Feb+2013

flat =Feb 25 - March 1 Stephan Langer=

=Mar 1=

1. [|arXiv:1302.7217] [ [|pdf], [|ps] , [|other] ] Number and spin densities in the ground state of a trapped mixture of two pseudospin-1/2 Bose gases with interspecies spin-exchange interaction [|Jinlong Wang], [|Yu Shi] We consider the ground state of a mixture of two pseudospin-$\1/2$ Bose gases with interspecies spin exchange in a trapping potential. In the mean field approach, the ground state can be described in terms of four wave functions governed by a set of coupled Gross-Pitaevskii-like (GP-like) equations, which differ from the usual GP equations in the existence of an interference term due to spin-exchange coupling between the two species. Using these GP-like equations, we calculate such ground state properties as chemical potentials, density profiles and spin density profiles, which are directly observable in experiments. We compare the cases with and without spin exchange. It is demonstrated that the spin exchange between the two species lowers the chemical potentials, tends to equalize the wave functions of the two pseudospin components of each species, and thus homogenizes the spin density. The novel features of the density and spin density profiles can serve as experimental probes of this novel Bose system.

2. [|arXiv:1302.7290] [ [|pdf], [|other] ] Charge density distribution and optical response of the LaAlO3/SrTiO3 interface [|Se Young Park], [|Andrew J. Millis]

We present calculations of the charge density profile, subband occupancy and ellipsometry spectra of the electron gas at the LaAlO3/SrTiO3 interface. The calculations employ self-consistent Hartree and random phase approximations, a tight binding parametrization of the band structure and a model for the optical phonon of SrTiO3. The dependence of the spatial structure and occupancy of subbands on the magnitude of the polarization charge at the interface and the dielectric function is determined. The interface-confined subbands may be labelled by the symmetry (xy, xz, or yz) of the Ti d-orbitals from which they mainly derive. The xy-derived band nearest the interface contains the major proportion of the electronic charge, but a large number of more distant, slightly occupied xy-derived bands are also found. Depending on the magnitude of the polarization charge, zero, one, or two xz/yz derived subbands are found. When present, these xz/yz bands give the dominant contribution to the long-distance tail of the interface charge. The response to applied ac electric fields polarized parallel and perpendicular to the interface is calculated and the results are presented in terms of ellipsometry angles. Two features are found: a dip in the spectrum near the LO feature of the STO phonon and a peak at the higher energy. We show that the form and magnitude of the dip is related to the Drude response of carriers moving in the plane of the interface while the peak arises from the plasmon excitation of the xz and yz electrons. The relation of the features of the subband occupancies and the in-plane conductivities is given.

3. [|arXiv:1111.3275] [ [|pdf], [|other] ] Information storage capacity of discrete spin systems [|Beni Yoshida]

Understanding the limits imposed on information storage capacity of physical systems is a problem of fundamental and practical importance which bridges physics and information science. There is a well-known upper bound on the amount of information that can be stored reliably in a given volume of discrete spin systems which are supported by gapped local Hamiltonians. However, all the previously known systems were far below this theoretical bound, and it remained open whether there exists a gapped spin system that saturates this bound. Here, we present a construction of spin systems which saturate this theoretical limit asymptotically by borrowing an idea from fractal properties arising in the Sierpinski triangle. Our construction provides not only the best classical error-correcting code which is physically realizable as the energy ground space of gapped frustration-free Hamiltonians, but also a new research avenue for correlated spin phases with fractal spin configurations.

=Feb 28=

1. [|arXiv:1302.6654] [ [|pdf], [|ps] , [|other] ] Dynamics of double-well Bose-Einstein Condensates subject to external Gaussian white noise [|Hanlei Zheng], [|Yajiang Hao] , [|Qiang Gu]

Dynamical properties of the Bose-Einstein condensate in double-well potential subject to Gaussian white noise are investigated by numerically solving the time-dependent Gross-Pitaevskii equation. The Gaussian white noise is used to describe influence of the random environmental disturbance on the double-well condensate. Dynamical evolutions from three different initial states, the Josephson oscillation state, the running phase and $\pi$-mode macroscopic quantum self-trapping states are considered. It is shown that the system is rather robust with respect to the weak noise whose strength is small and change rate is high. If the evolution time is sufficiently long, the weak noise will finally drive the system to evolve from high energy states to low energy states, but in a manner rather different from the energy-dissipation effect. In presence of strong noise with either large strength or slow change rate, the double-well condensate may exhibit very irregular dynamical behaviors.

2. [|arXiv:1302.6761] [ [|pdf], [|other] ] Supercell Gutzwiller method for bosonic lattice systems [|Dirk-Sören Lühmann]

A versatile and numerically inexpensive method is presented allowing the accurate calculation of phase diagrams for bosonic lattice models. By treating clusters within the Gutzwiller theory, a surprisingly good description of quantum fluctuations beyond the mean-field theory is achieved approaching quantum Monte-Carlo predictions for large clusters. Applying this powerful method to the Bose-Hubbard model, we demonstrate that it yields precise results for the superfluid to Mott-insulator transition in square, honeycomb, and cubic lattices. Due to the exact treatment within a cluster, the method can be effortlessly adapted to more complicated Hamiltonians in the fast progressing field of optical lattice experiments. This includes state- and site-dependent superlattices, large confined atomic systems and disordered potentials, as well as various types of extended Hubbard models. Furthermore, the approach allows an excellent treatment of systems with arbitrary filling factors. We discuss the perspectives that allow for the computation of large, spatially-varying lattices, low-lying excitations, and time evolution.

