Dec+2014

flat =Dec 1-Dec 5, Zhenyu zhou & Jinlong Yu, Dec 8-Dec 12, Jiyao chen & Jianhui zhou, Dec 15-Dec 19, Haiyuan Zou & Ahmet Keles, Dec 22-Dec 26, Zhifang Xu & Xuguang Yue, Dec 29-Jan 2 Bo Liu & Max=

=Dec 31= 

1. [|arXiv:1412.8472] [[|pdf], [|other]] **Exotic roton excitations in quadrupolar Bose-Einstein condensates** [|M. Lahrz], [|Mikhail Lemeshko], [|L. Mathey] We investigate the occurrence of rotons in a quadrupolar Bose-Einstein condensate confined to two dimensions. Depending on the particle density, the ratio of the contact and quadrupole-quadrupole interactions, and the alignment of the quadrupole moments with respect to the confinement plane, the dispersion relation features two or four point-like roton minima, or one ring-shaped minimum. We map out the entire parameter space of the roton behavior and identify the instability regions. We propose to observe the exotic rotons by monitoring the characteristic density wave dynamics resulting from a short local perturbation, and discuss the possibilities to detect the predicted effects in state-of-the-art experiments with ultracold homonuclear molecules.  2. [|arXiv:1412.8507] (cross-list from hep-th) [[|pdf], [|ps], [|other]] **OPE convergence in non-relativistic conformal field theories** [|Walter D. Goldberger], [|Zuhair U. Khandker], [|Siddharth Prabhu] Motivated by applications to the study of ultracold atomic gases near the unitarity limit, we investigate the structure of the operator product expansion (OPE) in non-relativistic conformal field theories (NRCFTs). The main tool used in our analysis is the representation theory of charged (i.e. non-zero particle number) operators in the NRCFT, in particular the mapping between operators and states in a non-relativistic "radial quantization" Hilbert space. Our results include: a determination of the OPE coefficients of descendant operators in terms of those of the underlying primary state, a demonstration of convergence of the (imaginary time) OPE in certain kinematic limits, and an estimate of the decay rate of the OPE tail inside matrix elements which, as in relativistic CFTs, depends exponentially on operator dimensions. To illustrate our results we consider several examples, including a strongly interacting field theory of bosons tuned to the unitarity limit, as well as a class of holographic models. Given the similarity with known statements about the OPE in SO(2,d) invariant field theories, our results suggest the existence of a bootstrap approach to constraining NRCFTs, with applications to bound state spectra and interactions. We briefly comment on a possible implementation of this non-relativistic conformal bootstrap program. 3. [|arXiv:1412.8477] (cross-list from quant-ph) [[|pdf], [|ps], [|other]] **Photon-mediated interactions: a scalable tool to create and sustain entangled many-body states** [|Camille Aron], [|Manas Kulkarni], [|Hakan E. Türeci] Generation and sustenance of entangled many-body states is of fundamental and applied interest. Recent experimental progress in the stabilization of two-qubit Bell states in superconducting quantum circuits using an autonomous feedback scheme [S. Shankar et al., Nature 504, 419 (2013)] has demonstrated the effectiveness and robustness of driven-dissipative approaches, i.e. engineering a fine balance between driven-unitary and dissipative dynamics. Despite the remarkable theoretical and experimental progress in those approaches for superconducting circuits, no demonstrably scalable scheme exists to drive an arbitrary number of spatially separated qubits to a desired entangled quantum many-body state. Here we propose and study such a scalable scheme, based on engineering photon-mediated interactions, for driving a register of spatially separated qubits into multipartite entangled states. We demonstrate how generalized W-states can be generated with remarkable fidelities and the entanglement sustained for an indefinite time. The protocol is primarily discussed for a superconducting circuit architecture but is ideally realized in any platform that permits controllable delivery of coherent light to specified locations in a network of Cavity QED systems.

