[ { "text": "Feshbach resonances of harmonically trapped atoms: Employing a short-range two-channel description we derive an analytic model\nof atoms in isotropic and anisotropic harmonic traps at a Feshbach resonance.\nOn this basis we obtain a new parameterization of the energy-dependent\nscattering length which differs from the one previously employed. We validate\nthe model by comparison to full numerical calculations for Li-Rb and explain\nquantitatively the experimental observation of a resonance shift and\ntrap-induced molecules in exited bands. Finally, we analyze the bound state\nadmixture and Landau-Zener transition probabilities.", "category": "cond-mat_quant-gas" }, { "text": "Curving the space by non-Hermiticity: Quantum systems are often classified into Hermitian and non-Hermitian ones.\nExtraordinary non-Hermitian phenomena, ranging from the non-Hermitian skin\neffect to the supersensitivity to boundary conditions, have been widely\nexplored. Whereas these intriguing phenomena have been considered peculiar to\nnon-Hermitian systems, we show that they can be naturally explained by a\nduality between non-Hermitian models in flat spaces and their counterparts,\nwhich could be Hermitian, in curved spaces. For instance, prototypical\none-dimensional (1D) chains with uniform chiral tunnelings are equivalent to\ntheir duals in two-dimensional (2D) hyperbolic spaces with or without magnetic\nfields, and non-uniform tunnelings could further tailor local curvatures. Such\na duality unfolds deep geometric roots of non-Hermitian phenomena, delivers an\nunprecedented routine connecting Hermitian and non-Hermitian physics, and gives\nrise to a theoretical perspective reformulating our understandings of\ncurvatures and distance. In practice, it provides experimentalists with a\npowerful two-fold application, using non-Hermiticity as a new protocol to\nengineer curvatures or implementing synthetic curved spaces to explore\nnon-Hermitian quantum physics.", "category": "cond-mat_quant-gas" }, { "text": "Two-dimensional dynamics of expansion of a degenerate Bose gas: Expansion of a degenerate Bose gas released from a pancakelike trap is\nnumerically simulated under the assumption of separation of the motion in the\nplane of the loose initial trapping and the motion in the direction of the\ninitial tight trapping. The initial conditions for the phase fluctuations are\ngenerated using the extension to the two-dimensional case of the description of\nthe phase noise by the Ornstein-Uhlenbeck stochastic process. The numerical\nsimulations, taking into account both the finite size of the two-dimensional\nsystem and the atomic interactions, which cannot be neglected on the early\nstage of expansion, did not reproduce the scaling law for the peaks in the\ndensity fluctuation spectra experimentally observed by Choi, Seo, Kwon, and\nShin [Phys. Rev. Lett. 109, 125301 (2012)]. The latter experimental results may\nthus require an explanation beyond our current assumptions.", "category": "cond-mat_quant-gas" }, { "text": "Spin 1 microcondensate in a magnetic field: semiclassics and exact\n solution: We study a spin 1 Bose condensate small enough to be treated as a single\nmagnetic `domain': a system that we term a microcondensate. Because all\nparticles occupy a single spatial mode, this quantum many body system has a\nwell defined classical limit consisting of three degrees of freedom,\ncorresponding to the three macroscopically occupied spin states. We study both\nthe classical limit and its quantization, finding an integrable system in both\ncases. Depending on the sign of the ratio of the spin interaction energy and\nthe quadratic Zeeman energy, the classical limit displays either a separartrix\nin phase space, or Hamiltonian monodromy corresponding to non-trivial phase\nspace topology. We discuss the quantum signatures of these classical phenomena\nusing semiclassical quantization as well as an exact solution using the Bethe\nansatz.", "category": "cond-mat_quant-gas" }, { "text": "Multi-wavelength holography with a single Spatial Light Modulator for\n ultracold atom experiments: We demonstrate a method to create arbitrary intensity distributions of\nmultiple wavelengths of light, which can be useful for ultracold atom\nexperiments, by using regional phase-calculation algorithms to find a single\nhologram which is illuminated with overlapped laser beams. The regionality of\nthe algorithms is used to program spatially distinct features in the calculated\nintensity distribution, which then overlap in the Fourier plane due to the\ndependence of diffraction angle on wavelength. This technique is easily\nintegrated into cold atom experiments, requiring little optical access. We\ndemonstrate the method and two possible experimental scenarios by generating\nlight patterns with 670nm, 780nm and 1064nm laser light which are accurate to\nthe level of a few percent.", "category": "cond-mat_quant-gas" }, { "text": "Thermalized Abrikosov lattices from decaying turbulence in rotating BECs: We study the long-time decay of rotating turbulence in Bose-Einstein\ncondensates (BECs). We consider the Gross-Pitaevskii equation in a rotating\nframe of reference, and review different formulations for the Hamiltonian of a\nrotating BEC. We discuss how the energy can be decomposed, and present a method\nto generate out-of-equilibrium initial conditions. We also present a method to\ngenerate finite-temperature states of rotating BECs compatible with the\nCanonical or the Grand canonical ensembles. Finally, we integrate numerically\nrotating BECs in cigar-shaped traps. A transition is found in the system\ndynamics as the rotation rate is increased, with a final state of the decay of\nthe turbulent flow compatible with an Abrikosov lattice in a finite-temperature\nthermalized state.", "category": "cond-mat_quant-gas" }, { "text": "Ultracold Atomic Gases: Novel States of Matter: Article to appear in the Encyclopedia of Complexity and Systems Science, Dr.\nR. A. Meyers (Ed.) (Springer Heidelberg, 2009).", "category": "cond-mat_quant-gas" }, { "text": "Faraday patterns in spin-orbit coupled Bose-Einstein condensates: We study the Faraday patterns generated by spin-orbit-coupling induced\nparametric resonance in a spinor Bose-Einstein condensate with repulsive\ninteraction. The collective elementary excitations of the Bose-Einstein\ncondensate, including density waves and spin waves, are coupled as the result\nof the Raman-induced spin-orbit coupling and a quench of the relative phase of\ntwo Raman lasers without the modulation of any of the system's parameters. We\nobserved several higher parametric resonance tongues at integer multiples of\nthe driving frequency and investigated the interplay between Faraday\ninstabilities and modulation instabilities when we quench the\nspin-orbit-coupled Bose-Einstein condensate from zero-momentum phase to\nplane-wave phase. If the detuning is equal to zero, the wave number of\ncombination resonance barely changes as the strength of spin-orbit coupling\nincreases. If the detuning is not equal to zero after a quench, a single\ncombination resonance tongue will split into two parts.", "category": "cond-mat_quant-gas" }, { "text": "Entanglement prethermalization: Locally thermal but non-locally\n non-thermal states in a one-dimensional Bose gas: A well-isolated system often shows relaxation to a quasi-stationary state\nbefore reaching thermal equilibrium. Such a prethermalization has attracted\nconsiderable interest recently in association with closely related fundamental\nproblems of relaxation and thermalization of isolated quantum systems.\nMotivated by the recent experiment in ultracold atoms, we study the dynamics of\na one-dimensional Bose gas which is split into two subsystems, and find that\nindividual subsystems relax to Gibbs states, yet the entire system does not due\nto quantum entanglement. In view of recent experimental realization on a small\nwell-defined number of ultracold atoms, our prediction based on exact few-body\ncalculations is amenable to experimental test.", "category": "cond-mat_quant-gas" }, { "text": "Persistent oscillations of the order parameter and interaction quench\n phase diagram for a confined Bardeen-Cooper-Schrieffer Fermi gas: We present a numerical study of the interaction quench dynamics in a\nsuperfluid ultracold Fermi gas confined in a three-dimensional cigar-shaped\nharmonic trap. In the present paper we investigate the amplitude mode of the\nsuperfluid order parameter after interaction quenches which start deep in the\nBCS phase and end in the BCS-BEC crossover regime. To this end, we exploit the\nBogoliubov-de Gennes formalism which takes the confinement potential explicitly\ninto account and provides a microscopic fully coherent description of the\nsystem. We find an anharmonic nonlinear oscillation of the modulus of the\nsuperfluid order parameter, i.e., of the Higgs mode. This oscillation persists\nfor large times with only a small amplitude modulation being visible. We\nconnect the frequency and the mean value of this oscillation with the breaking\nof Cooper pairs in the superfluid phase. Additionally, we demonstrate that the\noccurrence of this persistent oscillation is connected to the onset of chaotic\ndynamics in our model. Finally, we calculate an interaction quench phase\ndiagram of the Higgs mode for quenches on the BCS side of the BCS-BEC crossover\nand discuss its properties as a function of the aspect ratio of the\ncigar-shaped trap.", "category": "cond-mat_quant-gas" }, { "text": "Stability of Excited Dressed States with Spin-Orbit Coupling: We study the decay behaviors of ultracold atoms in metastable states with\nspin-orbit coupling (SOC), and demonstrate that there are two SOC-induced decay\nmechanisms. One arises from the trapping potential and the other is due to\ninteratomic collision. We present general schemes for calculating decay rates\nfrom these two mechanisms, and illustrate how the decay rates can be controlled\nby experimental parameters.We experimentally measure the decay rates over a\nbroad parameter region, and the results agree well with theoretical\ncalculations. This work provides an insight for both quantum simulation\ninvolving metastable dressed states and studies on few-body problems with SO\ncoupling.", "category": "cond-mat_quant-gas" }, { "text": "Quantum theory of bright matter wave solitons in harmonic confinement: This paper investigates bright quantum-matter-wave solitons beyond the\nGross-Pitaevskii equation (GPE). As proposals for interferometry and creating\nnonlocal quantum superpositions have been formed, it has become necessary to\ninvestigate effects not present in mean-field models. We investigate the effect\nof harmonic confinement on the internal degrees of freedom, as the ratio of\nzero-point harmonic oscillator length to classical soliton length, for\ndifferent numbers of atoms. We derive a first-order energy correction for the\naddition of a harmonic potential to the many-body wave function and use this to\ncreate a variational technique based on energy minimization of this wave\nfunction for an arbitrary number of atoms, and include numerics based on\ndiagonalization of the Hamiltonian in a basis of harmonic oscillator Fock\nstates. Finally we compare agreement between a Hartree product ground state and\nthe Bethe ansatz solution with a Gaussian envelope localizing the center of\nmass and show a region of good agreement.", "category": "cond-mat_quant-gas" }, { "text": "Exact solutions to the four Goldstone modes around a dark soliton of the\n nonlinear Schroedinger equation: This article is concerned with the linearisation around a dark soliton\nsolution of the nonlinear Schr\\\"odinger equation. Crucially, we present\nanalytic expressions for the four linearly-independent zero eigenvalue\nsolutions (also known as Goldstone modes) to the linearised problem. These\nsolutions are then used to construct a Greens matrix which gives the\nfirst-order spatial response due to some perturbation. Finally we apply this\nGreens matrix to find the correction to the dark-soliton wavefunction of a\nBose-Einstein condensate in the presence of fluctuations.", "category": "cond-mat_quant-gas" }, { "text": "Interaction induced dynamical $\\mathcal{PT}$ symmetry breaking in\n dissipative Fermi-Hubbard models: We investigate the dynamical properties of one-dimensional dissipative\nFermi-Hubbard models, which are described by the Lindblad master equations with\nsite-dependent jump operators. The corresponding non-Hermitian effective\nHamiltonians with pure loss terms possess parity-time ($\\mathcal{PT}$) symmetry\nif we compensate the system additionally an overall gain term. By solving the\ntwo-site Lindblad equation with fixed dissipation exactly, we find that the\ndynamics of rescaled density matrix shows an instability as the interaction\nincreases over a threshold, which can be equivalently described in the scheme\nof non-Hermitian effective Hamiltonians. This instability is also observed in\nmulti-site systems and closely related to the $\\mathcal{PT}$ symmetry breaking\naccompanied by appearance of complex eigenvalues of the effective Hamiltonian.