proof of principle, we apply this method to detect the complete phase diagram displayed by a chain of (rotationally invariant) spin1 bosons.

O. RomeroIsart, A. C. Pflanzer, M. L. Juan, R. Quidant, N. Kiesel, M. Aspelmeyer, J. I. Cirac Optically levitating dielectrics in the quantum regime: Theory and protocols,
Phys. Rev. A 83 013803 (20110107),
http://dx.doi.org/10.1103/PhysRevA.83.013803 doi:10.1103/PhysRevA.83.013803 (ID: 718740)
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We provide a general quantum theory to describe the coupling of light with the motion of a dielectric object inside a highfinesse optical cavity. In particular, we derive the total Hamiltonian of the system as well as a master equation describing the state of the centerofmass mode of the dielectric and the cavityfield mode. In addition, a quantum theory of elasticity is used to study the coupling of the centerofmass motion with internal vibrational excitations of the dielectric. This general theory is applied to the recent proposal of using an optically levitating nanodielectric as a cavity optomechanical system [see RomeroIsart et al., New J. Phys. 12, 033015 (2010); Chang et al., Proc. Natl. Acad. Sci. USA 107, 1005 (2010)]. On this basis, we also design a lightmechanics interface to prepare nonGaussian states of the mechanical motion, such as quantum superpositions of Fock states. Finally, we introduce a direct mechanical tomography scheme to probe these genuine quantum states by timeof flight experiments.

O. RomeroIsart, M. L. Juan, R. Quidant, J. I. Cirac Toward quantum superposition of living organisms,
New J. Phys. 12 033015 (20100301),
http://dx.doi.org/10.1088/13672630/12/3/033015 doi:10.1088/13672630/12/3/033015 (ID: 718741)
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The most striking feature of quantum mechanics is the existence of superposition states, where an object appears to be in different situations at the same time. The existence of such states has been previously tested with small objects, such as atoms, ions, electrons and photons (Zoller et al 2005 Eur. Phys. J. D 36 203–28), and even with molecules (Arndt et al 1999 Nature 401 680–2). More recently, it has been shown that it is possible to create superpositions of collections of photons (Deléglise et al 2008 Nature 455 510–14), atoms (Hammerer et al 2008 arXiv:0807.3358) or Cooper pairs (Friedman et al 2000 Nature 406 43–6). Very recent progress in optomechanical systems may soon allow us to create superpositions of even larger objects, such as microsized mirrors or cantilevers (Marshall et al 2003 Phys. Rev. Lett. 91 130401; Kippenberg and Vahala 2008 Science 321 1172–6; Marquardt and Girvin 2009 Physics 2 40; Favero and Karrai 2009 Nature Photon. 3 201–5), and thus to test quantum mechanical phenomena at larger scales. Here we propose a method to cool down and create quantum superpositions of the motion of subwavelength, arbitrarily shaped dielectric objects trapped inside a highfinesse cavity at a very low pressure. Our method is ideally suited for the smallest living organisms, such as viruses, which survive under lowvacuum pressures (Rothschild and Mancinelli 2001 Nature 406 1092–101) and optically behave as dielectric objects (Ashkin and Dziedzic 1987 Science 235 1517–20). This opens up the possibility of testing the quantum nature of living organisms by creating quantum superposition states in very much the same spirit as the original Schrödinger's cat 'gedanken' paradigm (Schrödinger 1935 Naturwissenschaften 23 807–12, 823–8, 844–9). We anticipate that our paper will be a starting point for experimentally addressing fundamental questions, such as the role of life and consciousness in quantum mechanics.

A. Monras, O. RomeroIsart Quantum Information Processing with Quantum Zeno ManyBody Dynamics,
Quantum Information and Computation 10 201 (20100101),
(ID: 718747)
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We show how the quantum Zeno effect can be exploited to control quantum manybody dynamics for quantum information and computation purposes. In particular, we consider a one dimensional array of three level systems interacting via a nearestneighbour interaction. By encoding the qubit on two levels and using simple projective frequent measurements yielding the quantum Zeno effect, we demonstrate how to implement a well defined quantum register, quantum state transfer on demand, universal twoqubit gates and twoqubit parity measurements. Thus, we argue that the main ingredients for universal quantum computation can be achieved in a spin chain with an alwayson and constant manybody Hamiltonian. We also show some possible modifications of the initially assumed dynamics in order to create maximally entangled qubit pairs and single qubit gates.