=Dec 30= 1. [|arXiv:1412.8403] [[|pdf], [|other]] Traces of integrability in relaxation of one-dimensional two-mass mixtures [|Zaijong Hwang], [|Frank Cao], [|Maxim Olshanii] Subjects: Quantum Gases (cond-mat.quant-gas)

We study relaxation in a one-dimensional two-mass mixture of hard-core particles. A heavy-light-heavy triplet of three neighboring particles can form a little known unequal mass generalization of Newton's cradle at particular light-to-heavy mass ratios. An anomalous slow-down in the relaxation of the whole system is expected due to the presence of these triplets, and we provide numerical evidence to support this prediction. The expected experimental realization of our model involves mixtures of two internal states in optical lattices, where the ratio between effective masses can be controlled at will. = = =Dec 29= 1. [|arXiv:1412.7361] [[|pdf], [|other]] Non-equilibrium Berezinskii-Kosterlitz-Thouless transition in a driven open quantum system [|G. Dagvadorj], [|J. M. Fellows], [|S. Matyjaskiewicz], [|F. M. Marchetti], [|I. Carusotto], [|M. H. Szymanska] Subjects: Quantum Gases (cond-mat.quant-gas)

The Berezinskii-Kosterlitz-Thouless mechanism, in which a phase transition is mediated by the proliferation of topological defects, governs the critical behaviour of a wide range of equilibrium two-dimensional systems with a continuous symmetry, ranging from superconducting thin films to two-dimensional Bose fluids, such as liquid helium and ultracold atoms. We show here that this phenomenon is not restricted to thermal equilibrium, rather it survives more generally in a dissipative highly non-equilibrium system driven into a steady-state. By considering a light-matter superfluid of polaritons, in the so-called optical parametric oscillator regime, we demonstrate that it indeed undergoes a vortex binding-unbinding phase transition. Yet, the exponent of the power-law decay of the first order correlation function in the (algebraically) ordered phase can exceed the equilibrium upper limit -- a surprising occurrence, which has also been observed in a recent experiment. Thus we demonstrate that the ordered phase is somehow more robust against the quantum fluctuations of driven systems than thermal ones in equilibrium.

=Dec 25= 1. [|arXiv:1412.7615] [[|pdf], [|other]] Weyl points in three-dimensional optical lattices: synthetic magnetic monopoles in momentum space [|Tena Dubček], [|Colin J. Kennedy], [|Ling Lu], [|Wolfgang Ketterle], [|Marin Soljačić], [|Hrvoje Buljan] Comments: are welcome Subjects: Quantum Gases (cond-mat.quant-gas)

We show that Hamiltonians with Weyl points can be realized for ultracold atoms using laser-assisted tunneling in three-dimensional optical lattices. Weyl points are synthetic magnetic monopoles that exhibit a robust, three-dimensional linear dispersion. They are associated with many interesting topological states of matter, such as Weyl semimetals and chiral Weyl fermions. However, Weyl points have yet to be experimentally observed in any system. We show that this elusive goal is well-within experimental reach with an extension of the techniques recently used to obtain the Harper Hamiltonian. 2. [|arXiv:1412.7605] [[|pdf], [|other]] Proposal for feasible experiments of cold-atom quantum simulator of U(1) lattice gauge-Higgs model[|Yoshihito Kuno], [|Kenichi Kasamatsu], [|Yoshiro Takahashi], [|Ikuo Ichinose], [|Tetsuo Matsui] Comments: 11 pages, 7 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; High Energy Physics - Lattice (hep-lat); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph) Lattice gauge theory has provided us with a crucial non-perturbative method in studying canonical models in high-energy physics such as quantum chromodynamics. Among other models of lattice gauge theory, the lattice gauge-Higgs model is a quite important one because it describes wide variety of phenomena/models related to the Anderson-Higgs mechanism such as superconductivity, the standard model of particle physics, and inflation process of the early universe. In this paper, to realize a quantum simulator of the U(1) lattice gauge-Higgs model on an optical lattice filled by cold atoms, we propose two feasible methods: (i) Wannier states in the excited bands and (ii) dipolar atoms in a multilayer optical lattice. We pay attentions to respect the constraint of Gauss's law and avoid nonlocal gauge interactions. Numerical simulations of the time development of an electric flux by using the Gross-Pitaevskii equations reveal some interesting characteristics of dynamical aspect of the model.