\nMoreover, we unveil that the dynamical instability of the anti-ferromagnetic\nMott phase comes from the $\\mathcal{PT}$ symmetry breaking in highly excited\nbands, although the low-energy effective model of the non-Hermitian Hubbard\nmodel in the strongly interacting regime is always Hermitian. We also provide a\nquantitative estimation of the time for the observation of dynamical\n$\\mathcal{PT}$ symmetry breaking which could be probed in experiments.", "category": "cond-mat_quant-gas" }, { "text": "Coherent Interaction of a Single Fermion with a Small Bosonic Field: We have experimentally studied few-body impurity systems consisting of a\nsingle fermionic atom and a small bosonic field on the sites of an optical\nlattice. Quantum phase revival spectroscopy has allowed us to accurately\nmeasure the absolute strength of Bose-Fermi interactions as a function of the\ninterspecies scattering length. Furthermore, we observe the modification of\nBose-Bose interactions that is induced by the interacting fermion. Because of\nan interference between Bose-Bose and Bose-Fermi phase dynamics, we can infer\nthe mean fermionic filling of the mixture and quantify its increase (decrease)\nwhen the lattice is loaded with attractive (repulsive) interspecies\ninteractions.", "category": "cond-mat_quant-gas" }, { "text": "Asymmetric Particle Transport and Light-Cone Dynamics Induced by Anyonic\n Statistics: We study the non-equilibrium dynamics of Abelian anyons in a one-dimensional\nsystem. We find that the interplay of anyonic statistics and interactions gives\nrise to spatially asymmetric particle transport together with a novel dynamical\nsymmetry that depends on the anyonic statistical angle and the sign of\ninteractions. Moreover, we show that anyonic statistics induces asymmetric\nspreading of quantum information, characterized by asymmetric light cones of\nout-of-time-ordered correlators. Such asymmetric dynamics is in sharp contrast\nwith the dynamics of conventional fermions or bosons, where both the transport\nand information dynamics are spatially symmetric. We further discuss\nexperiments with cold atoms where the predicted phenomena can be observed using\nstate-of-the-art technologies. Our results pave the way toward experimentally\nprobing anyonic statistics through non-equilibrium dynamics.", "category": "cond-mat_quant-gas" }, { "text": "Spin and mass currents near a moving magnetic obstacle in a\n two-component Bose-Einstein condensate: We study the spatial distributions of the spin and mass currents generated by\na moving Gaussian magnetic obstacle in a symmetric, two-component Bose-Einstein\ncondensate in two dimensions. We analytically describe the current\ndistributions for a slow obstacle and show that the spin and the mass currents\nexhibit characteristic spatial structures resembling those of electromagnetic\nfields around dipole moments. When the obstacle's velocity increases, we\nnumerically observe that the flow pattern maintains its overall structure while\nthe spin polarization induced by the obstacle is enhanced with an increased\nspin current. We investigate the critical velocity of the magnetic obstacle\nbased on the local criterion of Landau energetic instability and find that it\ndecreases almost linearly as the magnitude of the obstacle's potential\nincreases, which can be directly tested in current experiments.", "category": "cond-mat_quant-gas" }, { "text": "Many-body approach to low-lying collective excitations in a BEC\n approaching collapse: An approximate many-body theory incorporating two-body correlations has been\nemployed to calculate low-lying collective multipole frequencies in a\nBose-Einstein condensate containing $A$ bosons, for different values of the\ninteraction parameter $\\lambda=\\frac{Aa_{s}}{a_{ho}}$. Significant difference\nfrom the variational estimate of the Gross-Pitaevskii equation has been found\nnear the collapse region. This is attributed to two-body correlations and\nfinite range attraction of the realistic interatomic interaction. A large\ndeviation from the hydrodynamic model is also seen for the second monopole\nbreathing mode and the quadrupole mode for large positive $\\lambda$.", "category": "cond-mat_quant-gas" }, { "text": "Optical lattice for tripod-like atomic level structure: Standard optical potentials use off-resonant laser standing wave induced\nAC-Stark shift. In a recent development [Phys. Rev. Lett. {\\bf 117}, 233001\n(2016)] a three-level scheme in $\\Lambda$ configuration coupled coherently by\nresonant laser fields was introduced leading to an effective lattice with\nsubwavelength potential peaks. Here as an extension of that work to a four\nlevel atomic setup in the tripod configuration is used to create spin\n$1/2$-like two-dimensional dark-space with 1D motion and the presence of\nexternal gauge fields. Most interestingly for a possible application, the\nlifetime for a dark subspace motion is up to two orders of magnitude larger\nthan for a similar $\\Lambda$ system. The model is quite flexible leading to\nlattices with significant nearest, next-nearest, or next-next-nearest hopping\nrates, $J_1,J_2,J_3$ opening up new intriguing possibilities to study, e.g.\nfrustrated systems. The characteristic Wannier functions lead also to new type\nof inter-site interactions not realizable in typical optical lattices.", "category": "cond-mat_quant-gas" }, { "text": "Quantum-granularity effect in the formation of supermixed solitons in\n ring lattices: We investigate a notable class of states peculiar to a bosonic binary mixture\nfeaturing repulsive intraspecies and attractive interspecies couplings. We\nevidence that, for small values of the hopping amplitudes, one can access\nparticular regimes marked by the fact that the interwell boson transfer occurs\nin a jerky fashion. This property is shown to be responsible for the emergence\nof a staircase-like structure in the phase diagram of a mixture confined in a\nring trimer and to strongly resemble the mechanism of the superfluid-Mott\ninsulator transition. Under certain conditions, in fact, we show that it is\npossible to interpret the interspecies attraction as an effective chemical\npotential and the supermixed soliton as an effective particle reservoir. Our\ninvestigation is developed both within a fully quantum approach based on the\nanalysis of several quantum indicators and by means of a simple analytical\napproximation scheme capable of capturing the essential features of this\nultraquantum effect.", "category": "cond-mat_quant-gas" }, { "text": "Quantum-torque-induced breaking of magnetic interfaces in ultracold\n gases: A rich variety of physical effects in spin dynamics arises at the interface\nbetween different magnetic materials. Engineered systems with interlaced\nmagnetic structures have been used to implement spin transistors, memories and\nother spintronic devices. However, experiments in solid state systems can be\ndifficult to interpret because of disorder and losses. Here, we realize\nanalogues of magnetic junctions using a coherently-coupled mixture of ultracold\nbosonic gases. The spatial inhomogeneity of the atomic gas makes the system\nchange its behavior from regions with oscillating magnetization -- resembling a\nmagnetic material in the presence of an external transverse field -- to regions\nwith a defined magnetization, as in magnetic materials with a ferromagnetic\nanisotropy stronger than external fields. Starting from a far-from-equilibrium\nfully polarized state, magnetic interfaces rapidly form. At the interfaces, we\nobserve the formation of short-wavelength magnetic waves. They are generated by\na quantum torque contribution to the spin current and produce strong spatial\nanticorrelations in the magnetization. Our results establish ultracold gases as\na platform for the study of far-from-equilibrium spin dynamics in regimes that\nare not easily accessible in solid-state systems.", "category": "cond-mat_quant-gas" }, { "text": "Tunable Wigner States with Dipolar Atoms and Molecules: We study the few-body physics of trapped atoms or molecules with electric or\nmagnetic dipole moments aligned by an external field. Using exact numerical\ndiagonalization appropriate for the strongly correlated regime, as well as a\nclassical analysis, we show how Wigner localization emerges with increasing\ncoupling strength. The Wigner states exhibit non-trivial geometries due to the\nanisotropy of the interaction. This leads to transitions between different\nWigner states as the tilt angle of the dipoles with the confining plane is\nchanged. Intriguingly, while the individual Wigner states are well described by\na classical analysis, the transitions between different Wigner states are\nstrongly affected by quantum statistics. This can be understood by considering\nthe interplay between quantum-mechanical and spatial symmetry properties.\nFinally, we demonstrate that our results are relevant to experimentally\nrealistic systems.", "category": "cond-mat_quant-gas" }, { "text": "Dissipation-induced dynamical phase transition in postselected quantum\n trajectories: It is known that effects of dissipation or measurement backreaction in\npostselected quantum trajectories are described by non-Hermitian Hamiltonian,\nbut their consequences in real-time dynamics of many-body systems are yet to be\nelucidated. Through a study of a non-Hermitian Hubbard model, we reveal a novel\ndissipation-induced dynamical phase transition in postselected quantum\ntrajectories, where time controls the strength of postselection and becomes the\nintrinsic parameter inducing the phase transition. Our findings are testable in\nultracold atom experiments and may open a new avenue in the dissipative\nengineering of quantum systems.", "category": "cond-mat_quant-gas" }, { "text": "Feynman path-integral treatment of the Bose polaron beyond the\n Fr\u00f6hlich model: An impurity immersed in a Bose-Einstein condensate is no longer accurately\ndescribed by the Fr\\\"ohlich Hamiltonian as the coupling between the impurity\nand the boson bath gets stronger. We study the dominant effects of the\ntwo-phonon terms beyond the Fr\\\"ohlich model on the ground-state properties of\nthe polaron using Feynman's variational path-integral approach. The previously\nreported discrepancy in the effective mass between the renormalization group\napproach and this theory is shown to be absent in the beyond-Fr\\\"ohlich model\non the positive side of the Feshbach resonance. Self-trapping, characterized by\na sharp and dramatic increase of the effective mass, is no longer observed for\nthe repulsive polaron once the two-phonon interactions are included. For the\nattractive polaron we find a divergence of the ground-state energy and\neffective mass at weaker couplings than previously observed within the\nFr\\\"ohlich model.", "category": "cond-mat_quant-gas" }, { "text": "Measuring molecular electric dipoles using trapped atomic ions and\n ultrafast laser pulses: We study a hybrid quantum system composed of an ion and an electric dipole.\nWe show how a trapped ion can be used to measure the small electric field\ngenerated by a classical dipole. We discuss the application of this scheme to\nmeasure the electric dipole moment of cold polar molecules, whose internal\nstate can be controlled with ultrafast laser pulses, by trapping them in the\nvicinity of a trapped ion.", "category": "cond-mat_quant-gas" }, { "text": "Finite-size effects on the Bose-Einstein condensation critical\n temperature in a harmonic trap: We obtain second and higher order corrections to the shift of the\nBose-Einstein critical temperature due to finite-size effects. The confinement\nis that of a harmonic trap with general anisotropy. Numerical work shows the\nhigh accuracy of our expressions. We draw attention to a subtlety involved in\nthe consideration of experimental values of the critical temperature in\nconnection with analytical expressions for the finite-size corrections.", "category": "cond-mat_quant-gas" }, { "text": "Controlling particle current in a many-body quantum system by external\n driving: We propose a method to control the particle current of a one-dimensional\nquantum system by resonating two many-body states through an external driving\nfield. We consider the Bose-Hubbard and spinless Fermi-Hubbard models with the\nPeierls phase which induces net particle currents in the many-body eigenstates.\nA driving field couples the ground state with one of the excited states having\nlarge net currents, enabling us to control the system's current via Rabi\noscillation. Employing the Floquet analysis, we find that the resonate excited\nstates are determined by the symmetry of the driving field, which allows us to\nselectively excite only certain states among the dense spectrum of a many-body\nquantum system.", "category": "cond-mat_quant-gas" }, { "text": "Few-to-many vortex states of density-angular-momentum coupled\n Bose-Einstein condensates: Motivated by recent experiments, we theoretically study a gas of atomic\nbosons confined in an elliptical harmonic trap; forming a quasi-two-dimensional\natomic Bose-Einstein condensate subject to a density-dependent gauge potential\nwhich realises an effective density-angular-momentum coupling. We present exact\nThomas-Fermi solutions which allows us to identify the stable regimes of the\nfull parameter space of the model. Accompanying numerical simulations reveal\nthe effect of the interplay of the rigid body and density-angular-momentum\ncoupling for the elliptically confined condensate. By varying the strength of\nthe gauge potential and trap anisotropy we explore how the superfluid state\nemerges in different experimentally accessible geometries, while for large\nrotation strengths dense vortex lattices and concentric vortex ring\narrangements are obtained.", "category": "cond-mat_quant-gas" }, { "text": "Orbital superfluidity in the $P$-band of a bipartite optical square\n lattice: The successful emulation of the Hubbard model in optical lattices has\nstimulated world wide efforts to extend their scope to also capture more\ncomplex, incompletely understood scenarios of many-body physics. Unfortunately,\nfor bosons, Feynmans fundamental \"no-node\" theorem under very general\ncircumstances predicts a positive definite ground state wave function with\nlimited relevance for many-body systems of interest. A promising way around\nFeynmans statement is to consider higher bands in optical lattices with more\nthan one dimension, where the orbital degree of freedom with its intrinsic\nanisotropy due to multiple orbital orientations gives rise to a structural\ndiversity, highly relevant, for example, in the area of strongly correlated\nelectronic matter. In homogeneous two-dimensional optical lattices, lifetimes\nof excited bands on the order of a hundred milliseconds are possible but the\ntunneling dynamics appears not to support cross-dimensional coherence. Here we\nreport the first observation of a superfluid in the $P$-band of a bipartite\noptical square lattice with $S$-orbits and $P$-orbits arranged in a\nchequerboard pattern. This permits us to establish full cross-dimensional\ncoherence with a life-time of several ten milliseconds. Depending on a small\nadjustable anisotropy of the lattice, we can realize real-valued striped\nsuperfluid order parameters with different orientations $P_x \\pm P_y$ or a\ncomplex-valued $P_x \\pm i P_y$ order parameter, which breaks time reversal\nsymmetry and resembles the $\\pi$-flux model proposed in the context of high\ntemperature superconductors. Our experiment opens up the realms of orbital\nsuperfluids to investigations with optical lattice models.", "category": "cond-mat_quant-gas" }, { "text": "Buckling transitions and clock order of two-dimensional Coulomb crystals: Crystals of repulsively interacting ions in planar traps form hexagonal\nlattices, which undergo a buckling instability towards a multi-layer structure\nas the transverse trap frequency is reduced. Numerical and experimental results\nindicate that the new structure is composed of three planes, whose separation\nincreases continuously from zero. We study the effects of thermal and quantum\nfluctuations by mapping this structural instability to the six-state clock\nmodel. A prominent implication of this mapping is that at finite temperature,\nfluctuations split the buckling instability into two thermal transitions,\naccompanied by the appearance of an intermediate critical phase. This phase is\ncharacterized by quasi-long-range order in the spatial tripartite pattern. It\nis manifested by broadened Bragg peaks at new wave vectors, whose line-shape\nprovides a direct measurement of the temperature dependent exponent $\\eta(T)$\ncharacteristic of the power-law correlations in the critical phase. A quantum\nphase transition is