Nov 2014
Nov 3-Nov 7, Bo Liu, Nov 10-Nov 14, Zhenyu Zhou & Jinlong Yu, Nov 17-Nov21, Jiyao chen & Jianhui zhou, Nov 24-Nov 28, Haiyuan Zou & Ahmet Keles
Nov 27
arXiv:1411.7375 [pdf, other]
An exact mobility edge in one dimension
Sriram Ganeshan, S. Das Sarma
Comments: 5 pages, 2 figures
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas)
We investigate localization properties of a family of deterministic (i.e. no disorder) nearest neighbor tight binding models with quasiperiodic onsite modulation. We prove that this family is self-dual under a generalized duality transformation. The self-dual condition for this general model turns out to be a simple closed form function of the model parameters and energy. We numerically verify that this self-dual line indeed defines a mobility edge in energy separating localized and extended states. Our model is a first example of a nearest neighbor tight binding model manifesting a mobility edge protected by a duality symmetry. Although primarily of fundamental theoretical significance as a one of a kind example of a true one-dimensional mobility edge, we also propose a simple scheme to experimentally realize our results in atomic optical lattices and photonic waveguides.
arXiv:1411.7316 [pdf, ps, other]
Weyl Knots in 3D Spin-Orbit Coupled Fulde-Ferrell Superfluids
Yong Xu, Chuanwei Zhang
Comments: 6 pages, 5 figures, including supplementary materials
Subjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)
Weyl points (or Weyl fermions), the band touching points possessing linear dispersion and integer topological charges in 3D non-degenerate energy spectrum, have been widely studied recently in solid state materials within the context of Weyl semimetals. In this Letter, we propose that a Weyl point in the linear Weyl Hamiltonian can evolve into a Weyl knot with a twisted surface structure and similar topological properties by adding additional linear momentum (spin-independent) and cubic spin-orbit coupling terms in the Hamiltonian. We show that such Weyl knots can be realized in the quasiparticle energy spectrum of a 3D spin-orbit coupled degenerate Fermi gas subject to Zeeman fields, which supports Fulde-Ferrell Superfluids. The rich phase diagram that contains Weyl knots is obtained. We find that a Weyl knot has a quantized topological charge only as a whole and two Weyl knots can be connected to form a Weyl knot ring. In experiments, Weyl knots may be probed by measuring their spectral density using momentum resolved photoemission spectroscopy.
Nov 26
arXiv:1411.6967 [pdf, other]
FFLO order in ultra-cold atoms in three-dimensional optical lattices
Peter Rosenberg, Simone Chiesa, Shiwei Zhang
Subjects: Quantum Gases (cond-mat.quant-gas)
We investigate different ground-state phases of attractive spin-imbalanced populations of fermions in 3-dimensional optical lattices. Detailed numerical calculations are performed using Hartree-Fock-Bogoliubov theory to determine the ground-state properties systematically for different values of density, spin polarization and interaction strength. We first consider the high density and low polarization regime, in which the effect of the optical lattice is most evident. We then proceed to the low density and high polarization regime where the effects of the underlying lattice are less significant and the system begins to resemble a continuum Fermi gas. We explore the effects of density, polarization and interaction on the character of the phases in each regime and highlight the qualitative differences between the two regimes. In the high-density regime, the order is found to be of Larkin-Ovchinnikov type, linearly oriented with one characteristic wave vector but varying in its direction with the parameters. At lower densities the order parameter develops more structures involving multiple wave vectors.
arXiv:1411.6895 [pdf, other]
Angular spin-orbit coupling in cold atoms
Michael DeMarco, Han Pu
Comments: 7 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We propose coupling two internal atomic states using a pair of Raman beams operated in Laguerre-Gaussian laser modes with unequal phase windings. This generates a coupling between the atom's pseudo-spin and its orbital angular momentum. We analyze the single-particle properties of the system using realistic parameters and provide detailed studies of the spin texture of the ground state. Finally, we consider a weakly interacting atomic condensate subject to this angular spin-orbit coupling and show how the inter-atomic interactions modifies the single-particle physics.
Nov 25
arXiv:1411.6019 [pdf, ps, other]
Chiral spin density wave order on frustrated honeycomb and bilayer triangle lattice Hubbard model at half-filling
Kun Jiang, Yi Zhang, Sen Zhou, Ziqiang Wang
Comments: 5 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We study the ground states of the Hubbard model on the frustrated honeycomb lattice with nearest-neighbor t1 and second nearest-neighbor hopping t2, which is isomorphic to the bilayer triangle lattice. We show that, at {\em half-filling}, the on-site Coulomb interaction U induces antiferromagnetic (AF) chiral spin-density wave (χ-SDW) order in a wide range of κ=t2/t1 where both the two-sublattice AF order at small κ and the decoupled three-sublattice 120∘ order at large κ are strongly frustrated, leading to three distinct phases with different anomalous Hall responses. Increasing U at fixed κ drives a continuous transition from a χ-SDW semimetal with anomalous Hall effect to a topological chiral Chern insulator exhibiting quantum anomalous Hall effect, which undergoes a discontinuous transition to a χ-SDW insulator with zero total Chern number but anomalous ac Hall effect. We obtain the phase diagram, discuss its properties, and argue that the AF χ-SDW state is a generic phase of strongly correlated and frustrated hexagonal lattice electrons.
Nov 24
[1] arXiv:1411.5684 [pdf, other]
Wess-Zumino-Witten terms in graphene Landau levels
Junhyun Lee, Subir Sachdev
We consider the interplay between the antiferromagnetic and Kekul\'e valence bond solid orderings in the zero energy Landau levels of neutral monolayer and bilayer graphene. We establish the presence of Wess-Zumino-Witten terms between these orders: this implies that their quantum fluctuations are described by the deconfined critical theories of quantum spin systems. We present implications for experiments, including the possible presence of excitonic superfluidity in bilayer graphene.