=Dec 24= 1. [|arXiv:1412.7165] [[|pdf], [|other]] Quantum quench phase diagrams of an s-wave BCS-BEC condensate[|Emil A. Yuzbashyan], [|Maxim Dzero], [|Victor Gurarie], [|Matthew S. Foster] Comments: 46 pages, 26 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Superconductivity (cond-mat.supr-con)

We study the dynamic response of an s-wave BCS-BEC (atomic-molecular) condensate to detuning quenches within the two channel model beyond the weak coupling BCS limit. At long times after the quench, the condensate ends up in one of three main asymptotic states (nonequilibrium phases), which are qualitatively similar to those in other fermionic condensates defined by a global complex order parameter. In phase I the amplitude of the order parameter vanishes as a power law, in phase II it goes to a nonzero constant, and in phase III it oscillates persistently. We construct exact quench phase diagrams that predict the asymptotic state (including the many-body wavefunction) depending on the initial and final detunings and on the Feshbach resonance width. Outside of the weak coupling regime, both the mechanism and the time dependence of the relaxation of the amplitude of the order parameter in phases I and II are modified. Also, quenches from arbitrarily weak initial to sufficiently strong final coupling do not produce persistent oscillations in contrast to the behavior in the BCS regime. The most remarkable feature of coherent condensate dynamics in various fermion superfluids is an effective reduction in the number of dynamic degrees of freedom as the evolution time goes to infinity. As a result, the long time dynamics can be fully described in terms of just a few new collective dynamical variables governed by the same Hamiltonian only with "renormalized" parameters. Combining this feature with the integrability of the underlying (e.g. the two channel) model, we develop and consistently present a general method that explicitly obtains the exact asymptotic state of the system.

=Dec 23= 1. [|arXiv:1412.6685] [[|pdf], [|ps], [|other]] Spin-Imbalanced Quasi-Two-Dimensional Fermi Gases [|W. Ong], [|C.-Y. Cheng], [|I. Arakelyan], [|J. E. Thomas] Comments: 14 pages, 12 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We measure the density profiles for a Fermi gas of ^6 Li containing N_1 spin-up atoms and N_2 spin-down atoms, confined in a quasi-two-dimensional geometry. The spatial profiles are measured as a function of spin-imbalance <span class="MathJax_Preview">N_2/N_1 and interaction strength, which is controlled by means of a collisional (Feshbach) resonance. The measured cloud radii and central densities are in disagreement with mean-field Bardeen-Cooper-Schrieffer theory for a true two-dimensional system. We find that the data for normal-fluid mixtures are reasonably well fit by a simple two-dimensional polaron model of the free energy. Not predicted by the model is a phase transition to a spin-balanced central core, which is observed above a critical value of <span class="MathJax_Preview">N_2/N_1. Our observations provide important benchmarks for predictions of the phase structure of quasi-two-dimensional Fermi gases. 2. [|arXiv:1412.6672] [[|pdf], [|ps], [|other]] Competing Orders in a Dipolar Bose-Fermi Mixture on a Square Optical Lattice: Mean-Field Perspective[|Jasen Scaramazza], [|Ben Kain], [|Hong Y. Ling] Comments: 18 pages, 9 figures Subjects: Quantum Gases (cond-mat.quant-gas) We consider a mixture of a two-component Fermi gas and a single-component dipolar Bose gas in a square optical lattice and reduce it into an effective Fermi system where the Fermi-Fermi interaction includes the attractive interaction induced by the phonons of a uniform dipolar Bose-Einstein condensate. Focusing on this effective Fermi system in the parameter regime that preserves the symmetry of <span class="MathJax_Preview">D_4, the point group of a square, we explore, within the Hartree-Fock-Bogoliubov mean-field theory, the phase competition among density wave orderings and superfluid pairings. We construct the matrix representation of the linearized gap equation in the irreducible representations of <span class="MathJax_Preview">D_4. We show that in the weak coupling regime, each matrix element, which is a four-dimensional (4D) integral in momentum space, can be put in a separable form involving a 1D integral, which is only a function of temperature and the chemical potential, and a pairing-specific "effective" interaction, which is an analytical function of the parameters that characterize the Fermi-Fermi interactions in our system. We analyze the critical temperatures of various competing orders as functions of different system parameters in both the absence and presence of the dipolar interaction. We find that close to half filling, the <span class="MathJax_Preview">d_{x^{2}-y^{2}} -wave pairing with a critical temperature in the order of a fraction of Fermi energy (at half filling) may dominate all other phases, and at a higher filling factor, the <span class="MathJax_Preview">p -wave pairing with a critical temperature in the order of a hundredth of Fermi energy may emerge as a winner. We find that tuning a dipolar interaction can dramatically enhance the pairings with <span class="MathJax_Preview">d_{xy} - and <span class="MathJax_Preview">g -wave symmetries but not enough for them to dominate other competing phases.