3. [|arXiv:1302.6901] [ [|pdf], [|ps] , [|other] ] Ferromagnetic to antiferromagnetic transition of one-dimensional spinor Bose gases with spin-orbit coupling [|Xing Chen], [|Haiping Hu] , [|Yuzhu Jiang] , [|Shu Chen] We have analytically solved one-dimensional interacting two-component bosonic gases with spin-orbit (SO) coupling by the Bethe-ansatz method. Through a gauge transformation, the effect of SO coupling is incorporated into a spin-dependent twisted boundary condition. Our result shows that the SO coupling can influence the eigenenergy in a periodical pattern. The interplay between interaction and SO coupling may induce the energy level crossing for the ground state, which leads to a transition from the ferromagnetic to antiferromagnetic state.

=Feb 27=

1. [|arXiv:1302.6596] [ [|pdf], [|ps] , [|other] ] Quantum Simulation Architecture for Lattice Bosons in Arbitrary, Tunable External Gauge Fields [|Eliot Kapit]

We describe a lattice of asymmetrical qubit pairs in one or two dimensions, with couplings arranged so that the motion of single-qubit excited states mimics the behavior of charged lattice bosons hopping in a magnetic field. We show in particular that one can choose the parameters of the many-body circuit to reach a regime where the complex hopping phase between any two elements can be tuned to any value by simply adjusting the relative phases of two applied oscillating voltage signals. We also propose a specific realization of our model using coupled three junction flux qubits, in which one can reach the strongly interacting bosonic quantum Hall limit where one will find anyonic excitations. The circuits could be used for topological quantum computation.

2. [|arXiv:1302.6694] [ [|pdf], [|other] ] Numerical study of unitary fermions in one spatial dimension [|Michael G. Endres]

I perform lattice Monte Carlo studies of universal four-component fermion systems in one spatial dimension. Continuum few-body observables (i.e., ground state energies and integrated contact densities) are determined for both unpolarized and polarized systems of up to eight fermions confined to a harmonic trap. Estimates of the continuum energies for four and five trapped fermions show agreement with exact analytic calculations to within approximately one percent statistical uncertainties. Continuum many-body observables are determined for unpolarized systems of up to 88 fermions confined to a finite box, and 56 fermions confined to a harmonic trap. Results are reported for universal quantities such as the Bertsch parameter, defined as the energy of the untrapped many-body system in units of the corresponding free-gas energy, and its subleading correction at large but finite scattering length. Two independent estimates of these quantities are obtained from thermodynamic limit extrapolations of continuum extrapolated observables. A third estimate of the Bertsch parameter is obtained by combining estimates of the untrapped and trapped integrated contact densities with additional theoretical input from a calculation based on Thomas-Fermi theory. All estimates of the Bertsch parameter and its subleading correction are found to be consistent to within approximately one percent statistical uncertainties. Finally, the continuum restoration of virial theorems is confirmed for both few- and many-body systems confined to a trap.

3. [|arXiv:1302.6610] [ [|pdf], [|other] ] Manipulation and coherence of ultra-cold atoms on a superconducting atom chip [|S. Bernon], [|H. Hattermann] , [|D. Bothner] , [|M. Knufinke] , [|P. Weiss] , [|F. Jessen] , [|D. Cano] , [|M. Kemmler] , [|R. Kleiner] , [|D. Koelle] , [|J. Fortágh]

The coherence of quantum systems is crucial to quantum information processing. While it has been demonstrated that superconducting qubits can process quantum information at microelectronics rates, it remains a challenge to preserve the coherence and therefore the quantum character of the information in these systems. An alternative is to share the tasks between different quantum platforms, e.g. cold atoms storing the quantum information processed by superconducting circuits. In our experiment, we characterize the coherence of superposition states of 87Rb atoms magnetically trapped on a superconducting atom-chip. We load atoms into a persistent-current trap engineered in the vicinity of an off-resonance coplanar resonator, and observe that the coherence of hyperfine ground states is preserved for several seconds. We show that large ensembles of a million of thermal atoms below 350 nK temperature and pure Bose-Einstein condensates with 3.5 x 10^5 atoms can be prepared and manipulated at the superconducting interface. This opens the path towards the rich dynamics of strong collective coupling regimes.