[2]arXiv:1411.5710 (cross-list from quant-ph) [pdf, other]
Quantum annealing: the fastest route to quantum computation?
C. R. Laumann, R. Moessner, A. Scardicchio, S. L. Sondhi
In this review we consider the performance of the quantum adiabatic algorithm for the solution of decision problems. We divide the possible failure mechanisms into two sets: small gaps due to quantum phase transitions and small gaps due to avoided crossings inside a phase. We argue that the thermodynamic order of the phase transitions is not predictive of the scaling of the gap with the system size. On the contrary, we also argue that, if the phase surrounding the problem Hamiltonian is a Many-Body Localized (MBL) phase, the gaps are going to be typically exponentially small and that this follows naturally from the existence of local integrals of motion in the MBL phase.
[3] arXiv:1411.5802 [pdf, other]
Strongly interacting Majorana fermions
Ching-Kai Chiu, D.I. Pikulin, M. Franz
Interesting phases of quantum matter often arise when the constituent particles -- electrons in solids -- interact strongly. Such strongly interacting systems are however quite rare and occur only in extreme environments of low spatial dimension, low temperatures or intense magnetic fields. Here we introduce a new system in which the fundamental electrons interact only weakly but the low energy effective theory is described by strongly interacting Majorana fermions. The system consists of an Abrikosov vortex lattice in the surface of a strong topological insulator and is accessible experimentally using presently available technology. The simplest interactions between the Majorana degrees of freedom exhibit an unusual nonlocal structure that involves four distinct Majorana sites. We formulate simple lattice models with this type of interaction and find exact solutions in certain physically relevant one- and two-dimensional geometries. In other cases we show how our construction allows for the experimental realization of interesting spin models previously only theoretically contemplated.
[4] arXiv:1411.6004 [pdf, other]
Mesoscopic Bose-Einstein condensates as quantum simulators
A. Gallemí, M. Guilleumas, R. Mayol, A. Sanpera
Mesoscopic interacting Bose-Einstein condensates confined in a few traps display phase transitions that cannot be explained with a mean field theory. By describing each trap as an effective site of a Bose-Hubbard model and using the Schwinger representation of spin operators, these systems can be mapped to spin models. We show that it is possible to define correlations between bosons in such a way that critical behavior is associated to the divergence of a correlation length accompanied by a gapless spectrum in the thermodynamic limit. The latter is now defined as the limit in which the mean field analysis becomes valid. Such description provides critical exponents to the associated phase transitions and encompasses the notion of universality demonstrating thus the potential use of mesoscopic Bose-Einstein condensates as quantum simulators of condensed matter systems.
[5] arXiv:1411.5962 [pdf, other]
Strongly correlated states of trapped ultracold fermions in deformed Landau levels
M. Burrello, M. Rizzi, M. Roncaglia, A. Trombettoni
We analyze the strongly correlated regime of a two-component trapped ultracold fermionic gas in a synthetic non-Abelian U(2) gauge potential, that consists of both a magnetic field and a homogeneous spin-orbit coupling. This gauge potential deforms the Landau levels (LLs) with respect to the Abelian case and exchanges their ordering as a function of the spin-orbit coupling. In view of experimental realizations, we show that a harmonic potential combined with a Zeeman term, gives rise to an angular momentum term, which can be used to test the stability of the correlated states obtained through interactions. We derive the Haldane pseudopotentials (HPs) describing the interspecies contact interaction within a lowest LL approximation. Unlike ordinary fractional quantum Hall systems and ultracold bosons with short-range interactions in the same gauge potential, the HPs for sufficiently strong non-Abelian fields show an unconventional non-monotonic behaviour in the relative angular momentum. Exploiting this property, we study the occurrence of new incompressible ground states as a function of the total angular momentum. In the first deformed Landau level (DLL) we obtain Laughlin and Jain states. Instead, in the second DLL three classes of stabilized states appear: Laughlin states, a series of intermediate strongly correlated states and finally vortices of the integer quantum Hall state. Remarkably, in the intermediate regime, the non-monotonic HPs of the second DLL induce two-particle correlations which are reminiscent of paired states such as the Haffnian state. Via exact diagonalization in the disk geometry, we compute experimentally relevant observables like density profiles and correlations, and we study the entanglement spectra as a further tool to characterize the obtained strongly correlated states.
[6] arXiv:1411.5868 [pdf, ps, other]
Topological Condensate in an Interaction Induced Gauge Potential
Jun-hui Zheng, Bo Xiong, G. Juzeliunas, Daw-Wei Wang
We systematically investigate the ground state and elementary excitations of a Bose-Einstein Condensate with a synthetic vector potential, which is induced by the many-body effects and atom-light coupling. For a sufficiently strong inter-atom interaction, we find the condensate undergoes a Stoner-type ferromagnetic transition through the self-consistent coupling with the vector potential. For a weak interaction, the critical velocity of a supercurrent is found anisotropic due to the density fluctuations affecting the gauge field. We further analytically demonstrate the topological ground state with a coreless vortex ring in a 3D harmonic trap and a coreless vortex-antivortex pair in a 2D trap. The circulating persistent current is measurable in the time-of-flight experiment or in the dipolar oscillation through the violation of Kohn theorem.