=Dec 22= 1. [|arXiv:1412.6377] [[|pdf], [|other]] Non-equilibrium dynamics of coupled Luttinger liquids[|Laura Foini], [|Thierry Giamarchi] Comments: 8 pages Subjects: Quantum Gases (cond-mat.quant-gas)

In this work we consider the dynamics of two tunnel coupled chains after a quench in the tunneling strength is performed and the two systems are let evolve independently. We describe the form of the initial state comparing with previous results concerning the dynamics after the splitting of a one-dimensional gas of bosons into two phase coherent systems. We compute different correlation functions, among which those that are relevant for interference measurements, and discuss the emergence of effective temperatures also in connection with previous works.

2. [|arXiv:1412.6186] [[|pdf], [|other]] A route to non-Abelian quantum turbulence in spinor Bose-Einstein condensates[|Thomas Mawson], [|Gary Ruben], [|Tapio Simula] Comments: 11 pages, 10 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph)

We have studied computationally the collision dynamics of spin-2 Bose--Einstein condensates initially confined in a triple-well trap. Depending on the intra- and inter-component relative phases of the initial state spinor wave function, the collision of the three condensate segments produces one of many possible vortex-antivortex lattices after which the system transitions to quantum turbulence. We find that the emerging vortex lattice structures can be described in terms of multi-wave interference. We show that the three-segment collisions can be used to systematically produce staggered vortex-antivortex honeycomb lattices of fractional-charge vortices, whose collision dynamics are known to be non-Abelian. Such condensate collider experiments could potentially be used as a controllable pathway to generating non-Abelian superfluid turbulence with networks of vortex rungs.

3. [|arXiv:1412.6342] (cross-list from physics.optics) [[|pdf], [|other]] Observation of a localized flat-band state in a photonic Lieb lattice[|Sebabrata Mukherjee], [|Alexander Spracklen], [|Debaditya Choudhury], [|Nathan Goldman], [|Patrik Öhberg], [|Erika Andersson], [|Robert R. Thomson] Comments: 6 pages + supplementary material. Comments are welcome Subjects: Optics (physics.optics) ; Quantum Gases (cond-mat.quant-gas)

We show experimentally how a non-diffracting state can be excited in a photonic Lieb lattice. This lattice supports three energy bands, including a perfectly flat middle band, which corresponds to an infinite effective mass with zero dispersion. We show that a suitable optical input state can be prepared so as to only excite the flat band. We analyse, both experimentally and theoretically, the evolution of such photonic flat-band states, and show their remarkable robustness, even in the presence of disorder.

=Dec 19= [|arXiv:1412.6059] [ [|pdf], [|ps] , [|other] ] Artificial magnetic fields in spin chains with long-range interactions - fractal energy spectra and topological phases [|Tobias Grass], [|Christine Muschik] , [|Alessio Celi] , [|Ravindra Chhajlany] , [|Maciej Lewenstein]   Comments: 4+4 pages, 4+1 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Spin chains with tunable interaction range have become available in systems of trapped ions or atoms coupled to waveguides. The spin chain can be mapped onto hard-core bosons moving on a higher-dimensional graph, where the dimensionality of the graph is given by the range of the interactions. We propose to use shaking techniques to mimic magnetic fields. This can lead to a fractal energy spectrum, and the appearance of topological phases, which we reveal by calculating edge states and Chern numbers.