found at the largest value of the critical transverse\nfrequency: here the critical intermediate phase shrinks to zero. Moreover,\nwithin the ordered phase, we predict a crossover from classical to quantum\nbehavior, signifying the emergence of an additional characteristic scale for\nclock order. We discuss experimental realizations with trapped ions and\npolarized dipolar gases, and propose that within accessible technology, such\nexperiments can provide a direct probe of the rich phase diagram of the quantum\nclock model, not easily observable in condensed matter analogues. Therefore,\nthis works highlights the potential for ionic and dipolar systems to serve as\nsimulators for complex models in statistical mechanics and condensed matter\nphysics.", "category": "cond-mat_quant-gas" }, { "text": "Spin-orbit coupling in the presence of strong atomic correlations: We explore the influence of contact interactions on a synthetically\nspin-orbit coupled system of two ultracold trapped atoms. Even though the\nsystem we consider is bosonic, we show that a regime exists in which the\ncompetition between the contact and spin-orbit interactions results in the\nemergence of a ground state that contains a significant contribution from the\nanti-symmetric spin state. This ground state is unique to few-particle systems\nand does not exist in the mean-field regime. The transition to this state is\nsignalled by an inversion in the average momentum from being dominated by\ncentre-of-mass momentum to relative momentum and also affects the global\nentanglement shared between the real- and pseudo-spin spaces. Indeed,\ncompetition between the interactions can also result in avoided crossings in\nthe groundstate which further enhances these correlations. However, we find\nthat correlations shared between the pseudo-spin states are strongly depressed\ndue to the spin-orbit coupling and therefore the system does not contain\nspin-spin entanglement.", "category": "cond-mat_quant-gas" }, { "text": "Generalized Hydrodynamics in the 1D Bose gas: theory and experiments: We review the recent theoretical and experimental progress regarding the\nGeneralized Hydrodynamics (GHD) behavior of the one-dimensional Bose gas with\ncontact repulsive interactions, also known as the Lieb-Liniger gas. In the\nfirst section, we review the theory of the Lieb-Liniger gas, introducing the\nkey notions of the rapidities and of the rapidity distribution. The latter\ncharacterizes the Lieb-Liniger gas after relaxation and is at the heart of GHD.\nWe also present the asymptotic regimes of the Lieb-Liniger gas with their\ndedicated approximate descriptions. In the second section we enter the core of\nthe subject and review the theoretical results on GHD in 1D Bose gases. The\nthird and fourth sections are dedicated to experimental results obtained in\ncold atoms experiments: the experimental realization of the Lieb-Liniger model\nis presented in section 3, with a selection of key results for systems at\nequilibrium, and section 4 presents the experimental tests of the GHD theory.\nIn section 5 we review the effects of atom losses, which, assuming slow loss\nprocesses, can be described within the GHD framework. We conclude with a few\nopen questions.", "category": "cond-mat_quant-gas" }, { "text": "Multiorder topological superfluid phase transitions in a two-dimensional\n optical superlattice: Higher-order topological superfluids have gapped bulk and symmetry-protected\nMajorana zero modes with various localizations. Motivated by recent advances,\nwe present a proposal for synthesizing multi-order topological superfluids that\nsupport various Majorana zero modes in ultracold atomic gases. For this\npurpose, we use the two-dimensional optical superlattice that introduces a\nspatial modulation to the spin-orbit coupling in one direction, providing an\nextra degree of freedom for the emergent higher-order topological state. We\nfind the topologically trivial superfluids, first-order and second-order\ntopological superfluids, as well as different topological phase transitions\namong them with respect to the experimentally tunable parameters. Besides the\nconventional transition characterized by the Chern number associated with the\nbulk gap closing and reopening, we find the system can support the topological\nsuperfluids with Majorana corner modes, but the topological phase transition\nundergoes no gap-closing of bulk bands. Instead, the transition is refined by\nthe quadrupole moment and signaled out by the gap-closing of edge states. The\nproposal is based on the $s$-wave interaction and is valid using existing\nexperimental techniques, which unifies multi-order topological phase\ntransitions in a simple but realistic system.", "category": "cond-mat_quant-gas" }, { "text": "Dipolar fermions in a multilayer geometry: We investigate the behavior of identical dipolar fermions with aligned dipole\nmoments in two-dimensional multilayers at zero temperature. We consider density\ninstabilities that are driven by the attractive part of the dipolar interaction\nand, for the case of bilayers, we elucidate the properties of the stripe phase\nrecently predicted to exist in this interaction regime. When the number of\nlayers is increased, we find that this \"attractive\" stripe phase exists for an\nincreasingly larger range of dipole angles, and if the interlayer distance is\nsufficiently small, the stripe phase eventually spans the full range of angles,\nincluding the situation where the dipole moments are aligned perpendicular to\nthe planes. In the limit of an infinite number of layers, we derive an analytic\nexpression for the interlayer effects in the density-density response function\nand, using this result, we find that the stripe phase is replaced by a collapse\nof the dipolar system.", "category": "cond-mat_quant-gas" }, { "text": "Flowing bosonization in the nonperturbative functional\n renormalization-group approach: Bosonization allows one to describe the low-energy physics of one-dimensional\nquantum fluids within a bosonic effective field theory formulated in terms of\ntwo fields: the \"density\" field $\\varphi$ and its conjugate partner, the phase\n$\\vartheta$ of the superfluid order parameter. We discuss the implementation of\nthe nonperturbative functional renormalization group in this formalism,\nconsidering a Luttinger liquid in a periodic potential as an example. We show\nthat in order for $\\vartheta$ and $\\varphi$ to remain conjugate variables at\nall energy scales, one must dynamically redefine the field $\\vartheta$ along\nthe renormalization-group flow. We derive explicit flow equations using a\nderivative expansion of the scale-dependent effective action to second order\nand show that they reproduce the flow equations of the sine-Gordon model\n(obtained by integrating out the field $\\vartheta$ from the outset) derived\nwithin the same approximation. Only with the scale-dependent (flowing)\nreparametrization of the phase field $\\vartheta$ do we obtain the standard\nphenomenology of the Luttinger liquid (when the periodic potential is\nsufficiently weak so as to avoid the Mott-insulating phase) characterized by\ntwo low-energy parameters, the velocity of the sound mode and the renormalized\nLuttinger parameter.", "category": "cond-mat_quant-gas" }, { "text": "Emergent patterns in a spin-orbit coupled spin-2 Bose-Einstein\n condensate: The ground-state phases of a spin-orbit (SO) coupled atomic spin-2\nBose-Einstein condensate (BEC) are studied. Interesting density patterns\nspontaneously formed are widespread due to the competition between SO coupling\nand spin-dependent interactions like in a SO coupled spin-1 condensate. Unlike\nthe case of spin-1 condensates, which are characterized by either ferromagnetic\nor polar phase in the absence of SO, spin-2 condensates can take a cyclic\nphase, where we find the patterns formed due to SO are square or triangular in\ntheir spin component densities for axial symmetric SO interaction. Both\npatterns are found to continuously evolve into striped forms with increased\nasymmetry of the SO coupling.", "category": "cond-mat_quant-gas" }, { "text": "Coherent phase slips in coupled matter-wave circuits: Quantum Phase slips are dual process of particle tunneling in coherent\nnetworks. Besides to be of central interest for condensed matter physics,\nquantum phase slips are resources that are sought to be manipulated in quantum\ncircuits. Here, we devise a specific matter-wave circuit enlightening quantum\nphase slips. Specifically, we investigate the quantum many body dynamics of two\nside-by-side ring-shaped neutral bosonic systems coupled through a weak link.\nBy imparting a suitable magnetic flux, persistent currents flow in each ring\nwith given winding numbers. We demonstrate that coherent phase slips occur as\nwinding number transfer among the two rings, with the populations in each ring\nremaining nearly constant. Such a phenomenon occurs as a result of a specific\nentanglement of circulating states, that, as such cannot be captured by a mean\nfield treatment of the system. Our work can be relevant for the observation of\nquantum phase slips in cold atoms experiments and their manipulation in\nmatter-wave circuits. To make contact with the field, we show that the\nphenomenon has clear signatures in the momentum distribution of the system\nproviding the time of flight image of the condensate.", "category": "cond-mat_quant-gas" }, { "text": "BCS-BEC crossover at finite temperature in spin-orbit coupled Fermi\n gases: By adopting a $T$-matrix-based method within the $G_0G$ approximation for the\npair susceptibility, we study the effects of the pairing fluctuation on the\nthree-dimensional spin-orbit coupled Fermi gases at finite temperature. The\ncritical temperatures of the superfluid/normal phase transition are determined\nfor three different types of spin-orbit coupling (SOC): (1) the extreme oblate\n(EO) or Rashba SOC, (2) the extreme prolate (EP) or equal Rashba-Dresselhaus\nSOC, and (3) the spherical (S) SOC. For EO- and S-type SOC, the SOC dependence\nof the critical temperature signals a crossover from BCS to BEC state; at\nstrong SOC limit, the critical temperature recover those of ideal BEC of\nrashbons. The pairing fluctuation induces a pseudogap in the fermionic\nexcitation spectrum in both superfluid and normal phases. We find that, for EO-\nand S-type SOC, even at weak coupling, sufficiently strong SOC can induce\nsizable pseudogap. Our research suggests that the spin-orbit coupled Fermi\ngases may open new means to the study of the pseudogap formation in fermionic\nsystems.", "category": "cond-mat_quant-gas" }, { "text": "Superdiffusive nonequilibrium motion of an impurity in a Fermi sea: We treat the nonequilibrium motion of a single impurity atom in a\nlow-temperature single-species Fermi sea, interacting via a contact\ninteraction. In the nonequilibrium regime, the impurity does a superdiffusive\ngeometric random walk where the typical distance traveled grows with time as\n$\\sim t^{d/(d+1)}$ for the $d$-dimensional system with $d\\geq 2$. For nonzero\ntemperature $T$, this crosses over to diffusive motion at long times with\ndiffusivity $D\\sim T^{-(d-1)/2}$. These results apply also to a nonzero\nconcentration of impurity atoms as long as they remain dilute and\nnondegenerate.", "category": "cond-mat_quant-gas" }, { "text": "Creating quantum many-body scars through topological pumping of a 1D\n dipolar gas: Quantum many-body scars, long-lived excited states of correlated quantum\nchaotic systems that evade thermalization, are of great fundamental and\ntechnological interest. We create novel scar states in a bosonic 1D quantum gas\nof dysprosium by stabilizing a super-Tonks-Girardeau gas against collapse and\nthermalization with repulsive long-range dipolar interactions. Stiffness and\nenergy density measurements show that the system is dynamically stable\nregardless of contact interaction strength. This enables us to cycle contact\ninteractions from weakly to strongly repulsive, then strongly attractive, and\nfinally weakly attractive. We show that this cycle is an energy-space\ntopological pump (due to a quantum holonomy). Iterating this cycle offers an\nunexplored topological pumping method to create a hierarchy of quantum\nmany-body scar states.", "category": "cond-mat_quant-gas" }, { "text": "Multi-particle composites in density-imbalanced quantum fluids: We consider two-component one-dimensional quantum gases with density\nimbalance. While generically such fluids are two-component Luttinger liquids,\nwe show that if the ratio of the densities is a rational number, p/q, and mass\nasymmetry between components is sufficiently strong, one of the two eigenmodes\nacquires a gap. The gapped phase corresponds to (algebraic) ordering of\n(p+q)-particle composites. In particular, for attractive mixtures, this implies\nthat the superconducting correlations are destroyed. We illustrate our\npredictions by numerical simulations of the fermionic Hubbard model with\nhopping asymmetry.", "category": "cond-mat_quant-gas" }, { "text": "Flux enhanced localization and reentrant delocalization in the quantum\n walk of interacting bosons on two-leg ladder: We study the quantum walk of two bosons possessing onsite repulsive\ninteraction on a two-leg ladder and show that the presence of uniform flux\npiercing through the plaquettes of the ladder favors the localization of the\nbound states in the dynamics. We find that when the two bosons are\nsymmetrically initialized on the edge rung of the ladder, they tend to\nedge-localize in their quantum walk - a phenomenon which is not possible in the\nabsence of flux. On the other hand, when the bosons are initialized on the bulk\nrung they never localize and exhibit linear spreading in their quantum walk.\nInterestingly, however, we find that in the later case a finite flux favours\nlocalization of the bulk bound states in the presence of sufficiently weak\nquasiperiodic disorder which is otherwise insufficient to localize the\nparticles in the absence of flux. In both the cases, we obtain that the\nlocalization in the dynamics strongly depends on the combined effect of the\nflux and interaction strengths, as a result which we obtain a signature of\nre-entrant delocalization as a function of flux (interaction) for fixed\ninteraction (flux) strengths.", "category": "cond-mat_quant-gas" }, { "text": "Dynamical Equilibration of Topological Properties: We study the dynamical process of equilibration of topological properties in\nquantum many-body systems undergoing a parameter quench between two\ntopologically inequivalent Hamiltonians. This scenario is motivated by recent\nexperiments on ultracold atomic gases, where a trivial initial state is\nprepared before the Hamiltonian is ramped into a topological insulator phase.