[7] arXiv:1411.6967 [pdf, other]
FFLO order in ultra-cold atoms in three-dimensional optical lattices
Peter Rosenberg, Simone Chiesa, Shiwei Zhang
We investigate different ground-state phases of attractive spin-imbalanced populations of fermions in 3-dimensional optical lattices. Detailed numerical calculations are performed using Hartree-Fock-Bogoliubov theory to determine the ground-state properties systematically for different values of density, spin polarization and interaction strength. We first consider the high density and low polarization regime, in which the effect of the optical lattice is most evident. We then proceed to the low density and high polarization regime where the effects of the underlying lattice are less significant and the system begins to resemble a continuum Fermi gas. We explore the effects of density, polarization and interaction on the character of the phases in each regime and highlight the qualitative differences between the two regimes. In the high-density regime, the order is found to be of Larkin-Ovchinnikov type, linearly oriented with one characteristic wave vector but varying in its direction with the parameters. At lower densities the order parameter develops more structures involving multiple wave vectors.
Nov 21
1. arXiv:1411.5632 [pdf, other]
Negative differential conductivity in an interacting quantum gas
Ralf Labouvie, Bodhaditya Santra, Simon Heun, Sandro Wimberger, Herwig Ott
Subjects: Quantum Gases (cond-mat.quant-gas)
Negative differential conductivity (NDC) is a widely exploited effect in modern electronic components. Here, a proof-of-principle is given for the observation of NDC in a quantum transport device for neutral atoms employing a multi-mode tunneling junction. The transport of the many-body quantum system is governed by the interplay between the tunnel coupling, the interaction energy and the thermodynamics of intrinsic collisions, which turn the coherent coupling into a hopping process. The resulting current voltage characteristics exhibit NDC, for which we identify a new microscopic physical mechanism. Our study opens new ways for the future implementation and control of complex neutral atom quantum circuits.
2. arXiv:1411.5593 [pdf, other]
Metastable Bose-Einstein Condensation in a Strongly Correlated Optical Lattice
David McKay, Ushnish Ray, Stefan Natu, Philip Russ, David Ceperley, Brian DeMarcoWe experimentally and theoretically study the peak fraction of a Bose-Einstein condensate loaded into a cubic optical lattice as the lattice potential depth and entropy per particle are varied. This system is well-described by the superfluid regime of the Bose-Hubbard model, which allows for comparison with mean-field theories and exact quantum Monte Carlo (QMC) simulations. Despite correcting for systematic discrepancies between condensate fraction and peak fraction, we discover that the experiment consistently shows the presence of a condensate at temperatures higher than the critical temperature predicted by QMC simulations. This metastability suggests that turning on the lattice potential is non-adiabatic. To confirm this behavior, we compute the timescales for relaxation in this system, and find that equilibration times are comparable with the known heating rates. The similarity of these timescales implies that turning on the lattice potential adiabatically may be impossible. Our results point to the urgent need for a better theoretical and experimental understanding of the timescales for relaxation and adiabaticity in strongly interacting quantum gases, and the importance of model-independent probes of thermometry in optical lattices.
3. arXiv:1411.5544 [pdf, other]
Repulsive to attractive interaction quenches of 1D Bose gas in a harmonic trap
Wladimir Tschischik, Masudul HaqueWe consider quantum quenches of harmonically trapped one-dimensional bosons from repulsive to attractive interactions, and the resulting breathing dynamics of the so-called `super-Tonks-Girardeau' (sTG) state. This state is highly excited compared to the ground state of the attractive gas, and is the lowest eigenstate where the particles are not bound or clustered. We analyze the dynamics from a spectral point of view, identifying the relevant eigenstates of the interacting trapped many-body system, and analyzing the nature of these quantum eigenstates. To obtain explicit eigenspectra, we use Hamiltonians with finite-dimensional Hilbert spaces to approximate the Lieb-Liniger system. We employ two very different approximate approaches: an expansion in a truncated single-particle harmonic-trap basis and a lattice (Bose-Hubbard) model. We show how the breathing frequency, identified with the energy difference between the sTG state and another particular eigenstate, varies with interaction.
Nov 20
1. arXiv:1411.5352 [pdf, other]
Generalized Gibbs Ensembles for Quantum Field Theories
F.H.L. Essler, G. Mussardo, M. Panfil
We consider the non-equilibrium dynamics in isolated systems, described by quantum field theories (QFTs). After being prepared in a density matrix that is not an eigenstate of the Hamiltonian, such systems are expected to relax locally to a stationary state. In a presence of local conservation laws, these stationary states are believed to be described by appropriate generalized Gibbs ensembles. Here we demonstrate that in order to obtain a correct description of the stationary state, it is necessary to take into account conservation laws that are not (ultra-)local in the usual sense of QFT, but fulfil a significantly weaker form of locality. We discuss implications of our results for integrable QFTs in one spatial dimension.
Nov 19
1. arXiv:1411.4946 [pdf, other]
Dissipative cooling of degenerate Bose gases
Pjotrs Grišins, Bernhard Rauer, Tim Langen, Jörg Schmiedmayer, Igor E. Mazets
We introduce a dissipative cooling schema for degenerate Bose gases based on coherent outcoupling of atoms from the condensate. In the universal phononic limit the system evolves towards an asymptotic dissipative state where the temperature is set by the quantum noise of the outcoupling process. The proposed mechanism supplements conventional evaporative cooling and dominates in settings where thermalization is highly suppressed, such as in a one-dimensional quasicondensate, where it can be readily observed in experiment.
2. arXiv:1411.4831 [pdf, other]
Superoperators vs. Trajectories for Matrix Product State Simulations of Open Quantum System: A Case Study
Lars Bonnes, Andreas M. LäuchliQuantum trajectories and superoperator algorithms implemented within the matrix product state (MPS) framework are powerful tools to simulate the real-time dynamics of open dissipative quantum systems. As for the unitary case, the reachable time-scales as well as system sizes are limited by the (possible) build-up of entanglement entropy. The aforementioned methods constitute complementary approaches how Lindblad master equations can be integrated relying either on a quasi-exact representation of the full density matrix or a stochastic unraveling of the density matrix in terms of pure states. In this work, we systematically benchmark both methods by studying the dynamics of a Bose-Hubbard chain in the presence of local as well as global dephasing. The build-up as well as system-size scaling of entanglement entropy strongly depends on the method and the parameter regime and we discuss the applicability of the methods for these cases as well as study the distribution of observables and time discretization errors that can become a limiting factor for global dissipation.