=Dec 17=

[|arXiv:1412.4788] [ [|pdf], [|ps] , [|other] ] Photonic Architectures for Equilibrium High-Temperature Bose-Einstein Condensation in Dichalcogenide Monolayers [|Jian-Hua Jiang], [|Sajeev John]   Journal-ref: Scientific Reports 4, 7432 (2014) Subjects: Quantum Gases (cond-mat.quant-gas) ; Materials Science (cond-mat.mtrl-sci); Optics (physics.optics) Semiconductor-microcavity polaritons are composite quasiparticles of excitons and photons, emerging in the strong coupling regime. As quantum superpositions of matter and light, polaritons have much stronger interparticle interactions compared with photons, enabling rapid equilibration and Bose-Einstein condensation (BEC). Current realizations based on 1D photonic structures, such as Fabry-P\'erot microcavities, have limited light-trapping ability resulting in picosecond polariton lifetime. We demonstrate, theoretically, above-room-temperature (up to 590 K) BEC of long-lived polaritons in MoSe$_2$ monolayers sandwiched by simple TiO$_2$ based 3D photonic band gap (PBG) materials. The 3D PBG induces very strong coupling of 40 meV (Rabi splitting of 62 meV) for as few as three dichalcogenide monolayers. Strong light-trapping in the 3D PBG enables the long-lived polariton superfluid to be robust against fabrication-induced disorder and exciton line-broadening.

[|arXiv:1412.5095] [ [|pdf], [|other] ] Long Distance Coupling of a Quantum Mechanical Oscillator to the Internal States of an Atomic Ensemble [|B. Vogell], [|T. Kampschulte] , [|M. T. Rakher] , [|A. Faber] , [|P. Treutlein] , [|K. Hammerer] , [|P. Zoller]   Comments: 14 pages, 5 figures Subjects: Quantum Physics (quant-ph) We propose and investigate a hybrid optomechanical system consisting of a micro-mechanical oscillator coupled to the internal states of a distant ensemble of atoms. The interaction between the systems is mediated by a light field which allows to couple the two systems in a modular way over long distances. Coupling to internal degrees of freedom of atoms opens up the possibility to employ high-frequency mechanical resonators in the MHz to GHz regime, such as optomechanical crystal structures, and to benefit from the rich toolbox of quantum control over internal atomic states. Previous schemes involving atomic motional states are rather limited in both of these aspects. We derive a full quantum model for the effective coupling including the main sources of decoherence. As an application we show that sympathetic ground-state cooling and strong coupling between the two systems is possible.

=Dec 16=

[|arXiv:1412.4377] [ [|pdf], [|ps] , [|other] ] Spatially partitioned many-body vortices [|Shachar Klaiman], [|Ofir E. Alon]   Comments: 5+eps+3 pages, 3+3 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) A vortex in Bose-Einstein condensates is a localized object which looks much like a tiny tornado storm. It is well described by mean-field theory. In the present work we go beyond the current paradigm and introduce many-body vortices. These are made of {\it spatially-partitioned} clouds, carry definite total angular momentum, and are fragmented rather than condensed objects which can only be described beyond mean-field theory. A phase diagram based on a mean-field model assists in predicting the parameters where many-body vortices occur. Implications are briefly discussed.

[|arXiv:1412.4420] [ [|pdf], [|ps] , [|other] ] General ultracold scattering formalism with isotropic spin orbit coupling [|Su-Ju Wang], [|Chris H. Greene]   Comments: 10 pages, 9 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph) A general treatment of ultracold two-body scattering in the presence of isotropic spin-orbit coupling (SOC) is presented. Owing to the mixing of different partial wave channels, scattering with SOC is in general a coupled multichannel problem. A systematic method is introduced to analytically solve a class of coupled differential equations by recasting the coupled channel problem as a simple eigenvalue problem. The exact Green's matrix in the presence of SOC is found, which readily gives the scattering solutions for any two identical particles in any total angular momentum subspace having negligible center of mass momentum. Application of this formalism to two spin-1 bosons shows the ubiquitous low energy threshold behavior for systems with isotropic SOC. A modified threshold behavior shows up, which does not occur for the spin-orbit coupled spin-1/2 system. We also confirm the parity-breaking mechanism for the spontaneous emergency of handedness, that has been proposed by Duan et. al. [1]. Additionally, a two-body bound state is found for any arbitrarily small and negative scattering length. Our study sheds light on the few-body side of SOC physics and provides one step towards understanding ultracold scattering in a non-Abelian gauge field.