=Feb 26=

1. [|arXiv:1302.6234] [ [|pdf], [|ps] , [|other] ] Boson topological insulators: A window into highly entangled quantum phases [|Chong Wang], [|T. Senthil]

We study several aspects of the realization of global symmetries in highly entangled phases of quantum matter. Examples include gapped topological ordered phases, gapless quantum spin liquids and non-fermi liquid phases. An insightful window into such phases is provided by recent developments in the theory of short ranged entangled Symmetry Protected Topological (SPT) phases. First they generate useful no-go constraints on how global symmetry may be implemented in a highly entangled phase. Possible symmetry implementation in gapped topological phases and some proposed gapless spin/bose liquids are examined in this light. We show that some previously proposed spin liquid states for 2d quantum magnets do not in fact have consistent symmetry implementation unless they occur as the surface of a 3d SPT phase. A second SPT-based insight into highly entangled states is the development of a view point of such states as SPT phases of one of the emergent excitations. We describe this in the specific context of time reversal symmetric 3d U(1) quantum spin liquids with an emergent photon. Different such spin liquids are shown to be equivalent to different SPT insulating phases of the emergent monopole excitation of such phases. The highly entangled states also in turn enrich our understanding of SPT phases. We use the insights obtained from our results to provide an explicit construction of bosonic SPT phases in 3d in a system of coupled layers. This includes construction of a time reversal symmetric SPT state that is not currently part of the cohomology classification of such states.

2. [|arXiv:1302.6444] [ [|pdf], [|other] ] Entanglement creation in cold molecular gases using strong laser pulses [|Felipe Herrera], [|Sabre Kais] , [|K. Birgitta Whaley]

While many-particle entanglement can be found in natural solids and strongly interacting atomic and molecular gases, generating highly entangled states between weakly interacting particles in a controlled and scalable way presents a significant challenge. We describe here a one-step method to generate entanglement in a dilute gas of cold polar molecules. For molecules in optical traps separated by a few micrometers, we show that maximally entangled states can be created using the strong off-resonant pulses that are routinely used in molecular alignment experiments. We show that the resulting alignment-mediated entanglement can be detected by measuring laser-induced fluorescence with single-site resolution and that signatures of this molecular entanglement also appear in the microwave absorption spectra of the molecular ensemble. We analyze the robustness of these entangled molecular states with respect to intensity fluctuations of the trapping laser and discuss possible applications of the system for quantum information processing.

3. [|arXiv:1302.6251] [ [|pdf], [|ps] , [|other] ] Frustration, Entanglement, and Correlations in Quantum Many Body Systems [|U. Marzolino], [|S. M. Giampaolo] , [|F. Illuminati]

We derive an exact lower bound to a universal measure of frustration in globally degenerate ground states of many body systems. The bound results in the sum of two contributions, one due to entanglement and one due to classical correlations after local generalized measurements on arbitrarily selected subsystems. The existence of classical statistical correlations is thus identified as a third necessary source of frustration, beyond geometry and entanglement, in the presence of ground-state degeneracy. We show that the global, on average, frustration properties of quantum systems are characterized by the behavior of the maximally mixed ground state, that is the convex combination with equal a priori probabilities of all the degenerate pure ground states. We determine sufficient conditions for a quantum spin system to saturate the bound, and for models with twofold ground-state degeneracy we prove that the local and on average characterizations of frustration coincide, in the sense that all degenerate ground states possess the same frustration properties of the maximally mixed ground state.

=Feb 25=

1. [|arXiv:1302.6126] [ [|pdf], [|ps] , [|other] ] Interacting Non-equilibrium Systems with Two Temperatures [|Roberto C. Alamino], [|Amit Chattopadhyay] , [|David Saad]

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We investigate a simplified model of two fully connected magnetic systems maintained at different temperatures by virtue of being connected to two independent thermal baths while simultaneously being inter-connected with each other. Using generating functional analysis, commonly used in statistical mechanics, we find exactly soluble expressions for their individual magnetisations that define a two-dimensional non-linear map, the equations of which have the same form as those obtained for densely connected equilibrium systems. Steady states correspond to the fixed points of this map, separating the parameter space into a rich set of non-equilibrium phases that we analyse in asymptotically high and low (non-equilibrium) temperature limits. The theoretical formalism is shown to subvert to the classical non-equilibrium steady state problem for two interacting systems with a non-zero heat transfer between them that catalyses a phase transition between ambient non-equilibrium states.

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">2. [|arXiv:1302.6136] [ [|pdf], [|other] ] Nonlocal Long Range Orders in 1D Fermionic Systems [|Luca Barbiero], [|Arianna Montorsi] , [|Marco Roncaglia]

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We prove on a prototype Hamiltonian that hidden long range order is always present in the gapped phases of interacting fermionic systems on one dimensional lattices. It is captured by correlation functions of appropriate nonlocal charge and/or spin operators, which remain asymptotically finite. The corresponding microscopic orders are classified. The results are confirmed by DMRG numerical simulation of the phase diagram of the extended Hubbard model, and of a Haldane insulator phase.

3. [|arXiv:1302.6222] [ [|pdf], [|other] ]

Entanglement Entropies in Conformal Systems with Boundaries [|L. Taddia], [|J. C. Xavier] , [|F. C. Alcaraz] , [|G. Sierra]

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We study the entanglement entropies in one-dimensional open critical systems, whose effective description is a conformal field theory, both for the ground state and the excited states associated to primary fields. The analytical results are checked numerically finding excellent agreement for the $c=1/2$ minimal model and the $c=1$ compactified massless free boson.