Nov 18
1. arXiv:1411.4634 [pdf, ps, other]
A chromium dipolar Fermi sea
B. Naylor, A. Reigue, E. Maréchal, O. Gorceix, B. Laburthe-Tolra, L. VernacWe report on the production of a degenerate Fermi gas of 53Cr atoms, polarized in the state F=9/2, m_F=-9/2, by sympathetic cooling with bosonic S=3, m_S=-3 52Cr atoms. We load in an optical dipole trap 3.10^4 53Cr atoms with 10^6 52Cr atoms. Despite this initial small number of fermionic atoms, we reach a final temperature of T=0.6 T_f (Fermi temperature), with up to 10^3 53Cr atoms. This surprisingly efficient evaporation stems from an inter-isotope scattering length |a_{BF}| = (85+/- 10) a_{Bohr} which is small enough to reduce evaporative losses of the fermionic isotope, but large enough to insure thermalization.
2. arXiv:1411.4258 [pdf, other]
Spinor Bose-Einstein Condensates of Rotating Polar Molecules
Y. Deng, S. YiWe propose a scheme to realize a pseudospin-1/2 model of the 1Σ(v=0) bialkali polar molecules with the spin states corresponding to two sublevels of the first excited rotational level. We show that the effective dipole-dipole interaction between two spin-1/2 molecules couples the rotational and orbital angular momenta and is highly tunable via a microwave field. We also investigate the ground state properties of a spin-1/2 molecular condensate. A variety of nontrivial quantum phases, including the doubly-quantized vortex states, are discovered. Our scheme can also be used to create spin-1 model of polar molecules. Thus, we show that the ultracold gases of bialkali polar molecules provide a unique platform for studying the spinor condensates of rotating molecules.
3. arXiv:1411.4168 [pdf, other]
Signatures of Fractional Exclusion Statistics in the Spectroscopy of Quantum Hall Droplets
Nigel R. Cooper, Steven H. Simon
We show how spectroscopic experiments on a small Laughlin droplet of rotating bosons can directly demonstrate Haldane fractional exclusion statistics of quasihole excitations. The characteristic signatures appear in the single-particle excitation spectrum. We show that the transitions are governed by a "many-body selection rule" which allows one to relate the number of allowed transitions to the number of quasihole states on a finite geometry. We illustrate the theory with numerically exact simulations of small numbers of particles.
4. arXiv:1411.4635 (cross-list from cond-mat.mes-hall) [pdf, other]
Unconventional localisation transition in high dimensions
S.V. Syzranov, V. Gurarie, L. RadzihovskyWe study non-interacting systems with a power-law quasiparticle dispersion ξk∝kα and a random short-range-correlated potential. We show that, unlike the case of lower dimensions, for d>2α there exists a critical disorder strength (set by the band width), at which the system exhibits a disorder-driven quantum phase transition at the bottom of the band, that lies in a universality class distinct from the Anderson transition. In contrast to the conventional wisdom, it manifests itself in, e.g., the disorder-averaged density of states. For systems in symmetry classes that permit localisation, the striking signature of this transition is a non-analytic behaviour of the mobility edge, that is pinned to the bottom of the band for subcritical disorder and grows for disorder exceeding a critical strength. Focussing on the density of states, we calculate the critical behaviour (exponents and scaling functions) at this novel transition, using a renormalisation group, controlled by an ε=d−2α expansion. We also apply our analysis to Dirac materials, e.g., Weyl semimetal, where this transition takes place in physically interesting three dimensions.
Nov 17
1. arXiv:1411.3937 (cross-list from quant-ph) [pdf, other]
Entanglement Generation and Dynamics for a Bose-Hubbard model in a Double-Well Potential
Fabio Gentile, Arianna Montorsi, Marco RoncagliaThe study of entanglement between bosonic systems is of primary importance for establishing feasible resources needed for implementing quantum information protocols, both in their interacting atomic or photonic realizations. Atomic systems are particularly efficient in the production of large amounts of entanglement, providing higher information density than conventional qubit entangled states. Such increased quantum resources pave the way to novel fundamental tests of nature and efficient applications in quantum information, metrology and sensing. We consider a basic setup made up of two parties A and B, each one populated by a single level bosonic variable. The bosons are interacting and can hop between A and B, thus describing a two-site Bose-Hubbard Hamiltonian. We consider the generation of quantum states in several situations that cover the majority of physical realizations: ground state, finite temperature, unitary dynamics, dissipation through dephasing and loss of particles. The system is analyzed through truncated exact diagonalization, as a function of the microscopic parameters. The non separability of the obtained quantum states is estimated by means of the negativity, which has recently been proven to be a suitable measure of entanglement. Finally, we calculate lower bounds of the entanglement of formation, an indicator that quantifies the minimal amount of entanglement resources needed to build up such states.
Nov 14
1. arXiv:1411.3577 [pdf, other]
Emergence of coherence in a uniform quasi-two-dimensional Bose gas
Lauriane Chomaz, Laura Corman, Tom Bienaimé, Rémi Desbuquois, Christof Weitenberg, Sylvain Nascimbène, Jérôme Beugnon, Jean Dalibard
Phase transitions are ubiquitous in our three-dimensional world. By contrast most conventional transitions do not occur in infinite uniform two-dimensional systems because of the increased role of thermal fluctuations. Here we explore the dimensional crossover of Bose-Einstein condensation (BEC) for a weakly interacting atomic gas confined in a novel quasi-two-dimensional geometry, with a flat in-plane trap bottom. We detect the onset of an extended phase coherence, using velocity distribution measurements and matter-wave interferometry. We relate this coherence to the transverse condensation phenomenon, in which a significant fraction of atoms accumulate in the ground state of the motion perpendicular to the atom plane. We also investigate the dynamical aspects of the transition through the detection of topological defects that are nucleated in a quench cooling of the gas, and we compare our results to the predictions of the Kibble-Zurek theory for the conventional BEC second-order phase transition.