\nWhile the many-body wave function must stay topologically trivial in the\ncoherent post-quench dynamics, here we show how the topological properties of\nthe single particle density matrix dynamically change and equilibrate in the\npresence of interactions. In this process, the single particle density matrix\ngoes through a characteristic level crossing as a function of time, which plays\nan analogous role to the gap closing of a Hamiltonian in an equilibrium\ntopological quantum phase transition. As an exact case study exemplifying this\nmechanism, we numerically solve the quench dynamics of an interacting\none-dimensional topological insulator.", "category": "cond-mat_quant-gas" }, { "text": "Controlling Dipolar Exchange Interactions in a Dense 3D Array of Large\n Spin Fermions: Dipolar interactions are ubiquitous in nature and rule the behavior of a\nbroad range of systems spanning from energy transfer in biological systems to\nquantum magnetism. Here, we study magnetization-conserving dipolar induced\nspin-exchange dynamics in dense arrays of fermionic erbium atoms confined in a\ndeep three-dimensional lattice. Harnessing the special atomic properties of\nerbium, we demonstrate control over the spin dynamics by tuning the dipole\norientation and changing the initial spin state within the large 20 spin\nhyperfine manifold. Furthermore, we demonstrate the capability to quickly turn\non and off the dipolar exchange dynamics via optical control. The experimental\nobservations are in excellent quantitative agreement with numerical\ncalculations based on discrete phase-space methods, which capture entanglement\nand beyond-mean field effects. Our experiment sets the stage for future\nexplorations of rich magnetic behaviors in long-range interacting dipoles,\nincluding exotic phases of matter and applications for quantum information\nprocessing.", "category": "cond-mat_quant-gas" }, { "text": "How is the density of quasi-two-dimensional uniform dipolar quantum Bose\n gases affected by trap imperfections?: We theoretically investigate the impact of weak perturbations of a flat\npotential on the density of a quasi-two-dimensional dipolar Bose gas. We use a\nmean-field perturbative treatment of the potential defects and derive their\neffects at first order in the mean-field stable regime. We first focus on\ndefects containing a single spatial frequency and study the wavevector\ndependence of the density perturbation. A qualitative modification of the\nwavenumber dependence with the interaction parameters and a sensitivity in the\nexcitation direction reveal the long-range and anisotropic dipolar effects.\nThese effects are found to be most important at intermediate wavenumbers and\ncan give rise to a local maximum in the density perturbation reminiscent of the\nroton mode softening and local instabilities. The dependence on the gas and\ninteraction parameters is studied. The case of a flat potential perturbed with\nwhite noise on a certain momentum range is then examined. Here it is found that\nthe strength perturbation becomes independent of the mean density when\nsufficiently large. Our study touches upon experimentally relevant issues,\ngiving hints on how flat a uniform potential should be to achieve uniform\nquasi-two-dimensional dipolar Bose gases.", "category": "cond-mat_quant-gas" }, { "text": "Spin conductivity spectrum and spin superfluidity in a binary Bose\n mixture: We investigate the spectrum of spin conductivity for a miscible two-component\nBose-Einstein condensate (BEC) that exhibits spin superfluidity. By using the\nBogoliubov theory, the regular part being the spin conductivity at finite ac\nfrequency and the spin Drude weight characterizing the delta-function peak at\nzero frequency are analytically computed. We demonstrate that the spectrum\nexhibits a power-law behavior at low frequency, reflecting gapless density and\nspin modes specific to the binary BEC. At the phase transition points into\nimmiscible and quantum-droplet states, the change in quasiparticle dispersion\nrelations modifies the power law. In addition, the spin Drude weight becomes\nfinite, indicating zero spin resistivity due to spin superfluidity. Our results\nalso suggest that the Andreev-Bashkin drag density is accessible by measuring\nthe spin conductivity spectrum.", "category": "cond-mat_quant-gas" }, { "text": "Singular mean-field states: A brief review of recent results: This article provides a focused review of recent findings which demonstrate,\nin some cases quite counter-intuitively, the existence of bound states with a\nsingularity of the density pattern at the center, while the states are\nphysically meaningful because their total norm converges. One model of this\ntype is based on the 2D Gross-Pitaevskii equation (GPE) which combines the\nattractive potential ~ 1/r^2 and the quartic self-repulsive nonlinearity,\ninduced by the Lee-Huang-Yang effect (quantum fluctuations around the\nmean-field state). The GPE demonstrates suppression of the 2D quantum collapse,\ndriven by the attractive potential, and emergence of a stable ground state\n(GS), whose density features an integrable singularity ~1/r^{4/3} at r --> 0.\nModes with embedded angular momentum exist too, and they have their stability\nregions. A counter-intuitive peculiarity of the model is that the GS exists\neven if the sign of the potential is reversed from attraction to repulsion,\nprovided that its strength is small enough. This peculiarity finds a relevant\nexplanation. The other model outlined in the review includes 1D, 2D, and 3D\nGPEs, with the septimal (seventh-order), quintic, and cubic self-repulsive\nterms, respectively. These equations give rise to stable singular solitons,\nwhich represent the GS for each dimension D, with the density singularity\n~1/r^{2/(4-D). Such states may be considered as a result of screening of a\n\"bare\" delta-functional attractive potential by the respective nonlinearity.", "category": "cond-mat_quant-gas" }, { "text": "Dynamics of spatial coherence and momentum distribution of polaritons in\n a semiconductor microcavity under conditions of Bose-Einstein condensation: The dynamics of spatial coherence and momentum distribution of polaritons in\nthe regime of Bose-Einstein condensation are investigated in a GaAs microcavity\nwith embedded quantum wells under nonresonant excitation with picosecond laser\npulses. It is shown that the onset of the condensate first order sparial\ncoherence is accompanied by narrowing of the polariton momentum distribution.\nAt the same time, at sufficiently high excitation densities, there is\nsignificant qualitative discrepancy between the dynamic behavior of the width\nof the polariton momentum distribution determined from direct measurements and\nthat calculated from the coherence spatial distribution. This discrepancy is\nobserved at the fast initial stage of the polariton system kinetics and,\napparently, results from the strong spatial nonuniformity of the phase of the\ncondensate wave function, which equilibrates on a much longer time scale.", "category": "cond-mat_quant-gas" }, { "text": "Quantum information theoretic measures to distinguish fermionized bosons\n from non-interacting fermions: We study the dynamical fermionization of strongly interacting one-dimensional\nbosons in Tonks-Girardeau limit by solving the time dependent many-boson\nSchr\\\"odinger equation numerically exactly. We establish that the one-body\nmomentum distribution approaches the ideal Fermi gas distribution at the time\nof dynamical fermionization. The analysis is further complemented by the\nmeasures on two-body level. Investigation on two-body momentum distribution,\ntwo-body local and non-local correlation clearly distinguish the fermionized\nbosons from non-interacting fermions. The magnitude of distinguishablity\nbetween the two systems is further discussed employing suitable measures of\ninformation theory, i.e., the well known Kullback-Leibler relative entropy and\nthe Jensen-Shannon divergence entropy. We also observe very rich structure in\nthe higher-body density for strongly correlated bosons whereas non-interacting\nfermions do not possess any higher order correlation beyond two-body.", "category": "cond-mat_quant-gas" }, { "text": "Dynamics of exciton-polaritons in a Josephson double dimer: We study the dynamics of exciton-polaritons in a double-well configuration.\nThe system consists of two weakly coupled Bose-Josephson junctions, each\ncorresponding to a different circular polarization of the polaritons, forming a\n{\\it Josephson double dimer}. We show that the Josephson oscillation between\nthe wells is strongly coupled to the polarization rotation and that\nconsequently Josephson excitation is periodically exchanged between the two\npolarizations. Linearized analysis agrees well with numerical simulations using\ntypical experimental parameters.", "category": "cond-mat_quant-gas" }, { "text": "Evidence of a liquid phase in interacting Bosons at intermediate\n densities: In this paper, we present evidence for a liquid-like phase in systems of many\ninteracting Bosons at intermediate densities. The interacting Bose gas has been\nstudied extensively in the low and high density regimes, in which interactions\ndo not play a physically significant role, and the system behaves similarly to\nthe ideal quantum gas. Instead, we will turn our attention to the intermediate\ndensity regime, and report evidence that the system enters a strongly\ncorrelated phase where its behavior is markedly different from that of the\nideal quantum gas. To do so, we use the Simplified approach to the Bose gas,\nwhich was introduced by Lieb in 1963 and recently found to provide very\naccurate predictions for many-Boson systems at all densities. Using this tool,\nwe will compute predictions for the radial distribution function, structure\nfactor, condensate fraction and momentum distribution, and show that they are\nconsistent with liquid-type behavior.", "category": "cond-mat_quant-gas" }, { "text": "Bidirectional dynamic scaling in an isolated Bose gas far from\n equilibrium: Understanding and classifying nonequilibrium many-body phenomena, analogous\nto the classification of equilibrium states of matter into universality\nclasses, is an outstanding problem in physics. Any many-body system, from\nstellar matter to financial markets, can be out of equilibrium in a myriad of\nways; since many are also difficult to experiment on, it is a major goal to\nestablish universal principles that apply to different phenomena and physical\nsystems. At the heart of the classification of equilibrium states is the\nuniversality seen in the self-similar spatial scaling of systems close to phase\ntransitions. Recent theoretical work, and first experimental evidence, suggest\nthat isolated many-body systems far from equilibrium generically exhibit\ndynamic (spatiotemporal) self-similar scaling, akin to turbulent cascades and\nthe Family-Vicsek scaling in classical surface growth. Here we observe\nbidirectional dynamic scaling in an isolated quench-cooled atomic Bose gas; as\nthe gas thermalises and undergoes Bose-Einstein condensation, it shows\nself-similar net flows of particles towards the infrared (smaller momenta) and\nenergy towards the ultraviolet (smaller lengthscales). For both infrared (IR)\nand ultraviolet (UV) dynamics we find that the scaling exponents are\nindependent of the strength of the interparticle interactions that drive the\nthermalisation.", "category": "cond-mat_quant-gas" }, { "text": "Rotational pendulum dynamics of a vortex molecule in a channel geometry: A vortex molecule is a topological excitation in two coherently coupled\nsuperfluids consisting of a vortex in each superfluid connected by a domain\nwall of the relative phase, also known as a Josephson vortex. We investigate\nthe dynamics of this excitation in a quasi-two-dimensional geometry with slab\nor channel boundary conditions using an extended point vortex framework\ncomplemented by Gross-Pitaevskii simulations. Apart from translational motion\nalong the channel, the vortex molecule is found to exhibit intriguing internal\ndynamics including rotation and rotational-pendulum-like dynamics. Trajectories\nleading to a boundary-induced break-up of the vortex molecule are also\ndescribed qualitatively by the simplified model. We classify the stable and\nunstable fixed points as well as separatrices that characterize the vortex\nmolecule dynamics.", "category": "cond-mat_quant-gas" }, { "text": "Bose-Einstein Condensation of 84-Sr: We report Bose-Einstein condensation of 84-Sr in an optical dipole trap.\nEfficient laser cooling on the narrow intercombination line and an ideal s-wave\nscattering length allow creation of large condensates (N0 ~ 3x10^5) even though\nthe natural abundance of this isotope is only 0.6%. Condensation is heralded by\nthe emergence of a low-velocity component in time-of-flight images.", "category": "cond-mat_quant-gas" }, { "text": "Dynamic Structure Factor of Normal Fermi Gas from Collisionless to\n Hydrodynamic Regime: The dynamic structure factor of a normal Fermi gas is investigated by using\nthe moment method for the Boltzmann equation. We determine the spectral\nfunction at finite temperatures over the full range of crossover from the\ncollisionless regime to the hydrodynamic regime. We find that the Brillouin\npeak in the dynamic structure factor exhibits a smooth crossover from zero to\nfirst sound as functions of temperature and interaction strength. The dynamic\nstructure factor obtained using the moment method also exhibits a definite\nRayleigh peak ($/omega /sim 0$), which is a characteristic of the hydrodynamic\nregime. We compare the dynamic structure factor obtained by the moment method\nwith that obtained from the hydrodynamic equations.", "category": "cond-mat_quant-gas" }, { "text": "Small two-component Fermi gases in a cubic box with periodic boundary\n conditions: The properties of two-component Fermi gases become universal if the\ninterspecies s-wave scattering length $a_s$ and the average interparticle\nspacing are much larger than the range of the underlying two-body potential.