=Feb 18 - Feb 22 Jin-Long Yu=

=Feb 22=

1. [|arXiv:1302.5135] [[|pdf], [|other]] Topology by dissipation [|C.-E. Bardyn], [|M. A. Baranov], [|C. V. Kraus], [|E. Rico], [|A. Imamoglu], [|P. Zoller], [|S. Diehl] Topological states of fermionic matter can be induced by means of a suitably engineered dissipative dynamics. Dissipation then does not occur as a perturbation, but rather as the main resource for many-body dynamics, providing a targeted cooling into a topological phase starting from an arbitrary initial state. We explore the concept of topological order in this setting, developing and applying a general theoretical framework based on the system density matrix which replaces the wave function appropriate for the discussion of Hamiltonian ground-state physics. We identify key analogies and differences to the more conventional Hamiltonian scenario. Differences mainly arise from the fact that the properties of the spectrum and of the state of the system are not as tightly related as in a Hamiltonian context. We provide a symmetry-based topological classification of bulk steady states and identify the classes that are achievable by means of quasi-local dissipative processes driving into superfluid paired states. We also explore the fate of the bulk-edge correspondence in the dissipative setting, and demonstrate the emergence of Majorana edge modes. We illustrate our findings in one- and two-dimensional models that are experimentally realistic in the context of cold atoms.

=Feb 21=

1. [|arXiv:1302.4747] [[|pdf], [|ps], [|other]] Thermodynamic signatures for topological phase transitions to Majorana and Weyl superfluids in ultracold Fermi gases [|Kangjun Seo], [|Chuanwei Zhang], [|Sumanta Tewari] We discuss the thermodynamic signatures for the topological phase transitions into Majorana and Weyl superfluid phases in ultracold Fermi gases in two and three dimensions in the presence of Rashba spin-orbit coupling and a Zeeman field. We analyze the thermodynamic properties exhibiting the distinct nature of the topological phase transitions linked with the Majorana fermions (2D Fermi gas) and Weyl fermions (3D Fermi gas) which can be observed experimentally, including pressure, chemical potential, isothermal compressibility, entropy, and specific heat, as a function of the interaction and the Zeeman field at both zero and finite temperatures. We conclude that among the various thermodynamic quantities, the isothermal compressibility and the chemical potential as a function of the artificial Zeeman field have the strongest signatures of the topological transitions in both two and three dimensions.

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-align: -webkit-auto;">2. [|arXiv:1302.4897] [[|pdf], [|other]] Entanglement of bosons in optical lattices [|M. Cramer], [|A. Bernard], [|N. Fabbri], [|L. Fallani], [|C. Fort], [|S. Rosi], [|F. Caruso], [|M. Inguscio], [|M.B. Plenio] Entanglement is a fundamental resource for quantum information processing which occurs naturally in many-body systems at low temperatures. The presence of entanglement and, in particular, its scaling with the size of system partitions underlies the complexity of quantum many-body states. The quantitative estimation of entanglement in many-body systems represents a major challenge as it is held to require either full state tomography, which scales exponentially in the system size, or the assumption of unverified system characteristics such as its Hamiltonian or its temperature. We adopt recently developed approaches for the determination of rigorous lower entanglement bounds from readily accessible measurements and apply them in an experiment of ultracold interacting bosons in optical lattices of approximately $10^5$ lattice sites. We use this approach to study the behaviour of spatial entanglement between the sites when crossing the superfluid to Mott insulator transition and when varying the temperature. This constitutes the first rigorous experimental large-scale entanglement quantification in a scalable quantum simulator.

=Feb 20=

1. <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: initial;">[|arXiv:1302.4736] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> [ <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: initial;">[|pdf] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">, <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: initial;">[|other] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">] [**Experiment**] Heavy Solitons in a Fermionic Superfluid [|Tarik Yefsah], [|Ariel T. Sommer], [|Mark J.H. Ku], [|Lawrence W. Cheuk], [|Wenjie Ji], [|Waseem S. Bakr], [|Martin W. Zwierlein] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Topological excitations are found throughout nature, in proteins and DNA, as dislocations in crystals, as vortices and solitons in superfluids and superconductors, and generally in the wake of symmetry-breaking phase transitions. In fermionic systems, topological defects may provide bound states for fermions that often play a crucial role for the system's transport properties. Famous examples are Andreev bound states inside vortex cores, fractionally charged solitons in relativistic quantum field theory, and the spinless charged solitons responsible for the high conductivity of polymers. However, the free motion of topological defects in electronic systems is hindered by pinning at impurities. Here we create long-lived solitons in a strongly interacting fermionic superfluid by imprinting a phase step into the superfluid wavefunction, and directly observe their oscillatory motion in the trapped superfluid. As the interactions are tuned from the regime of Bose-Einstein condensation (BEC) of tightly bound molecules towards the Bardeen-Cooper-Schrieffer (BCS) limit of long-range Cooper pairs, the effective mass of the solitons increases dramatically to more than 200 times their bare mass. This signals their filling with Andreev states and strong quantum fluctuations. For the unitary Fermi gas, the mass enhancement is more than fifty times larger than expectations from mean-field Bogoliubov-de Gennes theory. Our work paves the way towards the experimental study and control of Andreev bound states in ultracold atomic gases. In the presence of spin imbalance, the solitons created here represent one limit of the long sought-after Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state of mobile Cooper pairs.