2. arXiv:1411.3483 [pdf, other]
Observation of chiral superfluid order by matter wave heterodyning
T. Kock, M. Ölschläger, A. Ewerbeck, W.-M. Huang, L. Mathey, A. Hemmerich
The breaking of time reversal symmetry via the spontaneous formation of chiral order is ubiquitous in nature. Here, we present an unambiguous demonstration of this phenomenon for atoms Bose-Einstein condensed in the second Bloch band of an optical lattice. As a key tool we use a matter wave heterodyning technique, which lets us directly observe the phase properties of the superfluid order parameter and allows us to reconstruct the spatial geometry of certain low energy excitations, associated with the formation of domains of different chirality. Our work marks a new era of optical lattices where orbital degrees of freedom play an essential role for the formation of exotic quantum matter, similarly as in electronic systems.
3. arXiv:1411.3345 [pdf, other]
An interaction-driven topological insulator in fermionic cold atoms on an optical lattice: A design with a density functional formalism
Sota Kitamura, Naoto Tsuji, Hideo Aoki
We design an interaction-driven topological insulator for fermionic cold atoms in an optical lattice, i.e., we pose a question whether we can realize in a continuous space a spontaneous symmetry breaking induced by the inter-atom interaction into a topological Chern insulator. Such a state, sometimes called topological Mott insulator (TMI), has yet to be realized in solid-state systems, since this requires, in the tight-binding model, large off-site interactions on top of a small on-site interaction. Here we overcome the difficulty by introducing a spin-dependent potential, where a spin-selective occupation of fermions in A and B sublattices makes the on-site interaction absent, while a sizeable inter-site interaction is achieved by a shallow optical potential with a large overlap between neighboring Wannier orbitals. This puts the system away from the tight-binding model, so that we adopt the density functional theory for cold-atoms, here extended to accommodate non-collinear spin structures emerging in the topological regime, to quantitatively demonstrate the phase transition to TMI.
Nov 13
1. arXiv:1411.2990 [pdf, ps, other]
Striped Ferronematic ground states in a spin-orbit coupled $S=1$ Bose gas
Stefan S. Natu, Xiaopeng Li, William S. Cole
We theoretically establish the mean-field phase diagram of a homogeneous spin-1, spin-orbit coupled Bose gas as a function of the spin-dependent interaction parameter, the Raman coupling strength and the quadratic Zeeman shift. We find that the interplay between spin-orbit coupling and spin-dependent interactions leads to the occurrence of ferromagnetic or ferronematic phases which also break translational symmetry. For weak Raman coupling, increasing attractive spin-dependent interactions (as in 87Rb or 7Li) induces a transition from a uniform to a stripe XY ferromagnet (with no nematic order). For repulsive spin-dependent interactions however (as in 23Na), we find a transition from an XY spin spiral phase (<Sz>=0and uniform total density) with uniaxial nematic order, to a biaxial ferronematic, where the total density, spin vector and nematic director oscillate in real space. We investigate the stability of these phases against the quadratic Zeeman effect, which generally tends to favor uniform phases with either ferromagnetic or nematic order but not both. We discuss the relevance of our results to ongoing experiments on spin-orbit coupled, spinor Bose gases.
2. arXiv:1411.3216 [pdf, ps, other]
Antiferromagnetic Order in a Spin-Orbit Coupled Bose-Einstein Condensate
Zhongbo Yan, Shaolong Wan
Spin-orbit coupling related new physics and quantum magnetism are two branches of great interest both in condensed matter physics and in cold atomic physics. With the introduction of a Rashba-like SOC into a Bose-Einstein condensate (BEC) loaded in a two-dimensional bipartite optical square lattice, we find that the ground state of the BEC always favors a coherent condensate than a fragmented condensate and always exhibits very large degeneracy, and most importantly, an antiferromagnetic order of quantum nature emerges when parameters satisfy certain condition. This provides an ideal platform to study the interplay of antiferromagnetic phase and superfluid phase.
3. arXiv:1411.3118 [pdf, other]
Quantum magnetism and topological ordering via Rydberg-dressing near F?örster-resonances
R. M. W. van Bijnen, T. Pohl
We devise a cold-atom approach to realizing a broad range of bi-linear quantum magnets. Our scheme is based on off-resonant single-photon excitation of Rydberg P-states, whose strong interactions and state-mixing are shown to yield controllable XYZ-interactions between effective spins, represented by different atomic ground states. Exploiting distinctive features of F?\"orster-resonant Rydberg atom interactions, we obtain large spin-interactions, up to three orders of magnitude in excess of corresponding decoherence rates. We illustrate the concept on a spin-1 chain implemented with cold Rubidium atoms, and demonstrate that this permits the dynamical preparation of topological magnetic phases. Generally, the described approach provides a viable route to exploring quantum magnetism with dynamically tuneable (an)isotropic interactions as well as variable space- and spin-dimensions in cold-atom experiments.
4. arXiv:1411.3069 [pdf, other]
Bose-Einstein condensation of 162Dy and 160Dy
Yijun Tang, Nathaniel Q. Burdick, Kristian Baumann, Benjamin L. Lev
We report Bose-Einstein condensation of two isotopes of the highly magnetic element dysprosium: 162Dy and 160Dy. For 162Dy, condensates with 10^5 atoms form below T = 50 nK. We find the evaporation efficiency for the isotope 160Dy to be poor; however, by utilizing a low-field Fano-Feshbach resonance to carefully change the scattering properties, it is possible to produce a Bose-Einstein condensate of 160Dy with 10^3 atoms. The 162Dy BEC reported is an order of magnitude larger in atom number than that of the previously reported 164Dy BEC, and it may be produced within 18 s.