[|arXiv:1412.4263] [ [|pdf], [|ps] , [|other] ] Quantum phases of the frustrated XY models on the honeycomb lattice [|Zhenyue Zhu], [|Steven R. White]   Comments: Invited review paper for modern physics letter B, including 22 pages, 10 figures Journal-ref: Mod. Phys. Lett. B, Vol. 28, No. 31 (2014) 1430016 Subjects: Strongly Correlated Electrons (cond-mat.str-el) Searching for spin liquid states has long been attracting both experimentalists and theorists. In this article, we review recent density matrix renormalization group studies of the spin-1/2 XY model on the honeycomb lattice, with first-neighbor ($J_1 = 1$) and frustrating second-neighbor ($J_2>0$) interactions. For the intermediate frustration regime $0.22\lesssim J_2\lesssim0.36$, there exists a surprising antiferromagnetic Ising phase, with ordered moments pointing along the $z$ axis, despite the absence of any $S_z S_z$ interactions in the Hamiltonian. Surrounding this phase as a function of $J_2$ are antiferromagnetic phases with the moments pointing in the $\textit{x-y}$ plane for small $J_2$ and a close competition between an $\textit{x-y}$ plane magnetic collinear phase and a dimer phase for large values of $J_2$. No spin-liquid phases was found in the XY model even with the third neighbor ($J_3>0$) interactions.

=Dec 15= [|arXiv:1412.3992] [ [|pdf], [|ps] , [|other] ] Destabilization effect of exchange dipole-dipole interaction on the spectrum of electric dipolar ultracold Fermi gas [|Pavel A. Andreev] Comments: 13 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) The self-consistent field approach for the electric dipolar ultracold spin-1/2 fermions is discussed. Contribution of the exchange part of the electric dipole interaction is found. Hence we obtain a model of dipolar fermions beyond the self-consistent field approximation. It is shown that the exchange interaction of electric dipolar fermions depends on the spin-polarisation of the system. For instance the electric dipole exchange interaction equals to zero for spin-unpolarised systems, namely all low laying quantum states occupied by two-particles with opposite spins. In opposite limit of the full spin polarisation of the degenerate fermions, then we have one particle in each quantum states, the exchange interaction has maximum value, which is comparable with the self-consistent field part of the dipole-dipole interaction. The self-consistent part of the electric dipole-dipole interaction gives a positive contribution into the spectrum of collective excitations, while the exchange part of the dipole-dipole interaction leads to a negative term in the spectrum. At angles between the equilibrium polarisation and the direction of wave propagation close to $\pi/2$ the full dipolar part of the spectrum becomes negative. At the electric dipole moment of fermions of order of several Debay the dipolar part is large enough to exceed the Fermi pressure, that reveals in an instability.

1. [|arXiv:1412.1625] [ [|pdf], [|ps], [|other] ]
Energy of strongly attractive Bose-Fermi mixtures [|Andrea Guidini], [|Elisa Fratini], [|Gianluca Bertaina], [|Pierbiagio Pieri] Comments: 5 pages, 2 figures, submitted to the EPJST volume "Novel Quantum Phases and Mesoscopic Physics in Quantum Gases" Subjects: Quantum Gases (cond-mat.quant-gas) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We discuss how approximate theories for Bose-Fermi mixtures recover in the molecular limit the expected expression for Fermi-Fermi mixtures of molecules and unpaired fermions. In particular, we compare the energy of the system resulting from a T-matrix diagrammatic approach with that obtained with Variational and Fixed-Node Diffusion Quantum Monte Carlo methods.

==2. [|arXiv:1412.1531] (cross-list from quant-ph) [ [|pdf], [|ps] , [|other] ] == Probabilistic Q-function distributions in fermionic phase-space [|Laura E. C. Rosales-Zarate], [|P. D. Drummond] Subjects: Quantum Physics (quant-ph) ; Quantum Gases (cond-mat.quant-gas) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We obtain a positive probability distribution or Q-function for an arbitrary fermionic many-body system. This is different to previous Q-function proposals, which were either restricted to a subspace of the overall Hilbert space, or used Grassmann methods that do not give probabilities. The fermionic Q-function obtained here is constructed using normally ordered Gaussian operators, which include both non-interacting thermal density matrices and BCS states. We prove that the Q-function exists for any density matrix, is real and positive, and has moments that correspond to Fermi operator moments. It is defined on a finite symmetric phase-space equivalent to the space of real, antisymmetric matrices. This has the natural SO(2M) symmetry expected for Majorana fermion operators. We show that there is a natural physical interpretation of the Q-function: it is the relative probability for observing a given Gaussian density matrix. The distribution has a uniform probability across the space at infinite temperature, while for pure states it has a maximum value on the phase-space boundary. The advantage of probabilistic representations is that they can be used for computational sampling without a sign problem.