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">2. [|arXiv:1302.4576] [[|pdf], [|other]] Defect-induced supersolidity with soft-core Bosons [|Fabio Cinti], [|Tommaso Macrì], [|Wolfgang Lechner], [|Guido Pupillo], [|Thomas Pohl] We determine the zero-temperature phase diagram of two-dimensional Bosons with finite-range soft-core interactions. For high particle densities, we demonstrate that the ground state can be a commensurate density-wave-type supersolid. For low densities, the system is shown to form a solid in which superfluidity is provided by delocalized zero-point defects. This provides the first example of continuous-space supersolidity consistent with the Andreev-Lifshitz-Chester scenario.

=Feb 19=

1. <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> [|arXiv:1302.4109] [ [|pdf], [|ps] , [|other] ] Dynamical Transition in Interaction Quenches of the One-Dimensional Hubbard Model [|Simone A. Hamerla], [|Götz S. Uhrig] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We show that the non-equilibrium time-evolution after interaction quenches in the one dimensional, integrable Hubbard model exhibits a dynamical transition in the half-filled case. This transition ceases to exist upon doping. Our study is based on systematically extended equations of motion. Thus it is controlled for small and moderate times; no relaxation effects are neglected. Remarkable similarities to the quench dynamics in the infinite dimensional Hubbard model are found suggesting dynamical transitions to be a general feature of quenches in such models.

=Feb 18=

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">1. [|arXiv:1302.4262] <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> [ <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">[|pdf] <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">, <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">[|other] <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">[ **<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Experiment **<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">] Direct measurement of the van der Waals interaction between two single atoms [|Lucas Béguin], [|Aline Vernier], [|Radu Chicireanu], [|Thierry Lahaye], [|Antoine Browaeys] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We report on the direct measurement of the van der Waals interaction between two isolated, single Rydberg atoms separated by a controlled distance of a few micrometers. By working in a regime where the single-atom Rabi frequency of the laser used for excitation to the Rydberg state is comparable to the interaction energy, we observe a \emph{partial} Rydberg blockade, whereby the time-dependent populations of the various two-atom states exhibit coherent oscillations with several frequencies. A quantitative comparison of the data with a simple model based on the optical Bloch equations allows us to extract the van der Waals energy, and to observe its characteristic $C_6/R^6$ dependence. The magnitude of the measured $C_6$ coefficient agrees well with an \emph{ab-initio} theoretical calculation, and we observe its dramatic increase with the principal quantum number $n$ of the Rydberg state. Our results not only allow to test an important physical law, but also demonstrate a degree of experimental control which opens new perspectives in quantum information processing and quantum simulation using long-range interactions between the atoms.

2. <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: initial;">[|arXiv:1302.4323] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> [ <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: initial;">[|pdf] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">, <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: initial;">[|other] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">] Engineering spin-waves in a high-spin ultracold Fermi gas

=
[|Jannes Heinze], [|Jasper Simon Krauser], [|Nick Fläschner], [|Klaus Sengstock], [|Christoph Becker], [|Ulrich Ebling], [|Andre Eckardt], [|Maciej Lewenstein] ====== <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We report on the detailed study of multi-component spin-waves in an s=3/2 Fermi gas where the high spin leads to novel tensorial degrees of freedom compared to s = 1/2 systems. The excitations of a spin-nematic state are investigated from the linear to the nonlinear regime, where the tensorial character is particularly pronounced. By tuning the initial state we engineer the tensorial spin-wave character, such that the magnitude and sign of the counterflow spin-currents are effectively controlled. A comparison of our data with numerical and analytical results shows excellent agreement.

=Feb 11 - Feb 15 Bo Liu=

Feb 15
1. [|arXiv:1302.3504] [[|pdf], [|other]] Superconducting Vortex Lattices for Ultracold Atoms [|O. Romero-Isart], [|C. Navau], [|A. Sanchez], [|P. Zoller], [|J. I. Cirac] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">The ability to trap and manipulate ultracold atoms in lattice structures has lead to a remarkable experimental progress to build quantum simulators for Hubbard models. A prominent example is atoms in optical lattices where lasers are used to create lattices with spacing set by the laser wavelength as well as to control and measure the many-body states. In contrast, here we propose and analyze a nanoengineered vortex array in a thin-film type-II superconductor as a magnetic lattice for ultracold atoms. This proposal addresses several of the key questions in the development of atomic quantum simulators. By trapping atoms close to the surface, tools of nanofabrication and structuring of lattices on the scale of few tens of nanometers become available with a corresponding benefit in energy scales and temperature requirements. This can be combined with the possibility of magnetic single site addressing and manipulation together with a favorable scaling of superconducting surface induced decoherence.