5. arXiv:1411.3043 [pdf, other]
Fermi Gases with Synthetic Spin-Orbit Coupling
Jing Zhang, Hui Hu, Xia-Ji Liu, Han Pu
We briefly review recent progress on ultracold atomic Fermi gases with different types of synthetic spin-orbit coupling, including the one-dimensional (1D) equal weight Rashba-Dresselhaus and two-dimensional (2D) Rasbha spin-orbit couplings. Theoretically, we show how the single-body, two-body and many-body properties of Fermi gases are dramatically changed by spin-orbit coupling. In particular, the interplay between spin-orbit coupling and interatomic interaction may lead to several long-sought exotic superfluid phases at low temperatures, such as anisotropic superfluid, topological superfluid and inhomogeneous superfluid. Experimentally, only the first type - equal weight combination of Rasbha and Dresselhaus spin-orbit couplings - has been realized very recently using a two-photon Raman process. We show how to characterize a normal spin-orbit coupled atomic Fermi gas in both non-interacting and strongly-interacting limits, using particularly momentum-resolved radio-frequency spectroscopy. The experimental demonstration of a strongly-interacting spin-orbit coupled Fermi gas opens a promising way to observe various exotic superfluid phases in the near future.
6. arXiv:1411.3036 [pdf, other]
Many-body theories of density response for a strongly correlated Fermi gas
Hui Hu
Recent breakthroughs in the creation of ultra-cold atomic gases in the laboratory have ushered in major changes in physical science. Many novel experiments are now possible, with an unprecedented control of interaction, geometry and purity. Quantum many-body theory is facing severe challenges in quantitatively understanding new experimental results. Here, we review some recently developed theoretical techniques that provide successful predictions for density response of a strongly correlated atomic Fermi gas. These include the strong-coupling random-phase approximation theory, high-temperature quantum virial expansion, and asymptotically exact Tan relations applicable at large momentum.
7. arXiv:1411.2993 [pdf, other]
Three-dimensional spin-orbit coupled Fermi gases: Fulde-Ferrell pairing, Majorana fermions, Weyl fermions and gapless topological superfluidity
Xia-Ji Liu, Hui Hu, Han Pu
We theoretically investigate a three-dimensional Fermi gas with Rashba spin-orbit coupling in the presence of both out-of-plane and in-plane Zeeman fields. We show that, driven by a sufficiently large Zeeman field, either out-of-plane or in-plane, the superfluid phase of this system exhibits a number of interesting features, including inhomogeneous Fulde-Ferrell pairing, gapped or gapless topological order and exotic quasi-particle excitations known as Weyl fermions that have linear energy dispersions in momentum space (i.e., massless Dirac fermions). The topological superfluid phase can have either four or two topologically protected Weyl nodes. We present the phase diagrams at both zero and finite temperatures and discuss the possibility of their observation in an atomic Fermi gas with synthetic spin-orbit coupling. In this context, topological superfluid phases with an imperfect Rashba spin-orbit coupling are also studied.
Nov 12
1. arXiv:1411.2678 [pdf, other]
Quantum Control by Imaging : The Zeno effect in an ultracold lattice gas
Y. S. Patil, S. Chakram, M. Vengalattore
We demonstrate the control of quantum tunneling in an ultracold lattice gas by the measurement backaction imposed by an imaging process. A {\em in situ} imaging technique is used to acquire repeated images of an ultracold gas confined in a shallow optical lattice. The backaction induced by these position measurements modifies the coherent quantum tunneling of atoms within the lattice. By smoothly varying the rate at which spatial information is extracted from the atomic ensemble, we observe the continuous crossover from the 'weak measurement regime' where position measurements have little influence on the tunneling dynamics, to the 'strong measurement regime' where measurement-induced localization causes a large suppression of tunneling. This suppression of coherent tunneling is a manifestation of the Quantum Zeno effect. Our study realizes an experimental demonstration of the paradigmatic Heisenberg microscope in a lattice gas and sheds light on the implications of quantum measurement on the coherent evolution of a mesoscopic quantum system. In addition, this demonstrates a powerful technique for the control of an interacting many-body quantum system via spatially resolved measurement backaction.
2. arXiv:1411.2784 [pdf, other]
Bloch dynamics in lattices with long range hoppings
J. Stockhofe, P. Schmelcher
We study a discrete Schr\"odinger equation with arbitrary long range hopping terms under the influence of an external force. The impact of long range hoppings on the single particle Bloch dynamics in the lattice is investigated. A closed expression for the propagator is given, based on which we analyze the dynamics of initially Gaussian wave packets. Our findings capture the anharmonic oscillations recently observed in zigzag lattices and furthermore provide a detailed quantitative description of the crossover between center of mass Bloch oscillations for wide wave packets and left-right symmetric width oscillations for narrow single site excitations. The analytical results are shown to be in agreement with numerical simulations. A helix lattice setup for ultracold atoms is proposed where such hopping terms to far neighbors can be experimentally tuned to sizable values.
Nov 11
1. arXiv:1411.2297 [pdf, other]
Spin-orbit Coupling in Optical lattices
Shizhong Zhang, William S. Cole, Arun Paramekanti, Nandini Trivedi
In this review, we discuss the physics of spin-orbit coupled quantum gases in optical lattices. After reviewing some relevant experimental techniques, we introduce the basic theoretical model and discuss some of its generic features. In particular, we concentrate on the interplay between spin-orbit coupling and strong interactions and show how it leads to various exotic quantum phases in both the Mott insulating and superfluid regimes. Phase transitions between the Mott and superfluid states are also discussed.