\nUsing an explicitly correlated Gaussian basis set expansion approach, we\ndetermine the eigen energies of two-component Fermi gases in a cubic box with\nperiodic boundary conditions as functions of the interspecies s-wave scattering\nlength and the effective range of the two-body potential. The universal\nproperties of systems consisting of up to four particles are determined by\nextrapolating the finite-range energies to the zero-range limit. We determine\nthe eigen energies of states with vanishing and finite momentum. In the\nweakly-attractive BCS regime, we analyze the energy spectra and degeneracies\nusing first-order degenerate perturbation theory. Excellent agreement between\nthe perturbative energy shifts and the numerically determined energies is\nobtained. For the infinitely large scattering length case, we compare our\nresults - where available - with those presented in the literature.", "category": "cond-mat_quant-gas" }, { "text": "Driven-dissipative many-body pairing states for cold fermionic atoms in\n an optical lattice: We discuss the preparation of many-body states of cold fermionic atoms in an\noptical lattice via controlled dissipative processes induced by coupling the\nsystem to a reservoir. Based on a mechanism combining Pauli blocking and phase\nlocking between adjacent sites, we construct complete sets of jump operators\ndescribing coupling to a reservoir that leads to dissipative preparation of\npairing states for fermions with various symmetries in the absence of direct\ninter-particle interactions. We discuss the uniqueness of these states, and\ndemonstrate it with small-scale numerical simulations. In the late time\ndissipative dynamics, we identify a \"dissipative gap\" that persists in the\nthermodynamic limit. This gap implies exponential convergence of all many-body\nobservables to their steady state values. We then investigate how these pairing\nstates can be used as a starting point for the preparation of the ground state\nof Fermi-Hubbard Hamiltonian via an adiabatic state preparation process also\ninvolving the parent Hamiltonian of the pairing state. We also provide a\nproof-of-principle example for implementing these dissipative processes and the\nparent Hamiltonians of the pairing states, based on Yb171 atoms in optical\nlattice potentials.", "category": "cond-mat_quant-gas" }, { "text": "AtomECS: Simulate laser cooling and magneto-optical traps: AtomECS is a software package that efficiently simulates the motion of\nneutral atoms experiencing forces exerted by laser radiation, such as in\nmagneto-optical traps and Zeeman slowers. The program is implemented using the\nEntity-Component-System pattern, which gives excellent performance, flexibility\nand scalability to parallel computing resources. The simulation package has\nbeen verified by comparison to analytic results, and extensively unit tested.", "category": "cond-mat_quant-gas" }, { "text": "Localization in spin chains with facilitation constraints and disordered\n interactions: Quantum many-body systems with kinetic constraints exhibit intriguing\nrelaxation dynamics. Recent experimental progress in the field of cold atomic\ngases offers a handle for probing collective behavior of such systems, in\nparticular for understanding the interplay between constraints and disorder.\nHere we explore a spin chain with facilitation constraints --- a feature which\nis often used to model classical glass formers --- together with disorder that\noriginates from spin-spin interactions. The specific model we study, which is\nrealized in a natural fashion in Rydberg quantum simulators, maps onto an\nXX-chain with non-local disorder. Our study shows that the combination of\nconstraints and seemingly unconventional disorder may lead to interesting\nnon-equilibrium behaviour in experimentally relevant setups.", "category": "cond-mat_quant-gas" }, { "text": "Gr\u00fcneisen Parameter for Gases: The Gr\\\"uneisen ratio ($\\Gamma$), i.e.\\,the ratio of the linear thermal\nexpansivity to the specific heat at constant pressure, quantifies the degree of\nanharmonicity of the potential governing the physical properties of a system.\nWhile $\\Gamma$ has been intensively explored in solid state physics, very\nlittle is known about its behavior for gases. This is most likely due to the\ndifficulties posed to carry out both thermal expansion and specific heat\nmeasurements in gases with high accuracy as a function of pressure and\ntemperature. Furthermore, to the best of our knowledge a comprehensive\ndiscussion about the peculiarities of the Gr\\\"uneisen ratio is still lacking in\nthe literature. Here we report on a detailed and comprehensive overview of the\nGr\\\"uneisen ratio. Particular emphasis is placed on the analysis of $\\Gamma$\nfor gases. The main findings of this work are: \\emph{i)} for the Van der Waals\ngas $\\Gamma$ depends only on the co-volume $b$ due to interaction effects, it\nis smaller than that for the ideal gas ($\\Gamma$ = 2/3) and diverges upon\napproaching the critical volume; \\emph{ii)} for the Bose-Einstein condensation\nof an ideal boson gas, assuming the transition as first-order $\\Gamma$ diverges\nupon approaching a critical volume, similarly to the Van der Waals gas;\n\\emph{iii)} for $^4$He at the superfluid transition $\\Gamma$ shows a singular\nbehavior. Our results reveal that $\\Gamma$ can be used as an appropriate\nexperimental tool to explore pressure-induced critical points.", "category": "cond-mat_quant-gas" }, { "text": "Signatures of Fractional Exclusion Statistics in the Spectroscopy of\n Quantum Hall Droplets: We show how spectroscopic experiments on a small Laughlin droplet of rotating\nbosons can directly demonstrate Haldane fractional exclusion statistics of\nquasihole excitations. The characteristic signatures appear in the\nsingle-particle excitation spectrum. We show that the transitions are governed\nby a \"many-body selection rule\" which allows one to relate the number of\nallowed transitions to the number of quasihole states on a finite geometry. We\nillustrate the theory with numerically exact simulations of small numbers of\nparticles.", "category": "cond-mat_quant-gas" }, { "text": "Rapidity distribution within the defocusing non-linear Schr\u00f6dinger\n equation model: We consider the classical field integrable system whose evolution equation is\nthe nonlinear Schr\\\"odinger equation with defocusing non-linearities, which is\nthe classical limit of the quantum Lieb-Liniger model. We propose a simple\nderivation of the relation between two sets of conserved quantities: on the one\nhand the trace of the monodromy matrix, parameterized by the spectral parameter\nand introduced in the inverse-scattering framework, and on the other hand the\nrapidity distribution, a concept imported from the Lieb-Liniger model. To do so\nwe use the definition of the rapidity distribution as the asymptotic momentum\ndistribution after an expansion. More precisely we use thought experiments\nimplementing an expansion and we present two different ways to derive our\nresult, based on different thought experiments which lead to different\ncalculations.", "category": "cond-mat_quant-gas" }, { "text": "Pairing and the spin susceptibility of the polarized unitary Fermi gas\n in the normal phase: We theoretically study the pairing behavior of the unitary Fermi gas in the\nnormal phase. Our analysis is based on the static spin susceptibility, which\ncharacterizes the response to an external magnetic field. We obtain this\nquantity by means of the complex Langevin approach and compare our calculations\nto available literature data in the spin-balanced case. Furthermore, we present\nresults for polarized systems, where we complement and expand our analysis at\nhigh temperature with high-order virial expansion results. The implications of\nour findings for the phase diagram of the spin-polarized unitary Fermi gas are\ndiscussed, in the context of the state of the art.", "category": "cond-mat_quant-gas" }, { "text": "Non-perturbative method to compute thermal correlations in\n one-dimensional systems: A brief overview: We develop a highly efficient method to numerically simulate thermal\nfluctuations and correlations in non-relativistic continuous bosonic\none-dimensional systems. We start by noticing the equivalence of their\ndescription through the transfer matrix formalism and a Fokker-Planck equation\nfor a distribution evolving in space. The corresponding stochastic differential\n(It\\={o}) equation is very suitable for computer simulations, allowing the\ncalculation of arbitrary correlation functions. As an illustration, we apply\nour method to the case of two tunnel-coupled quasicondensates of bosonic atoms.", "category": "cond-mat_quant-gas" }, { "text": "Miscibility-Immiscibility transition of strongly interacting bosonic\n mixtures in optical lattices: Interaction plays key role in the mixing properties of a multi-component\nsystem. The miscibility-immiscibility transition (MIT) in a weakly interacting\nmixture of Bose gases is predominantly determined by the strengths of the intra\nand inter-component two-body contact interactions. On the other hand, in the\nstrongly interacting regime interaction induced processes become relevant.\nDespite previous studies on bosonic mixtures in optical lattices, the effects\nof the interaction induced processes on the MIT remains unexplored. In this\nwork, we investigate the MIT in the strongly interacting phases of\ntwo-component bosonic mixture trapped in a homogeneous two-dimensional square\noptical lattice. Particularly we examine the transition when both the\ncomponents are in superfluid (SF), one-body staggered superfluid (OSSF) or\nsupersolid (SS) phases. Our study prevails that, similar to the contact\ninteractions, the MIT can be influenced by competing intra and inter-component\ndensity induced tunnelings and off-site interactions. To probe the MIT in the\nstrongly interacting regime, we study the extended version of the Bose-Hubbard\nmodel with the density induced tunneling and nearest-neighbouring interaction\nterms, and focus in the regime where the hopping processes are considerably\nweaker than the on-site interaction. We solve this model through\nsite-decoupling mean-field theory with Gutzwiller ansatz and characterize the\nmiscibility through the site-wise co-existence of the two-component across the\nlattice. Our study contributes to the better understanding of miscibility\nproperties of multi-component systems in the strongly interacting regime.", "category": "cond-mat_quant-gas" }, { "text": "Adiabatic spin cooling using high-spin Fermi gases: Spatial entropy redistribution plays a key role in adiabatic cooling of\nultra-cold lattice gases. We show that high-spin fermions with a spatially\nvariable quadratic Zeeman coupling may allow for the creation of an inner\nspin-1/2 core surrounded by high-spin wings. The latter are always more\nentropic than the core at high temperatures and, remarkably, at all\ntemperatures in the presence of frustration. Combining thermodynamic Bethe\nAnsatz with local density approximation, we study the spatial entropy\ndistribution for the particular case of one-dimensional spin-3/2 lattice\nfermions in the Mott phase. Interestingly, this spatially dependent entropy\nopens a possible path for an adiabatic cooling technique that, in contrast to\nprevious proposals, would specifically target the spin degree of freedom. We\ndiscuss a possible realization of this adiabatic cooling, which may allow for a\nhighly efficient entropy decrease in the spin-1/2 core and help access\nantiferromagnetic order in experiments on ultracold spinor fermions.", "category": "cond-mat_quant-gas" }, { "text": "Two-Stage Melting in Systems of Strongly Interacting Rydberg Atoms: We analyze the ground state properties of a one-dimensional cold atomic\nsystem in a lattice, where Rydberg excitations are created by an external laser\ndrive. In the classical limit, the ground state is characterized by a complete\ndevil's staircase for the commensurate solid structures of Rydberg excitations.\nUsing perturbation theory and a mapping onto an effective low energy\nHamiltonian, we find a transition of these commensurate solids into a floating\nsolid with algebraic correlations. For stronger quantum fluctuations the\nfloating solid eventually melts within a second quantum phase transition and\nthe ground state becomes paramagnetic.", "category": "cond-mat_quant-gas" }, { "text": "Probing quantum transport by engineering correlations in a speckle\n potential: We develop a procedure to modify the correlations of a speckle potential.\nThis procedure, that is suitable for spatial light modulator devices, allows\none to increase the localization efficiency of the speckle in a narrow energy\nregion whose position can be easily tuned. This peculiar energy-dependent\nlocalization behavior is explored by pulling the potential through a\ncigar-shaped Bose-Einstein condensate. We show that the percentage of dragged\natoms as a function of the pulling velocity depends on the potential\ncorrelations below a threshold of the disorder strength. Above this threshold,\ninterference effects are no longer clearly observable during the condensate\ndrag.", "category": "cond-mat_quant-gas" }, { "text": "Vortex gyroscope imaging of planar superfluids: We propose a robust imaging technique that makes it possible to distinguish\nvortices from antivortices in quasi-two-dimensional Bose--Einstein condensates\nfrom a single image of the density of the atoms. Tilting the planar condensate\nprior to standard absorption imaging excites a generalized gyroscopic mode of\nthe condensate revealing the sign and location of each vortex. This technique\nis anticipated to enable experimental measurement of the incompressible kinetic\nenergy spectrum of the condensate and the observation of a negative temperature\nphase transition of the vortex gas, driven by two-dimensional superfluid\nturbulence.", "category": "cond-mat_quant-gas" }, { "text": "Quantum interferometry at zero and finite temperature with two-mode\n bosonic Josephson junctions: We analyze phase interferometry realized with a bosonic Josephson junction\nmade of trapped dilute and ultracold atoms. By using a suitable phase\nsensitivity indicator we study the zero temperature junction states useful to\nachieve sub shot-noise precisions. Sub shot-noise phase shift sensitivities can\nbe reached even at finite temperature under a suitable choice of the junction\nstate. We infer a scaling law in terms of the size system (that is, the number\nof particles) for the temperature at which the shot-noise limit is not overcome\nanymore", "category": "cond-mat_quant-gas" }, { "text": "Topological semimetal in a fermionic optical lattice: Optical lattices play a versatile role in advancing our understanding of\ncorrelated quantum matter. The recent implementation of orbital degrees of\nfreedom in chequerboard and hexagonal optical lattices opens up a new thrust\ntowards discovering novel quantum states of matter, which have no prior analogs\nin solid state electronic materials. Here, we demonstrate that an exotic\ntopological semimetal emerges as a parity-protected gapless state in the\norbital bands of a two-dimensional fermionic optical lattice. The new quantum\nstate is characterized by a parabolic band-degeneracy point with Berry flux\n$2\\pi$, in sharp contrast to the $\\pi$ flux of Dirac points as in graphene. We\nprove that the appearance of this topological liquid is universal for all\nlattices with D$_4$ point group symmetry as long as orbitals with opposite\nparities hybridize strongly with each other and the band degeneracy is\nprotected by odd parity. Turning on inter-particle repulsive interactions, the\nsystem undergoes a phase transition to a topological insulator whose\nexperimental signature includes chiral gapless domain-wall modes, reminiscent\nof quantum Hall edge states.", "category": "cond-mat_quant-gas" }, { "text": "Resonant dipolar