2. [|arXiv:1302.3395] [[|pdf], [|ps], [|other]] Thermodynamic properties of correlated fermions in lattices with spin-dependent disorder [|K. Makuch], [|J. Skolimowski], [|P. B. Chakraborty], [|K. Byczuk], [|D. Vollhardt] <span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Motivated by the rapidly growing possibilities for experiments with ultracold atoms in optical lattices we investigate the thermodynamic properties of correlated lattice fermions in the presence of an external spin-dependent random potential. The corresponding model, a Hubbard model with spin-dependent local random potentials, is solved within dynamical mean-field theory. This allows us to present a comprehensive picture of the thermodynamic properties of this system. In particular, we show that for a fixed total number of fermions spin-dependent disorder induces a magnetic polarization. The magnetic response of the polarized system differs from that of a system with conventional disorder.
 * Feb 14 **

1. [|arXiv:1302.3165] [[|pdf], [|other]] Bloch oscillations in a periodic lattice under a strong (artificial) magnetic field [|Marco Cominotti], [|Iacopo Carusotto] Inspired by recent developments of artificial gauge fields for atoms and for photons, we study the semiclassical dynamics of a quantum particle in a two-dimensional square lattice, under the effect of crossed electric and magnetic fields. We provide an elementary derivation of how the Berry curvature of the Bloch bands modifies the usual semiclassical equations of motion. We then calculate the real-space trajectories of a wave packet and observe that the contribution of this anomalous Berry term can be as important as the usual group velocity one. The connection with the classical Hall effect is finally clarified.

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">2. <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">[|arXiv:1302.2947] [[|pdf], [|ps], [|other]] Topological Bose-Mott Insulators in a One-Dimensional Optical Superlattice [|Shi-Liang Zhu], [|Z. D. Wang], [|Y. -H. Chan], [|L. -M. Duan] We study topological properties of the Bose-Hubbard model with repulsive interactions in a one-dimensional optical superlattice. We find that the Mott insulator states of the single-component (two-component) Bose-Hubbard model under fractional fillings are topological insulators characterized by a nonzero charge (or spin) Chern number with nontrivial edge states. For ultracold atomic experiments, we show that the topological Chern number can be detected through measuring the density profiles of the bosonic atoms in a harmonic trap.

Feb 13
1. [|arXiv:1302.2817] [[|pdf], [|other]] Spontaneous self-ordered states of vortex-antivortex pairs in a Polariton Condensate [|F. Manni], [|T. C. H. Liew], [|K. G. Lagoudakis], [|C. Ouellet-Plamondon], [|R. André], [|V. Savona], [|B. Deveaud] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Polariton condensates have proved to be model systems to investigate topological defects, as they allow for direct and non-destructive imaging of the condensate complex order parameter. The fundamental topological excitations of such systems are quantized vortices. In specific configurations, further ordering can bring the formation of vortex lattices. In this work we demonstrate the spontaneous formation of ordered vortical states, consisting in geometrically self-arranged vortex-antivortex pairs. A mean-field generalized Gross-Pitaevskii model reproduces and supports the physics of the observed phenomenology.

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">2. <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">[|arXiv:1302.2871] [[|pdf], [|ps], [|other]] Second sound and the superfluid fraction in a resonantly interacting Fermi gas [|Leonid A. Sidorenkov], [|Meng Khoon Tey], [|Rudolf Grimm], [|Yan-Hua Hou], [|Lev Pitaevskii], [|Sandro Stringari] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Superfluidity is a macroscopic quantum phenomenon, which shows up below a critical temperature and leads to a peculiar behavior of matter, with frictionless flow, the formation of quantized vortices, and the quenching of the moment of inertia being intriguing examples. A remarkable explanation for many phenomena exhibited by a superfluid at finite temperature can be given in terms of a two-fluid mixture comprised of a normal component that behaves like a usual fluid and a superfluid component with zero viscosity and zero entropy. Important examples of superfluid systems are liquid helium and neutron stars. More recently, ultracold atomic gases have emerged as new superfluid systems with unprecedented possibilities to control interactions and external confinement. Here we report the first observation of `second sound' in an ultracold Fermi gas with resonant interactions. Second sound is a striking manifestation of the two-component nature of a superfluid and corresponds to an entropy wave, where the superfluid and the non-superfluid components oscillate in opposite phase, different from ordinary sound (`first sound'), where they oscillate in phase. The speed of second sound depends explicitly on the value of the superfluid fraction, a quantity sensitive to the spectrum of elementary excitations. Our measurements allow us to extract the temperature dependence of the superfluid fraction, which in strongly interacting quantum gases has been an inaccessible quantity so far.