2. arXiv:1411.2321 (cross-list from quant-ph) [pdf, ps, other]
Ultracold Three-body Recombination in Two Dimensions
J. P. D'Incao, Fatima Anis, B. D. Esry
We study three-body recombination in two dimensions for systems interacting via short-range two-body interactions in the regime of large scattering lengths. Using the adiabatic hyperspherical representation, we derive semi-analytical formulas for three-body recombination in both weakly and deeply bound diatom states. Our results demonstrate the importance of long-range corrections to the three-body potentials by showing how they alter the low-energy and scattering length dependence of the recombination rate for both bosonic and fermionic systems, which exhibit suppressed recombination if compared to the three-dimensional case. We verify these results through numerical calculations of recombination for systems with finite-range interactions and supporting a few two-body bound states. We also study finite-range effects for the energies of the universal three-identical-bosons states and found a slow approach to universal predictions as a function of the scattering length.
Nov 10
1. arXiv:1411.1847 [pdf, other]
Rotating a Bose-Einstein condensate by shaking an anharmonic axisymmetric magnetic potential
Seji Kang, J. Choi, S. W. Seo, W. J. Kwon, Y. Shin
We present an experimental method for rotating a Bose-Einstein condensate trapped in an axisymmetric magnetic potential. This method is based on the anharmonicity of the trapping potential, which couples the center-of-mass motion of the condensate to its internal motion. By circularly shaking the trapping potential, we generate a circular center-of-mass motion of the condensate around the trap center. The circulating condensate undergoes rotating shape deformation and eventually relaxes into a rotating condensate with a vortex lattice. We discuss the vortex nucleation mechanism and in particular, the role of the thermal cloud in the relaxation process. Finally, we investigate the dependence of the vortex nucleation on the elliptical polarization of the trap shaking. The response of the condensate is asymmetric with respect to the sign of the shaking polarization, demonstrating the gauge field effect due to the spin texture of the condensate in the magnetic potential.
Nov 7
1. arXiv:1411.1737 [pdf, other]
Spin - orbital angular momentum coupled Bose-Einstein condensates
Kuei Sun, Chunlei Qu, Chuanwei Zhang
Spin-orbit coupling (SOC) plays a crucial role in many branches of physics. In this context, the recent experimental realization of the coupling between spin and linear momentum of ultra-cold atoms opens a completely new avenue for exploring new spin-related superfluid physics. Here we propose that another important and fundamental SOC, the coupling between spin and orbital angular momentum (SOAM), can be implemented for ultra-cold atoms using higher order Laguerre-Gaussian laser beams to induce Raman coupling between two hyperfine spin states of atoms. We study the ground state phase diagrams of SOAM coupled Bose-Einstein condensates on a ring trap and explore their applications in gravitational force detection. Our results provide the basis for further investigation of intriguing superfluid physics induced by SOAM coupling.
2. arXiv:1411.1706 [pdf, other]
Size and shape of Mott regions for fermionic atoms in a two-dimensional optical lattice
Tiago Mendes-Santos, Thereza Paiva, Raimundo R. dos Santos
We investigate the harmonic-trap control of size and shape of Mott regions in the Fermi Hubbard model on a square optical lattice. The use of Lanczos diagonalization on clusters with twisted boundary conditions, followed by an average over 50-80 samples, drastically reduce finite-size effects in some ground state properties; calculations in the grand canonical ensemble together with a local-density approximation (LDA) allow us to simulate the radial density distribution. We have found that as the trap closes, the atomic cloud goes from a metallic state, to a Mott core, and to a Mott ring; the coverage of Mott atoms reaches a maximum at the core-ring transition. A `phase diagram' in terms of an effective density and the on-site repulsion is proposed, as a guide to maximize the Mott coverage. We also predict that the usual experimentally accessible quantities, the global compressibility and the average double occupancy (rather, its density derivative) display detectable signatures of the core-ring transition. Some spin correlation functions are also calculated, and predict the existence N\'eel ordering within Mott cores and rings.
3. arXiv:1411.1632 [pdf, ps, other]
Quasiparticle tunneling in a periodically driven bosonic Josephson junction
Bettina Gertjerenken, Martin Holthaus
A resonantly driven bosonic Josephson junction supports stable collective excitations, or quasiparticles, which constitute analogs of the Trojan wave packets previously explored with Rydberg atoms in strong microwave fields. We predict a quantum beating effect between such symmetryrelated many-body Trojan states taking place on time scales which are long in comparison with the driving period. Within a mean-field approximation, this quantum beating can be regarded as a manifestation of dynamical tunneling. On the full N-particle level, the beating phenomenon leads to an experimentally feasible, robust strategy for probing highly entangled mesoscopic states.
Nov 6
1. arXiv:1411.1203 [pdf, ps, other]
The Momentum-Space Harper-Hofstadter Model
Tomoki Ozawa, Hannah M. Price, Iacopo Carusotto
We show how the weakly trapped Harper-Hofstadter model can be mapped onto a Harper-Hofstadter model in momentum space: the Berry curvature plays the role of an effective magnetic field, the trap position sets the boundary conditions around the toroidal magnetic Brillouin zone, and spatially local interactions translate into non-local interactions in momentum space. Within a mean-field approximation, we show that increasing inter-particle interactions are responsible for a phase transition from a single rotationally-symmetric ground state to degenerate ground states that spontaneously break rotational symmetry.