collisions of ultracold molecules induced by microwave\n dressing: We demonstrate microwave dressing on ultracold, fermionic\n${}^{23}$Na${}^{40}$K ground-state molecules and observe resonant dipolar\ncollisions with cross sections exceeding three times the $s$-wave unitarity\nlimit. The origin of these collisions is the resonant alignment of the\napproaching molecules' dipoles along the intermolecular axis, which leads to\nstrong attraction. We explain our observations with a conceptually simple\ntwo-state picture based on the Condon approximation. Furthermore, we perform\ncoupled-channels calculations that agree well with the experimentally observed\ncollision rates. While collisions are observed here as laser-induced loss,\nmicrowave dressing on chemically stable molecules trapped in box potentials may\nenable the creation of strongly interacting dipolar gases of molecules.", "category": "cond-mat_quant-gas" }, { "text": "Effect of anisotropic spin-orbit coupling on condensation and\n superfluidity of a two dimensional Fermi gases: We investigated the ground state properties of a two dimensional Fermi\nsuperfluid with an anisotropic spin-orbit coupling (SOC) using path-integral\nfield theoretical method. Within the framework of mean-field theory, we\nobtained the condensed fraction including contributions from both singlet and\ntriple pairing fields. We found that for small interaction parameters and large\nanisotropic parameters, the total condensed fraction changes non-monotonically\nwhen increasing the strength of SOC and has a global maximum. But this feature\ndisappears with decreasing the anisotropic parameter and increasing the\ninteraction parameter. However, condensed fraction always decrease with\nincreasing the anisotropic parameters. Because of the anisotropy of the SOC,\nthe superfluid fraction becomes a tensor. We obtained the superfluid fraction\ntensor by deriving the effective action of the phase field of the order\nparameter. Our numerical results show that for small interaction parameters and\nlarge anisotropic parameters, superfluid fraction of the $x$ component\n$\\rho_{x}$ has a minimum as a function of the SOC strength. And this minimum of\n$\\rho_{x}$ disappears when decreasing the anisotropic parameters. In the strong\ninteraction regime, $\\rho_{x}$ always decreases with increasing the strength of\nSOC. While for the $y$ component of the superfluid fraction $\\rho_{y}$, no\nmatter how large the interaction parameters and anisotropic parameters are, it\nalways has a minimum as a function of the SOC strength. As a function of the\nanisotropic parameter, for strong SOC strength, $\\rho_{x}<\\rho_{y}$ with\n$\\rho_{x}$ having a minimum. For small SOC parameters $\\rho_{x}>\\rho_{y}$ with\n$\\rho_{y}$ developing a minimum only in the weak interaction limit.", "category": "cond-mat_quant-gas" }, { "text": "Imaginary Potential Induced Quantum Coherence for Bose-Einstein\n Condensates: The role of complex potentials in single-body Schr\\H{o}dinger equation has\nbeen studied intensively. We study the quantum coherence for degenerate Bose\ngases in complex potentials, when the exchange symmetry of identical bosons is\nconsidered. For initially independent Bose-Einstein condensates, it is shown\nthat even very weak imaginary potential can induce perfect quantum coherence\nbetween different condensates. The scheme to observe imaginary potential\ninduced quantum coherence is discussed.", "category": "cond-mat_quant-gas" }, { "text": "Two-body momentum correlations in a weakly interacting one-dimensional\n Bose gas: We analyze the two-body momentum correlation function for a uniform weakly\ninteracting one-dimensional Bose gas. We show that the strong positive\ncorrelation between opposite momenta, expected in a Bose-Einstein condensate\nwith a true long-range order, almost vanishes in a phase-fluctuating\nquasicondensate where the long-range order is destroyed. Using the Luttinger\nliquid approach, we derive an analytic expression for the momentum correlation\nfunction in the quasicondensate regime, showing (i) the reduction and\nbroadening of the opposite-momentum correlations (compared to the singular\nbehavior in a true condensate) and (ii) an emergence of anticorrelations at\nsmall momenta. We also numerically investigate the momentum correlations in the\ncrossover between the quasicondensate and the ideal Bose-gas regimes using a\nclassical field approach and show how the anticorrelations gradually disappear\nin the ideal-gas limit.", "category": "cond-mat_quant-gas" }, { "text": "Chiral confinement in quasirelativistic Bose-Einstein condensates: In the presence of a laser-induced spin-orbit coupling an interacting ultra\ncold spinor Bose-Einstein condensate may acquire a quasi-relativistic character\ndescribed by a non-linear Dirac-like equation. We show that as a result of the\nspin-orbit coupling and the non-linearity the condensate may become\nself-trapped, resembling the so-called chiral confinement, previously studied\nin the context of the massive Thirring model. We first consider 1D geometries\nwhere the self-confined condensates present an intriguing sinusoidal dependence\non the inter-particle interactions. We further show that multi-dimensional\nchiral-confinement is also possible under appropriate feasible laser\narrangements, and discuss the properties of 2D and 3D condensates, which differ\nsignificantly from the 1D case.", "category": "cond-mat_quant-gas" }, { "text": "Superfluid to Mott insulator transition in the one-dimensional\n Bose-Hubbard model for arbitrary integer filling factors: We study the quantum phase transition between the superfluid and the Mott\ninsulator in the one-dimensional (1D) Bose-Hubbard model. Using the\ntime-evolving block decimation method, we numerically calculate the tunneling\nsplitting of two macroscopically distinct states with different winding\nnumbers. From the scaling of the tunneling splitting with respect to the system\nsize, we determine the critical point of the superfluid to Mott insulator\ntransition for arbitrary integer filling factors. We find that the critical\nvalues versus the filling factor in 1D, 2D, and 3D are well approximated by a\nsimple analytical function. We also discuss the condition for determining the\ntransition point from a perspective of the instanton method.", "category": "cond-mat_quant-gas" }, { "text": "The spin evolution of spin-3 $^{52}$Cr Bose-Einstein condensate: The spin evolution of a Bose-Einstein condensate starting from a mixture of\ntwo or three groups of $^{52}$Cr (spin-3) atoms in an optical trap has been\nstudied theoretically. The initial state is so chosen that the system does not\ndistinguish up and down. In this choice, the deviation caused by the\nsingle-mode approximation is reduced. Moreover, since the particle number is\ngiven very small (N=20), the deviation caused by the neglect of the long-range\ndipole force is also reduced. Making use of these two simplifications, a\ntheoretical calculation beyond the mean field theory is performed. The\nnumerical results are help to evaluate the unknown strength $g_0$.", "category": "cond-mat_quant-gas" }, { "text": "Dynamics of first-order quantum phase transitions in extended\n Bose-Hubbard model: From density wave to superfluid and vice-versa: In this paper, we study the nonequilibrium dynamics of the Bose-Hubbard model\nwith the nearest-neighbor repulsion by using time-dependent Gutzwiller (GW)\nmethods. In particular, we vary the hopping parameters in the Hamiltonian as a\nfunction of time, and investigate the dynamics of the system from the density\nwave (DW) to the superfluid (SF) crossing a first-order phase transition and\nvice-versa. From the DW to SF, we find scaling laws for the correlation length\nand vortex density with respect to the quench time. This is a reminiscence of\nthe Kibble-Zurek scaling for continuous phase transitions and contradicts the\ncommon expectation. We give a possible explanation for this observation. On the\nother hand from the SF to DW, the system evolution depends on the initial SF\nstate. When the initial state is the ground-state obtained by the static GW\nmethods, a coexisting state of the SF and DW domains forms after passing\nthrough the critical point. Coherence of the SF order parameter is lost as the\nsystem evolves. This is a phenomenon similar to the glass transition in\nclassical systems. When the state starts from the SF with small local phase\nfluctuations, the system obtains a large-size DW-domain structure with thin\ndomain walls.", "category": "cond-mat_quant-gas" }, { "text": "Many-body exceptional points in colliding condensates: Exceptional points describe the coalescence of the eigenmodes of a\nnon-Hermitian matrix. When an exceptional point occurs in the unitary evolution\nof a many-body system, it generically leads to a dynamical instability with a\nfinite wavevector [N. Bernier \\etal, Phys. Rev. Lett. 113, 065303 (2014)].\nHere, we study exceptional points in the context of the counterflow instability\nof colliding Bose-Einstein condensates. We show that the instability of this\nsystem is due to an exceptional point in the Bogoliubov spectrum. We further\nclarify the connection of this effect to the Landau criterion of superfluidity\nand to the scattering of classical particles. We propose an experimental set-up\nto directly probe this exceptional point, and demonstrate its feasibility with\nthe aid of numerical calculations. Our work fosters the observation of\nexceptional points in nonequilibrium many-body quantum systems.", "category": "cond-mat_quant-gas" }, { "text": "Three Identical Fermions with Resonant p-wave Interactions in Two\n Dimensions: A new kind of \"super-Efimov\" states of binding energies scaling as\n$\\ln|E_n|\\sim-e^{3n\\pi/4}$ were predicted by a field theory calculation for\nthree fermions with resonant $p$-wave interactions in two dimensions [Phys.\nRev. Lett. \\textbf{110}, 235301 (2013)]. However, the universality of these\n\"super-Efimov\" states has not been proved independently. In this Letter, we\nstudy the three fermion system through the hyperspherical formalism. Within the\nadiabatic approximation, we find that at $p$-wave resonances, the low energy\nphysics of states of angular momentum $\\ell=\\pm1$ crucially depends on the\nvalue of an emergent dimensionless parameter $Y$ determined by the detail of\nthe inter-particle potential. Only if $Y$ is exactly zero, the predicted\n\"super-Efimov\" states exist. If $Y>0$, the scaling of the bound states changes\nto $\\ln|E_n|\\sim-(n\\pi)^2/2Y$, while there are no shallow bound states if\n$Y<0$.", "category": "cond-mat_quant-gas" }, { "text": "Relation between the noise correlations and the spin structure factor\n for Mott-insulating states in SU$(N)$ Hubbard models: It is well established that the noise correlations measured by time-of-flight\nimaging in cold-atom experiments, which correspond to the density-density\ncorrelations in the momentum space of trapped atomic gases, can probe the spin\nstructure factor deep in the Mott-insulating regime of SU(2) Hubbard models. We\nexplicitly derive the mathematical relation between the noise correlations and\nthe spin structure factor in the strong-interaction limit of SU$(N)$ Hubbard\nmodels at any integer filling $\\rho$. By calculating the ground states of\none-dimensional SU$(N)$ Fermi-Hubbard models for $2\\leq N\\leq 6$ with use of\nthe density-matrix renormalization-group method, we confirm the relation\nnumerically in the regime of strong interactions $U \\gg t$, where $U$ and $t$\ndenote the onsite interaction and the hopping energy. We show that the\ndeviation between the actual noise correlations and those obtained from the\nspin structure factor scales as approximately $(t/U)^2$ for $\\rho=1$ at\nintermediate and large lattice sizes on the basis of numeric and semi-analytic\narguments.", "category": "cond-mat_quant-gas" }, { "text": "Thermodynamics of a spin-1 Bose gas with fixed magnetization: We investigate the thermodynamics of a spin-1 Bose gas with fixed\nmagnetization including the quadratic Zeeman energy shift. Our calculations are\nbased on the grand canonical description for the ideal gas and the classical\nfields approximation for atoms with ferromagnetic and antiferromagnetic\ninteractions. We confirm the occurence of a double phase transition in the\nsystem that takes place due to two global constraints. We show analytically for\nthe ideal gas how critical temperatures and condensed fractions are changed by\na non-zero magnetic field. The interaction strongly affects the condensate\nscenario below the second critical temperature. The effect imposed by\ninteraction energies becomes diminished in high magnetic fields where\ncondensation, of both ferromagnetic and antiferromagnetic atoms, agree with the\nideal gas results.", "category": "cond-mat_quant-gas" }, { "text": "Dynamical Cluster Quantum Monte Carlo Study of the Single Particle\n Spectra of Strongly Interacting Fermion Gases: We study the single-particle spectral function of resonantly-interacting\nfermions in the unitary regime, as described by the three-dimensional\nattractive Hubbard model in the dilute limit. Our approach, based on the\nDynamical Cluster Approximation and the Maximum Entropy Method, shows the\nemergence of a gap with decreasing temperature, as reported in recent cold-atom\nphotoemission experiments, for coupling values that span the BEC-BCS crossover.\nBy comparing the behavior of the spectral function to that of the imaginary\ntime dynamical pairing susceptibility, we attribute the development of the gap\nto the formation of local bound atom pairs.", "category": "cond-mat_quant-gas" }, { "text": "Pseudopotential for the 2D contact interaction: We propose a smooth pseudopotential for the contact interaction acting\nbetween ultracold atoms confined to two dimensions. The pseudopotential\nreproduces the scattering properties of the repulsive contact interaction up to\n200 times more accurately than a hard disk potential, and in the attractive\nbranch gives a 10-fold improvement in accuracy over the square well potential.