Feb 12
1. [|arXiv:1302.2258] [[|pdf], [|ps], [|other]] Scaling solutions of the two fluid hydrodynamic equations in a harmonically trapped gas at unitarity [|Yan-Hua Hou], [|Lev P. Pitaevskii], [|Sandro Stringari] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We prove that the two fluid Landau hydrodynamic equations, when applied to a gas interacting with infinite scattering length (unitary gas) in the presence of harmonic trapping, admit exact scaling solutions of mixed compressional and surface nature. These solutions are characterized by a linear dependence of the velocity field on the spatial coordinates and a temperature independent frequency which is calculated in terms of the parameters of the trap. Our results are derived in the regime of small amplitude oscillations and hold both below and above the superfluid phase transition. They apply to isotropic as well as to deformed configurations, thereby providing a generalization of Castin's theorem (Y. Castin, C. R. Phys. \textbf{5}, 407 (2004)) holding for isotropic trapping. Our predictions agree with the experimental findings in resonantly interacting atomic Fermi gases. The breathing scaling solution, in the presence of isotropic trapping, is also used to prove the vanishing of two bulk viscosity coefficients in the superfluid phase.

Feb 11
1. <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> [|arXiv:1302.2121] [ [|pdf], [|ps] , [|other] ] Spin dynamics of cold fermions with synthetic spin-orbit coupling [|I.V. Tokatly],[|E.Ya. Sherman] We consider spin relaxation dynamics in cold Fermi gases with a pure-gauge spin-orbit coupling corresponding to recent experiments. We show that such experiments can give a direct access to the collisional spin drag rate, and establish conditions for the observation of spin drag effects. In the recent experiments the dynamics is found to be mainly ballistic leading to new regimes of reversible spin relaxation-like processes.

<span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;"><span style="font-family: arial,helvetica,sans-serif;">2. <span class="list-identifier" style="font-family: arial,helvetica,sans-serif;">[|arXiv:1302.1908] [[|pdf], [|ps], [|other]] <span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;"><span style="font-family: arial,helvetica,sans-serif;">Phase diagram of the quantum O(2)-model in 2+1 dimensions [|Kurt Langfeld] The quantum O(2) model in 2+1 dimensions is studied by simulating the 3d O(2) model near criticality. Finite densities are introduced by a non-zero chemical potential mu, and the worm algorithm is used to circumvent the sign problem. The renormalisation is discussed in some detail. We find that the onset value of the chemical potential coincides with the mass gap. The mu-dependence of the density rules out Bose-Einstein condensation and might be compatible with an interacting Fermi gas. The mu-T phase diagram is explored using the density and the magnetic susceptibility. In the cold, but dense regime of the phase diagram, we find a superfluid phase.

=Feb 4 - Feb 8 Xiaopeng Li=

Feb 8
1. [|arXiv:1302.1792] [[|pdf], [|other]] Theory of unitary Bose gases [|J.J.R.M. van Heugten], [|H.T.C. Stoof] We develop an analytical approach for the description of an atomic Bose gas at unitarity. By focusing in first instance on the evaluation of the single-particle density matrix, we derive several universal properties of the unitary Bose gas, such as the chemical potential, the contact, the speed of sound, the condensate density and the effective interatomic interaction. The theory is also generalized to describe Bose gases with a finite scattering length and then reduces to the Bogoliubov theory in the weak-coupling limit.

2. [|arXiv:1302.1824] (cross-list from quant-ph) [[|pdf], [|other]] Braiding of Atomic Majorana Fermions in Wire Networks and Implementation of the Deutsch-Josza Algorithm [|Christina V. Kraus], [|P. Zoller], [|Mikhail A. Baranov] We propose an efficient protocol for braiding atomic Majorana fermions in wire networks with AMO techniques and demonstrate its robustness against experimentally relevant errors. Based on this protocol we provide a topologically protected implementation of the Deutsch-Josza algorithm.

Feb 7
1. [|arXiv:1302.1408] [[|pdf], [|other]] Néel to valence-bond solid transition on the honeycomb lattice: Evidence for deconfined criticality [|Kedar Damle], [|Fabien Alet], [|Sumiran Pujari] We study a spin-1/2 Heisenberg model on the honeycomb lattice with nearest-neighbor antiferromagnetic exchange J that favors N\'eel order, and competing 6-spin interactions Q which favor a valence bond solid state in which the bond-energies order at the "columnar" wavevector K = (2\pi/3;-2\pi/3). Using projector Quantum Monte Carlo techniques, we find evidence for a direct continuous quantum phase transition between the N\'eel and VBS states, analogous to the deconfined critical point between these states on the square lattice. This implies that such deconfined critical points can exist on the honeycomb lattice although Berry phase effects allow tripled hedgehog defects of the N\'eel order parameter to play a role in the critical theory, unlike the square lattice case in which only quadrupled hedgehog defects are allowed.

2. [|arXiv:1302.1308] [[|pdf], [|other]] Driving dipolar fermions into the quantum Hall regime by spin-flip induced insertion of angular momentum [|David Peter], [|Axel Griesmaier], [|Tilman Pfau], [|Hans Peter Büchler] A new method to drive a system of neutral dipolar fermions into the lowest Landau level regime is proposed. By employing adiabatic spin-flip processes in combination with a diabatic transfer, the fermions are pumped to higher orbital angular momentum states in a repeated scheme that allows for the precise control over the final angular momentum. A simple analytical model is derived to quantify the transfer and compare the approach to rapidly rotating systems. Numerical simulations of the transfer process have been performed for small, interacting systems.