Nov 5
1. arXiv:1411.0749 [pdf, other]
Mott Transition in a Two Leg Bose Hubbard Ladder Under an Artificial Magnetic Field
Ahmet Keleş, M. Ö. Okte
We consider the Bose Hubbard model on a two leg ladder under an artificial magnetic field, and investigate the superfluid to Mott insulator transition in this setting. Recently, this system has been experimentally realized [M.Atala et.al, Nature Physics 10, 588-593 (2014)], albeit in a parameter regime that is far from the Mott transition boundary. Depending on the strength of the magnetic field, the single particle spectrum has either a single ground state or two degenerate ground states. The transition between these two phases is reflected in the many particle properties. We first investigate these phases through the Bogoliubov approximation in the superfluid regime and calculate the transition boundary for weak interactions. For stronger interactions the system is expected to form a Mott insulator. We calculate the Mott transition boundary as a function of magnetic field and inter--leg coupling with mean field theory, strong coupling expansion and density matrix renormalization group (DMRG). Finally, using DMRG, we investigate the particle-hole excitation gaps of this system at different filling factors and find peaks at simple fractions indicating the possibility of correlated phases.
2. arXiv:1411.0688 [pdf, ps, other]
Non-destructive selective probing of phononic excitations in a cold Bose gas using impurities
D. Hangleiter, M. T. Mitchison, T. H. Johnson, M. Bruderer, M. B. Plenio, D. Jaksch
We introduce a detector that selectively probes the phononic excitations of a cold Bose gas. The detector is composed of a single impurity atom confined by a double-well potential, where the two lowest eigenstates of the impurity form an effective probe qubit that is coupled to the phonons via density-density interactions with the bosons. The system is analogous to a two-level atom coupled to photons of the radiation field. We demonstrate that tracking the evolution of the qubit populations allows probing both thermal and coherent excitations in targeted phonon modes. The targeted modes are selected in both energy and momentum by adjusting the impurity's potential. We show how to use the detector to observe coherent density waves and to measure temperatures of the Bose gas down to the nano-Kelvin regime. We analyze how our scheme could be realized experimentally, including the possibility of using an array of multiple impurities to achieve greater precision from a single experimental run.
Nov 4
1. arXiv:1411.0429 [pdf, ps, other]
Superradiant Phase Transition of Fermi Gases in a Cavity across a Feshbach Resonance
Yu Chen, Hui Zhai, Zhenhua Yu
In this letter we consider the superradiant phase transition of a two-component Fermi gas in a cavity across a Feshbach resonance. It is known that quantum statistics plays a crucial role for the superradiant phase transition in atomic gases; in contrast to bosons, in a Fermi gas this transition exhibits strong density dependence. We show that across a Feshbach resonance, while the two-component Fermi gas passes through the BEC-BCS crossover, the superradiant phase transition undergoes a corresponding crossover from a fermionic behavior on the weakly interacting BCS side, to a bosonic behavior on the molecular BEC side. This intricate statistics crossover makes the superradiance maximally enhanced either in the unitary regime for low densities, in the BCS regime for moderate densities close to Fermi surface nesting, or in the BEC regime for high densities.
2. arXiv:1411.0191 [pdf, other]
Motional-state Bell inequality test with ultracold atoms
R. J. Lewis-Swan, K. V. Kheruntsyan
Bell inequalities have arguably been regarded as "the most profound discovery in science". They provide a fundamental distinction between local hidden-variable (LHV) descriptions of physical reality and the description based on quantum mechanics wherein the concept of nonlocal entanglement is a fundamental ingredient. Violations of Bell inequalities, which reject all LHV theories and attest for the validity of quantum mechanics, have been demonstrated in numerous experiments with massless photons, but in only a handful of experiments involving massive particles. In addition, all massive particle experiments have so far been restricted to exploiting entanglement between internal (spin) degrees of freedom, but never between external (motional) degrees of freedom such as translational momentum. Here, we propose and simulate a matter-wave experiment which, for the first time, can demonstrate a Bell inequality violation for pairs of momentum-entangled ultracold atoms produced in a collision of two Bose-Einstein condensates. In such a motional-state Bell inequality test, particle masses become directly relevant, thus enabling extensions of fundamental tests of quantum mechanics into regimes which may involve couplings to gravitational fields. Such regimes are relevant to theories of gravitational decoherence and can therefore shed light on theoretical constructs of quantum gravity.
Nov 3
1. arXiv:1410.8814 [pdf, other]
Integrated coherent matter wave circuits
C. Ryu, M. G. Boshie
An integrated coherent matter wave circuit is a single device, analogous to an integrated optical circuit, in which coherent de Broglie waves are created and then propagate freely in waveguides where they can be switched, divided, recombined, and detected. Applications of such circuits include guided atom interferometers, atomtronic circuits, and precisely controlled delivery of atoms. Here we report experiments demonstrating integrated matter wave circuits for guided coherent matter waves. The circuit elements are created with the painted potential technique, a form of time-averaged optical dipole potential in which a rapidly-moving, tightly-focused laser beam exerts forces on atoms through their electrical polarizability. The source of coherent matter waves is a Bose-Einstein condensate (BEC). We launch BECs into painted waveguides that guide them around bends and form switches, phase coherent beamsplitters, and closed circuits. These developments open the door to creating arbitrary and dynamic coherent matter wave circuits.
2. arXiv:1410.8716 [pdf, ps, other]
Twisted behavior of dipolar BECs: Dipole-dipole interaction beyond the self-consistent field approximation and exchange electric dipole interaction
Pavel A. Andreev
Dipole-dipole interaction is a long-range interaction, hence we could expect that the self-consistent field approximation might be applied. In most cases it is correct, but dipolar BECs reveal a surprise. Structure of the self-consistent field term requires that interacting particles are in different quantum states, while in BECs all particles in a single quantum state. This fact requires to consider the two-particle polarisation, which describes dipole-dipole interaction, in more details. We present this consideration and show an astonishing result that the two-particle quantum correlation in dipolar BECs reveals in the same form as the self-consistent field term.