\nFurthermore, the new potential enables diffusion Monte Carlo simulations of the\nultracold gas to be run 15 times quicker than was previously possible.", "category": "cond-mat_quant-gas" }, { "text": "Particle Fluctuations in Mesoscopic Bose Systems: Particle fluctuations in mesoscopic Bose systems of arbitrary spatial\ndimensionality are considered. Both ideal Bose gases and interacting Bose\nsystems are studied in the regions above the Bose-Einstein condensation\ntemperature $T_c$ as well as below this temperature. The strength of particle\nfluctuations defines whether the system is stable or not. Stability conditions\ndepend on the spatial dimensionality $d$ and on the confining dimension $D$ of\nthe system. The consideration shows that mesoscopic systems, experiencing\nBose-Einstein condensation, are stable when: (i) ideal Bose gas is confined in\na rectangular box of spatial dimension $d>2$ above $T_c$ and in a box of $d>4$\nbelow $T_c$; (ii) ideal Bose gas is confined in a power-law trap of a confining\ndimension $D>2$ above $T_c$ and of a confining dimension $D>4$ below $T_c$;\n(iii) interacting Bose system is confined in a rectangular box of dimension\n$d>2$ above $T_c$, while below $T_c$ particle interactions stabilize the\nBose-condensed system making it stable for $d=3$; (iv) nonlocal interactions\ndiminish the condensation temperature, as compared with the fluctuations in a\nsystem with contact interactions.", "category": "cond-mat_quant-gas" }, { "text": "How creating one additional well can generate Bose-Einstein condensation: The realization of Bose-Einstein condensation in ultracold trapped gases has\nled to a revival of interest in that fascinating quantum phenomenon. This\nexperimental achievement necessitated both extremely low temperatures and\nsufficiently weak interactions. Particularly in reduced spatial dimensionality\neven an infinitesimal interaction immediately leads to a departure to\nquasi-condensation. We propose a system of strongly interacting bosons which\novercomes those obstacles by exhibiting a number of intriguing related\nfeatures: (i) The tuning of just a single control parameter drives a transition\nfrom quasi-condensation to complete condensation, (ii) the destructive\ninfluence of strong interactions is compensated by the respective increased\nmobility, (iii) topology plays a crucial role since a crossover from one- to\n`infinite'-dimensionality is simulated, (iv) a ground state gap opens which\nmakes the condensation robust to thermal noise. Remarkably, all these features\ncan be derived by analytical and exact numerical means despite the\nnon-perturbative character of the system.", "category": "cond-mat_quant-gas" }, { "text": "Critical velocity, vortex shedding and drag in a unitary Fermi\n superfluid: We study the real-time motion of a microscopic object in a cold Fermi gas at\nunitary conditions by using an extended Thomas-Fermi density functional\napproach. We find that spontaneous creation of singly quantized\nvortex-antivortex pairs occurs as a critical velocity is exceeded, which leads\nto a drag between the moving object and the Fermi gas. The resulting force is\nlinear in the velocity for subsonic motion and becomes quadratic for supersonic\nmotion.", "category": "cond-mat_quant-gas" }, { "text": "Direct Observation of Fragmentation in a Disordered, Strongly\n Interacting Fermi Gas: Describing the behaviour of strongly interacting particles in the presence of\ndisorder is among the most challenging problems in quantum many-body physics.\nThe controlled setting of cold atom experiments provides a new avenue to\naddress these challenges [1], complementing studies in solid state physics,\nwhere a number of puzzling findings have emerged in experiments using\nsuperconducting thin films [2,3]. Here we investigate a strongly interacting\nthin film of an atomic Fermi gas subject to a random potential. We use\nhigh-resolution in-situ imaging [4-7] to resolve the atomic density at the\nlength scale of a single impurity, which would require scanning probe\ntechniques in solid state physics [8]. This allows us to directly observe the\nfragmentation of the density profile and to extract its percolation properties.\nTransport measurements in a two-terminal configuration indicate that the\nfragmentation process is accompanied by a breakdown of superfluidity. Our\nresults suggest that percolation of paired atoms is responsible for the loss of\nsuperfluidity, and that disorder is able to increase the binding energy of\npairs.", "category": "cond-mat_quant-gas" }, { "text": "Correlated quantum dynamics of graphene: Phase-space representations are a family of methods for dynamics of both\nbosonic and fermionic systems, that work by mapping the system's density matrix\nto a quasi-probability density and the Liouville-von Neumann equation of the\nHamiltonian to a corresponding density differential equation for the\nprobability. We investigate here the accuracy and the computational efficiency\nof one approximate phase-space representation, called the fermionic Truncated\nWigner Approximation (fTWA), applied to the Fermi-Hubbard model. On a many-body\n2D system, with hopping strength and Coulomb $U$ tuned to represent the\nelectronic structure of graphene, the method is found to be able to capture the\ntime evolution of first-order (site occupation) and second-order (correlation\nfunctions) moments significantly better than the mean-field, Hartree-Fock\nmethod. The fTWA was also compared to results from the exact diagonalization\nmethod for smaller systems, and in general the agreement was found to be good.\nThe fully parallel computational requirement of fTWA scales in the same order\nas the Hartree-Fock method, and the largest system considered here contained\n198 lattice sites.", "category": "cond-mat_quant-gas" }, { "text": "Strong Boundary and Trap Potential Effects on Emergent Physics in\n Ultra-Cold Fermionic Gases: The field of quantum simulations in ultra-cold atomic gases has been\nremarkably successful. In principle it allows for an exact treatment of a\nvariety of highly relevant lattice models and their emergent phases of matter.\nBut so far there is a lack in the theoretical literature concerning the\nsystematic study of the effects of the trap potential as well as the finite\nsize of the systems, as numerical studies of such non periodic, correlated\nfermionic lattices models are numerically demanding beyond one dimension. We\nuse the recently introduced real-space truncated unity functional\nrenormalization group to study these boundary and trap effects with a focus on\ntheir impact on the superconducting phase of the $2$D Hubbard model. We find\nthat in the experiments not only lower temperatures need to be reached compared\nto current capabilities, but also system size and trap potential shape play a\ncrucial role to simulate emergent phases of matter.", "category": "cond-mat_quant-gas" }, { "text": "The response to dynamical modulation of the optical lattice for fermions\n in the Hubbard model: Fermionic atoms in a periodic optical lattice provide a realization of the\nsingle-band Hubbard model. Using Quantum Monte Carlo simulations along with the\nMaximum Entropy Method, we evaluate the effect of a time-dependent perturbative\nmodulation of the optical lattice amplitude on atomic correlations, revealed in\nthe fraction of doubly-occupied sites. Our treatment extends previous\napproaches which neglected the time dependence of the on-site interaction, and\nshows that this term changes the results in a quantitatively significant way.\nThe effect of modulation depends strongly on the filling-- the response of the\ndouble occupation is significantly different in the half-filled Mott insulator\nfrom the doped Fermi liquid region.", "category": "cond-mat_quant-gas" }, { "text": "Decoherence of an impurity in a one-dimensional fermionic bath with mass\n imbalance: We study the transport, decoherence and dissipation of an impurity\ninteracting with a bath of free fermions in a one-dimensional lattice.\nNumerical simulations are made with the time-evolving block decimation method.\nWe introduce a mass imbalance between the impurity and bath particles and find\nthat the fastest decoherence occurs for a light impurity in a bath of heavy\nparticles. By contrast, the fastest dissipation of energy occurs when the\nmasses are equal. We present a simple model for decoherence in the heavy bath\nlimit, and a linear density response description of the interaction which\npredicts maximum dissipation for equal masses.", "category": "cond-mat_quant-gas" }, { "text": "Rotating Fulde-Ferrell-Larkin-Ovchinnikov state in cold Fermi gases: We study an effect of rotation on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)\nstate of two component Fermi superfluid gases in a toroidal trap. We\ninvestigate a stability of the FFLO states in the quasi-one-dimensional regime\non the basis of the Bogoliubov-de Gennes equation. We find that two novel FFLO\nphases, i.e., the half quantum vortex state and the intermediate state of\nFulde-Ferrell (FF) state and Larkin-Ovchinnikov (LO) state, are stabilized by\nthe rotation. The phase diagram for the FF state, LO state, intermediate state,\nand half quantum vortex state is shown in both T-P plane and T-h plane. We\ndemonstrate characteristic features of these states, such as the order\nparameter, flux quantization, and local polarization. Several related works are\ndiscussed, and the advantages of cold Fermi gases are indicated.", "category": "cond-mat_quant-gas" }, { "text": "Bloch oscillations of spin-orbit-coupled cold atoms in an optical\n lattice and spin current generation: We study the Bloch oscillation dynamics of a spin-orbit-coupled cold atomic\ngas trapped inside a one-dimensioanl optical lattice. The eigenspectra of the\nsystem is identified as two interpenetrating Wannier-Stark ladder. Based on\nthat, we carefully analyzed the Bloch oscillation dynamics and found out that\nintraladder coupling between neighboring rungs of Wannier-Stark ladder give\nrise to ordinary Bloch oscillation while interladder coupling lead to small\namplitude high frequency oscillation superimposed on it. Specifically\nspin-orbit interaction breaks Galilean invariance, which can be reflected by\nout-of-phase oscillation of the two spin components in the accelerated frame.\nThe possibility of generating spin current in this system are also explored.", "category": "cond-mat_quant-gas" }, { "text": "Mixtures of ultra-cold atoms in 1D disordered potentials: We study interacting 1D two-component mixtures of cold atoms in a random\npotential, and extend the results reported earlier [{\\it Phys. Rev. Lett.} {\\bf\n105}, 115301 (2010)]. We construct the phase diagram of a disordered Bose-Fermi\nmixture as a function of the strength of the Bose-Bose and Bose-Fermi\ninteractions, and the ratio of the bosonic sound velocity and the Fermi\nvelocity. Performing renormalization group and variational calculations, three\nphases are identified: (i) a fully delocalized two-component Luttinger liquid\nwith superfluid bosons and fermions (ii) a fully localized phase with both\ncomponents pinned by disorder, and (iii) an intermediate phase where fermions\nare localized but bosons are superfluid. Within the variational approach, each\nphase corresponds to a different level of replica symmetry breaking. In the\nfully localized phase we find that the bosonic and fermionic localization\nlengths can largely differ. We also compute the momentum distribution as well\nas the structure factor of the atoms (both experimentally accessible), and\ndiscuss how the three phases can be experimentally distinguished.", "category": "cond-mat_quant-gas" }, { "text": "Lattice bosons with infinite range checkerboard interactions: Motivated by experiments performed by Landig et al. [Nature 532, 476-479], we\nconsider a two dimensional Bose gas in an optical lattice, trapped inside a\nsingle mode superradiant Fabry Perot cavity. The cavity mediates infinite range\ncheckerboard interactions between the atoms, which produces competition between\nMott insulator, charge density wave, superfluid and supersolid phases. We\ncalculate the phase diagram of this Bose gas in a homogeneous system and in the\npresence of a harmonic trap.", "category": "cond-mat_quant-gas" }, { "text": "Theory of Non-Hermitian Fermionic Superfluidity with a Complex-Valued\n Interaction: Motivated by recent experimental advances in ultracold atoms, we analyze a\nnon-Hermitian (NH) BCS Hamiltonian with a complex-valued interaction arising\nfrom inelastic scattering between fermions. We develop a mean-field theory to\nobtain a NH gap equation for order parameters, which are different from the\nstandard BCS ones due to the inequivalence of left and right eigenstates in the\nNH physics. We find unconventional phase transitions unique to NH systems:\nsuperfluidity shows reentrant behavior with increasing dissipation, as a\nconsequence of non-diagonalizable exceptional points, lines, and surfaces in\nthe quasiparticle Hamiltonian for weak attractive interactions. For strong\nattractive interactions, the superfluid gap never collapses but is enhanced by\ndissipation due to an interplay between the BCS-BEC crossover and the quantum\nZeno effect. Our results lay the groundwork for studies of fermionic\nsuperfluidity subject to inelastic collisions.", "category": "cond-mat_quant-gas" }, { "text": "Quantum phases in spin-orbit-coupled Floquet spinor Bose gases: We propose a spin-orbit-coupled Floquet spinor Bose-Einstein condensate (BEC)\nwhich can be implemented by Floquet engineering of a quadratic Zeeman field.\nThe Floquet spinor BEC has a Bessel-function-modulated Rabi frequency and a\nFloquet-induced spin-exchange interaction. The quantum phase diagram of the\nspin-orbit-coupled Floquet spinor BEC is investigated by considering\nantiferromagnetic or ferromagnetic spin-spin interactions. In comparison with\nthe usual spin-orbit-coupled spin-1 BEC, we find that a stripe phase for\nantiferromagnetic interactions can exist in a large quadratic Zeeman field\nregime, and a different stripe phase with an experimentally favorable contrast\nfor ferromagnetic interactions is uncovered.", "category": "cond-mat_quant-gas" }, { "text": "First-order superfluid to Mott-insulator phase transitions in spinor\n condensates: We observe evidence of first-order superfluid to Mott-insulator quantum phase\ntransitions in a lattice-confined antiferromagnetic spinor Bose-Einstein\ncondensate. The observed signatures include hysteresis effect and significant\nheatings across the phase transitions. The nature of the phase transitions is\nfound to strongly depend on the ratio of the quadratic Zeeman energy to the\nspin-dependent interaction. Our observations are qualitatively understood by\nthe mean field theory, and in addition suggest tuning the quadratic Zeeman\nenergy is a new approach to realize superfluid to Mott-insulator phase\ntransitions.", "category": "cond-mat_quant-gas" }, { "text": "Quantum anomaly and 2D-3D crossover in strongly interacting Fermi gases: We present an experimental investigation of collective oscillations in\nharmonically trapped Fermi gases through the crossover from two to three\ndimensions. Specifically, we measure the frequency of the radial monopole or\nbreathing mode as a function of dimensionality in Fermi gases with tunable\ninteractions. The frequency of this mode is set by the adiabatic\ncompressibility and probes the thermodynamic equation of state. In 2D, a\ndynamical scaling symmetry for atoms interacting via a {\\delta}-potential\npredicts the breathing mode to occur at exactly twice the harmonic confinement\nfrequency. However, a renormalized quantum treatment introduces a new length\nscale which breaks this classical scale invariance resulting in a so-called\nquantum anomaly. Our measurements deep in the 2D regime lie above the\nscale-invariant prediction for a range of interaction strengths indicating the\nbreakdown of a {\\delta}-potential model for atomic interactions. As the\ndimensionality is tuned from 2D to 3D we see the breathing oscillation\nfrequency evolve smoothly towards the 3D limit.", "category": "cond-mat_quant-gas" }, { "text": "Notes on the Cluster Gutzwiller Method: Inhomogeneous Lattices,\n Excitations, and Cluster Time Evolution: Several perspectives of the cluster Gutzwiller method are briefly discussed.