3. [|arXiv:1302.1205] (cross-list from quant-ph) [[|pdf], [|ps], [|other]] Surface Entanglement in Quantum Spin Networks [|S. Zippilli], [|S. M. Giampaolo], [|F. Illuminat] We study the ground-state entanglement in systems of spins forming the boundary of a quantum spin network in arbitrary geometries and dimensionality. We show that as long as they are weakly coupled to the bulk of the network, the surface spins are strongly entangled, even when distant and non directly interacting, thereby generalizing the phenomenon of long-distance entanglement occurring in quantum spin chains. Depending on the structure of the couplings between surface and bulk spins, we discuss in detail how the patterns of surface entanglement can range from multi-pair bipartite to fully multipartite. In the context of quantum information and communication, these results find immediate application to the implementation of quantum routers, that is devices able to distribute quantum correlations on demand among multiple network nodes.

Feb 6
1. [|arXiv:1302.1187] [[|pdf], [|other]] Thermal vs. Entanglement Entropy: A Measurement Protocol for Fermionic Atoms with a Quantum Gas Microscope [|Hannes Pichler], [|Lars Bonnes], [|Andrew J. Daley], [|Andreas M. Läuchli], [|Peter Zoller] We show how to measure the order-two Renyi entropy of many-body states of spinful fermionic atoms in an optical lattice in equilibrium and non-equilibrium situations. The proposed scheme relies on the possibility to produce and couple two copies of the state under investigation, and to measure the occupation number in a site- and spin-resolved manner, e.g. with a quantum gas microscope. Such a protocol opens the possibility to measure entanglement and test a number of theoretical predictions, such as area laws and their corrections. As an illustration we discuss the interplay between thermal and entanglement entropy for a one dimensional Fermi-Hubbard model at finite temperature, and its possible measurement in an experiment using the present scheme.

2. [|arXiv:1302.1113] [[|pdf], [|other]] Entanglement negativity in the critical Ising chain [|Pasquale Calabrese], [|Luca Tagliacozzo], [|Erik Tonni] We study the scaling of the traces of the integer powers of the partially transposed reduced density matrix and of the entanglement negativity for two spin blocks as function of their length and separation in the critical Ising chain. For two adjacent blocks, we show that tensor network calculations agree with universal conformal field theory (CFT) predictions. In the case of two disjoint blocks the CFT predictions are recovered only after taking into account the finite size corrections induced by the finite length of the blocks.

3. [|arXiv:1302.0899] [[|pdf], [|ps], [|other]] Entanglement Entropy as a Portal to the Physics of Quantum Spin Liquids [|Tarun Grover], [|Yi Zhang], [|Ashvin Vishwanath] Quantum Spin Liquids (QSLs) are phases of interacting spins that do not order even at the absolute zero temperature, making it impossible to characterize them by a local order parameter. In this article, we review the unique view provided by the quantum entanglement on QSLs. We illustrate the crucial role of Topological Entanglement Entropy in diagnosing the non-local order in QSLs, using specific examples such as the Chiral Spin Liquid. We also demonstrate the detection of anyonic quasiparticles and their braiding statistics using quantum entanglement. In the context of gapless QSLs, we discuss the detection of emergent fermionic spinons in a bosonic wavefunction, by studying the size dependence of entanglement entropy.

Feb 5
1. [|arXiv:1302.0701] [[|pdf], [|other]] Majorana edge states in two atomic wires coupled by pair-hopping [|Christina V. Kraus], [|Marcello Dalmonte], [|Mikhail A. Baranov], [|Andreas M. Laeuchli], [|P. Zoller] We present evidence for the existence of Majorana edge states in a number conserving theory describing a system of spinless fermions on two wires that are coupled by a pair hopping. Our analysis is based on the combination of a qualitative low energy approach and numerical techniques using the Density Matrix Renormalization Group. We also discuss an experimental realization of pair-hopping interactions in cold atom gases confined in optical lattices, and its possible alternative applications to quantum simulation.

2. [|arXiv:1302.0559] [[|pdf], [|ps], [|other]] Many-body \textit{T}-matrix theory of a strongly interacting spin-orbit coupled Fermi gas: Momentum-resolved radio-frequency spectroscopy and fermionic pairing [|Hui Hu], [|Han Pu], [|Jing Zhang], [|Xia-Ji Liu] Interacting Fermi gases with spin-orbit coupling are responsible for many intriguing phenomena such as topological superfluids and Majorana fermions. Here we characterize theoretically fermionic pairing in a strongly interacting spin-orbit coupled Fermi gas, by using momentum-resolved radio-frequency spectroscopy. We develop a strong-coupling $T$-matrix theory and present a phase diagram near the unitary resonance limit. A smooth transition from atomic to molecular responses in the momentum-resolved spectroscopy is predicted, with a clear signature of anisotropic pairing at and below resonance. Our prediction with many-body pairing can be directly tested in a spin-orbit coupled Fermi gas of $^{40}$K or $^{6}$Li atoms near broad Feshbach resonances.

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