O. RomeroIsart, J. J. GarcíaRipoll Quantum ratchets for quantum communication with optical superlattices,
Phys. Rev. A 76 052304 (20071106),
http://dx.doi.org/10.1103/PhysRevA.76.052304 doi:10.1103/PhysRevA.76.052304 (ID: 718751)
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We propose to use a quantum ratchet to transport quantum information in a chain of atoms trapped in an optical superlattice. The quantum ratchet is created by a continuous modulation of the optical superlattice which is periodic in time and in space. Though there is zero average force acting on the atoms, we show that indeed the ratchet effect permits atoms on even and odd sites to move along opposite directions. By loading the optical lattice with twolevel bosonic atoms, this scheme permits us to perfectly transport a qubit or entangled state imprinted in one or more atoms to any desired position in the lattice. From the quantum computation point of view, the transport is achieved by a smooth concatenation of perfect swap gates. We analyze setups with noninteracting and interacting particles and in the latter case we use the tools of optimal control to design optimal modulations. We also discuss the feasibility of this method in current experiments.

O. RomeroIsart, K. Eckert, C. Rodó, A. Sanpera Transport and entanglement generation in the Bose–Hubbard model,
J. Phys. A: Math. Gen. 40 8019 (20070627),
http://dx.doi.org/10.1088/17518113/40/28/S11 doi:10.1088/17518113/40/28/S11 (ID: 718748)
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We study entanglement generation via particle transport across a onedimensional system described by the Bose–Hubbard Hamiltonian. We analyse how the competition between interactions and tunnelling affects transport properties and the creation of entanglement in the occupation number basis. Alternatively, we propose to use spatially delocalized quantum bits, where a quantum bit is defined by the presence of a particle either in a site or in the adjacent one. Our results can serve as guidance for future experiments to characterize entanglement of ultracold gases in onedimensional optical lattices.

O. RomeroIsart, K. Eckert, A. Sanpera Quantum state transfer in spin1 chains,
Phys. Rev. A 75 050303 (20070516),
http://dx.doi.org/10.1103/PhysRevA.75.050303 doi:10.1103/PhysRevA.75.050303 (ID: 718752)
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We study the transfer of a quantum state through a Heisenberg spin1 chain prepared in its ground state. We characterize the efficiency of the transfer via the fidelity of retrieving an arbitrarily prepared state and also via the transfer of quantum entanglement. The Heisenberg spin1 chain has a very rich quantum phase diagram. We show that the boundaries of the different quantum phases are reflected in sharp variations of the transfer efficiency. In the vicinity of the border between the dimer and the ferromagnetic phase (in the conjectured spinnematic region), we find strong indications for a qualitative change of the excitation spectrum. Moreover, we identify two regions of the phase diagram that give rise to particularly high transfer efficiency; the channel might be nonclassical even for chains of arbitrary length, in contrast to spin1/2 chains.

C. Rodó, O. RomeroIsart, K. Eckert, A. Sanpera Efficiency in Quantum Key Distribution Protocols with Entangled Gaussian States,
Open Syst. Inf. Dyn. 14 69 (20070301),
http://dx.doi.org/10.1007/s110800079030x doi:10.1007/s110800079030x (ID: 718749)
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Quantum key distribution (QKD) refers to specific quantum strategies which permit the secure distribution of a secret key between two parties that wish to communicate secretly. Quantum cryptography has proven unconditionally secure in ideal scenarios and has been successfully implemented using quantum states with finite (discrete) as well as infinite (continuous) degrees of freedom. Here, we analyze the efficiency of QKD protocols that use as a resource entangled gaussian states and gaussian operations only. In this framework, it has already been shown that QKD is possible [1] but the issue of its efficiency has not been considered. We propose a figure of merit (the efficiency E) to quantify the number of classical correlated bits that can be used to distill a key from a sample of N entangled states. We relate the efficiency of the protocol to the entanglement and purity of the states shared between the parties.

K. Eckert, O. RomeroIsart, A. Sanpera Efficient quantum state transfer in spin chains via adiabatic passage,
New J. Phys. 9 155 (20070301),
http://dx.doi.org/10.1088/13672630/9/5/155 doi:10.1088/13672630/9/5/155 (ID: 718750)
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We propose a method for quantum state transfer in spin chains using an adiabatic passage technique. Modifying even and odd nearestneighbour couplings in time allows transfer fidelities arbitrarily close to one to be achieved, without the need for precise control of coupling strengths and timing. We study in detail transfer by adiabatic passage in a spin1 chain governed by a generalized Heisenberg Hamiltonian. We consider optimization of the transfer process applying optimal control techniques. We discuss a realistic experimental implementation using cold atomic gases confined in deep optical lattices.