\nI show that the cluster mean-field method can be used for large inhomogeneous\nlattices, for computing local excitations, and for the time evolution of\ncorrelated quantum systems.", "category": "cond-mat_quant-gas" }, { "text": "Self-Assembled Chains and Solids of Dipolar Atoms in a Multilayer: We predict that ultracold bosonic dipolar gases, confined within a multilayer\ngeometry, may undergo self-assembling processes, leading to the formation of\nchain gases and solids. These dipolar chains, with dipoles aligned across\ndifferent layers, emerge at low densities and resemble phases observed in\nliquid crystals, such as nematic and smectic phases. We calculate the phase\ndiagram using quantum Monte Carlo methods, introducing a newly devised trial\nwave function designed for describing the chain gas, where dipoles from\ndifferent layers form chains without in-plane long-range order. We find gas,\nsolid, and chain phases, along with quantum phase transitions between these\nstates. Specifically, we predict a quantum phase transition from a gaseous to a\nself-ordered phase, which occurs at a critical interlayer distance. Remarkably,\nin the self-organized phases, the mean interparticle distance can significantly\nexceed the characteristic length of the interaction potential, yielding solids\nand chain gases with densities several orders of magnitude lower than those of\nconventional quantum solids.", "category": "cond-mat_quant-gas" }, { "text": "Chiral condensates in a polariton hexagonal ring: We model generation of vortex modes in exciton-polariton condensates in\nsemiconductor micropillars, arranged into a hexagonal ring molecule, in the\npresence of TE-TM splitting. This splitting lifts the degeneracy of azimuthally\nmodulated vortex modes with opposite topological charges supported by this\nstructure, so that a number of non-degenerate vortex states characterized by\ndifferent combinations of topological charges in two polarization components\nappears. We present a full bifurcation picture for such vortex modes and show\nthat because they have different energies, they can be selectively excited by\ncoherent pump beams with specific frequencies and spatial configurations. At\nhigh pumping intensity, polariton-polariton interactions give rise to the\ncoupling of different vortex resonances and a bistable regime is achieved.", "category": "cond-mat_quant-gas" }, { "text": "Hidden vortices in a Bose-Einstein condensate in a rotating double-well\n potential: We study vortex formation in a Bose-Einstein condensate in a rotating\ndouble-well potential. Besides the ordinary quantized vortices and elusive\nghost vortices, \"hidden\" vortices are found distributing along the central\nbarrier. These hidden vortices are invisible like ghost vortex but carry\nangular momentum. Moreover, their core size is not given by the healing length,\nbut is strongly influenced by the external potential. We find that the\nFeynman's rule can be well satisfied only after including the hidden vortices.\nThere is no critical rotating frequency for the formation of hidden vortex\nwhile there is one for the formation of ordinary visible vortices. Hidden\nvortices can be revealed in the free expansion of the Bose-Einstein\ncondensates. In addition, the hidden vortices in a Bose-Einstein condensate can\nappear in other external potentials, such as a rotating anisotropic toroidal\ntrap.", "category": "cond-mat_quant-gas" }, { "text": "Two types of dark solitons in a spin-orbit-coupled Fermi gas: Dark solitons in quantum fluids are well known nonlinear excitations that are\nusually characterized by a single length scale associated with the underlying\nbackground fluid. We show that in the presence of spin-orbit coupling and a\nlinear Zeeman field, superfluid Fermi gases support two different types of\nnonlinear excitations featured by corresponding length scales related to the\nexistence of two Fermi surfaces. Only one of these types, which occurs for\nfinite spin-orbit coupling and a Zeeman field, survives to the topological\nphase transition, and is therefore capable to sustain Majorana zero modes. At\nthe point of the emergence of this soliton for varying the Zeeman field, the\nassociated Andreev bound states present a minigap that vanishes for practical\npurposes, in spite of lacking the reality condition of Majorana modes.", "category": "cond-mat_quant-gas" }, { "text": "Effects of thermal and quantum fluctuations on the phase diagram of a\n spin-1 87Rb Bose-Einstein condensate: We investigate effects of thermal and quantum fluctuations on the phase\ndiagram of a spin-1 87Rb Bose-Einstein condensate (BEC) under a quadratic\nZeeman effect. Due to the large ratio of spinindependent to spin-dependent\ninteractions of 87Rb atoms, the effect of noncondensed atoms on the condensate\nis much more significant than that in scalar BECs. We find that the condensate\nand spontaneous magnetization emerge at different temperatures when the ground\nstate is in the brokenaxisymmetry phase. In this phase, a magnetized condensate\ninduces spin coherence of noncondensed atoms in different magnetic sublevels,\nresulting in temperature-dependent magnetization of the noncondensate. We also\nexamine the effect of quantum fluctuations on the order parameter at absolute\nzero, and find that the ground-state phase diagram is significantly altered by\nquantum depletion.", "category": "cond-mat_quant-gas" }, { "text": "Effective dipole-dipole interactions in multilayered dipolar\n Bose-Einstein condensates: We propose a two-dimensional model for a multilayer stack of dipolar\nBose-Einstein condensates formed by a strong optical lattice. We derive\neffective intra- and interlayer dipole-dipole interaction potentials and\nprovide simple analytical approximations for a given number of lattice sites at\narbitrary polarization. We find that the interlayer dipole-dipole interaction\nchanges the transverse aspect ratio of the ground state in the central layers\ndepending on its polarization and the number of lattice sites. The changing\naspect ratio should be observable in time of flight images. Furthermore, we\nshow that the interlayer dipole-dipole interaction reduces the excitation\nenergy of local perturbations, affecting the development of a roton minimum.", "category": "cond-mat_quant-gas" }, { "text": "Particle and spin transports of spin-orbit coupled Fermi gas through a\n Quantum Point Contact: The particle and spin transport through a quantum point contact between two\nFermi gases with Raman-induced spin-orbit coupling are investigated. We show\nthat the particle and spin conductances both demonstrate the structure of\nplateau due to the mesoscopic scale of the quantum point contact. Compared with\nthe normal Fermi gases the particle conductance can be significantly enhanced\nby the spin-orbit coupling effect. Furthermore, the conversion of the particle\nand spin currents can take place in the spin-orbit coupled system, and we find\nthat it is controlled by the parameter of two-photon detuning. When the\nparameter of two-photon detuning vanishes the particle and spin currents\ndecouple.", "category": "cond-mat_quant-gas" }, { "text": "Two-mode Dicke model from non-degenerate polarization modes: We realize a non-degenerate two-mode Dicke model with competing interactions\nin a Bose-Einstein condensate (BEC) coupled to two orthogonal polarization\nmodes of a single optical cavity. The BEC is coupled to the cavity modes via\nthe scalar and vectorial part of the atomic polarizability. We can\nindependently change these couplings and determine their effect on a\nself-organization phase transition. Measuring the phases of the system, we\ncharacterize a crossover from a single-mode to a two-mode Dicke model. This\nwork provides perspectives for the realization of coupled phases of spin and\ndensity.", "category": "cond-mat_quant-gas" }, { "text": "Gauge Violation Spectroscopy in Synthetic Gauge Theories: Recently synthetic gauge fields have been implemented on quantum simulators.\nUnlike the gauge fields in the real world, in synthetic gauge fields, the gauge\ncharge can fluctuate and gauge invariance can be violated, which leading rich\nphysics unexplored before. In this work, we propose the gauge violation\nspectroscopy as a useful experimentally accessible measurement in the synthetic\ngauge theories. We show that the gauge violation spectroscopy exhibits no\ndispersion. Using three models as examples, two of them can be exactly solved\nby bosonization, and one has been realized in experiment, we further\ndemonstrate the gauge violation spectroscopy can be used to detect the\nconfinement and deconfinement phases. In the confinement phase, it shows a\ndelta function behavior, while in the deconfinement phase, it has a finite\nwidth.", "category": "cond-mat_quant-gas" }, { "text": "From Nodal Ring Topological Superfluids to Spiral Majorana Modes in Cold\n Atomic Systems: In this work, we consider a 3D cubic optical lattice composed of coupled 1D\nwires with 1D spin-orbit coupling. When the s-wave pairing is induced through\nFeshbach resonance, the system becomes a topological superfluid with ring\nnodes, which are the ring nodal degeneracies in the bulk, and supports a large\nnumber of surface Majorana zero energy modes. The large number of surface\nMajorana modes remain at zero energy even in the presence of disorder due to\nthe protection from a chiral symmetry. When the chiral symmetry is broken, the\nsystem becomes a Weyl topological superfluid with Majorana arcs. With 3D\nspin-orbit coupling, the Weyl superfluid becomes a novel gapless phase with\nspiral Majorana modes on the surface. The spatial resolved radio frequency\nspectroscopy is suggested to detect this novel nodal ring topological\nsuperfluid phase.", "category": "cond-mat_quant-gas" }, { "text": "Interacting quantum walk on a two-leg flux ladder: Emergence of\n re-entrant dynamics: We study the quench dynamics of interacting bosons on a two-leg flux ladder\nby implementing the continuous-time quantum walk and explore the combined\neffect of the magnetic field and onsite interaction in the presence of uniform\nflux. We show that in the regime of weak interaction, the magnetic field\nsubstantially slows down the spreading of the particles' wavefunction during\nthe dynamics. However, in the presence of strong interaction, we obtain an\ninteresting re-entrant behaviour in the dynamics where the radial velocity\nassociated to the spreading first increases, then decreases, and increases\nagain as a function of the flux strength. We also find a re-entrant dynamics in\nthe chiral motion of the particles as a function of interaction for fixed flux\nstrengths.", "category": "cond-mat_quant-gas" }, { "text": "Induced interaction and crystallization of self-localized impurity\n fields in a Bose-Einstein condensate: We model the behavior of N classical impurity fields immersed in a larger\nBose-Einstein condensate by N+1 coupled nonlinear Schrodinger equations in 1,\n2, and 3 space dimensions. We discuss the stability of the uniform miscible\nsystem and show the importance of surface tension for self localization of the\nimpurity fields. We derive analytically the attractive tail of\nimpurity-impurity interaction due to mediation by the underlying condensate.\nAssuming all impurity fields interact with the same strength, we explore the\nresulting phase diagram, which contains four phases: {\\it I}) all fields are\nmiscible; {\\it II}) the impurity fields are miscible with each other but phase\nseparate from the condensate as a single bubble; {\\it III}) the localized\nimpurity fields stay miscible with the condensate, but not with each other; and\n{\\it IV}) the impurity fields phase separate from the condensate and each\nother, forming a crystalline structure within a bubble. Thus, we show that a\ncrystal can be constructed solely from superfluid components. Finally, we argue\nthat the crystalline phases maintain their superfluid behavior, i.e. they\npossess a nonclassical rotational inertia, which, combined with lattice order,\nis a characteristic of supersolidity.", "category": "cond-mat_quant-gas" }, { "text": "Bose-Einstein condensation of light in a cavity: The paper considers Bose-Einstein condensation (BEC) of light in a cavity\nwith medium. In the framework of two-level model we show the effect of gaseous\nmedium on the critical temperature of light condensation in the system.\nTransition of the system to the state with released light condensate is\nillustrated in consequent stages. Analytical expressions for a typical spatial\nextent of the condensed cloud of photons, as well for spectral characteristics\nof the condensate peak are derived. Energy and heat capacity of photons as\nfunctions of temperature are obtained. Finally, we demonstrate that the energy\nof light can be accumulated in the BEC state.", "category": "cond-mat_quant-gas" }, { "text": "Thermalisation of Local Observables in Small Hubbard Lattices: We present a study of thermalisation of a small isolated Hubbard lattice\ncluster prepared in a pure state with a well-defined energy. We examine how a\ntwo-site subsystem of the lattice thermalises with the rest of the system as\nits environment. We explore numerically the existence of thermalisation over a\nrange of system parameters, such as the interaction strength, system size and\nthe strength of the coupling between the subsystem and the rest of the lattice.\nWe find thermalisation over a wide range of parameters and that interactions\nare crucial for efficient thermalisation of small systems. We relate this\nthermalisation behaviour to the eigenstate thermalisation hypothesis and\nquantify numerically the extent to which eigenstate thermalisation holds. We\nalso verify our numerical results theoretically with the help of previously\nestablished results from random matrix theory for the local density of states,\nparticularly the finite-size scaling for the onset of thermalisation.", "category": "cond-mat_quant-gas" }, { "text": "Critical velocity of a mobile impurity in one-dimensional quantum\n liquids: We study the notion of superfluid critical velocity in one spatial dimension.\nIt is shown that for heavy impurities with mass $M$ exceeding a critical mass\n$M_\\mathrm{c}$, the dispersion develops periodic metastable branches resulting\nin dramatic changes of dynamics in the presence of an external driving force.\nIn contrast to smooth Bloch Oscillations for $M