1. [|arXiv:1412.0800] [ [|pdf], [|ps], [|other] ]
Dynamical generation of dark solitons in spin-orbit-coupled Bose-Einstein condensates [|Shuai Cao], [|Chuan-Jia Shan], [|Dan-Wei Zhang], [|Xizhou Qin], [|Jun Xu] Comments: 10 pages, 9 figure; accepted for publication in JOSA B Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We numerically investigate the ground state, the Raman-driving dynamics and the nonlinear excitations of a realized spin-orbit-coupled Bose-Einstein condensate in a one-dimensional harmonic trap. Depending on the Raman coupling and the interatomic interactions, three ground-state phases are identified: stripe, plane wave and zero-momentum phases. A narrow parameter regime with coexistence of stripe and zero-momentum or plane wave phases in real space is found. Several sweep progresses across different phases by driving the Raman coupling linearly in time is simulated and the non-equilibrium dynamics of the system in these sweeps are studied. We find kinds of nonlinear excitations, with the particular dark solitons excited in the sweep from the stripe phase to the plane wave or zero-momentum phase within the trap. Moreover, the number and the stability of the dark solitons can be controlled in the driving, which provide a direct and easy way to generate dark solitons and study their dynamics and interaction properties.

1. [|arXiv:1412.0138] (cross-list from nucl-ex) [ [|pdf] ]
Momentum sharing in imbalanced Fermi systems [|O. Hen], [|M. Sargsian] , [|L.B. Weinstein] , [|E. Piasetzky] , [|H. Hakobyan] , [|D. W. Higinbotham] , [|M. Braverman] , [|W.K. Brooks] , [|S. Gilad] , [|K. P. Adhikari] , [|J. Arrington] , [|G. Asryan] , [|H. Avakian] , [|J. Ball] , [|N. A. Baltzell] , [|M. Battaglieri] , [|A. Beck] , [|S. May-Tal Beck] , [|I. Bedlinskiy] , [|W. Bertozzi] , [|A. Biselli] , [|V. D. Burkert] , [|T. Cao] , [|D. S. Carman] , [|A. Celentano] , [|S. Chandavar] , [|L. Colaneri] , [|P. L. Cole] , [|V. Crede] , [|A. DAngelo] , [|R. De Vita] , [|A. Deur] , [|C. Djalali] , [|D. Doughty] , [|M. Dugger] , [|R. Dupre] , [|H. Egiyan] , [|A. El Alaoui] , [|L. El Fassi] , [|L. Elouadrhiri] , [|G. Fedotov] , [|S. Fegan] , [|T. Forest] , [|B. Garillon] , [|M. Garcon] , [|N. Gevorgyan] , [|Y. Ghandilyan] , [|G. P. Gilfoyle] , [|F. X. Girod] , [|J. T. Goetz] , [|R. W. Gothe] , [|K. A. Griffioen] , [|M. Guidal] , [|L. Guo] , [|K. Hafidi] , [|C. Hanretty] , [|M. Hattawy] , [|K. Hicks] , [|M. Holtrop] , [|C. E. Hyde] , [|Y. Ilieva] , [|D. G. Ireland] , [|B.I. Ishkanov] , et al. (78 additional authors not shown)  Comments: Published in Science. 10 pages, 3 figures Journal-ref: Science 346, 614 (2014) Subjects: Nuclear Experiment (nucl-ex) ; Quantum Gases (cond-mat.quant-gas); Nuclear Theory (nucl-th) The atomic nucleus is composed of two different kinds of fermions, protons and neutrons. If the protons and neutrons did not interact, the Pauli exclusion principle would force the majority fermions (usually neutrons) to have a higher average momentum. Our high-energy electron scattering measurements using 12C, 27Al, 56Fe and 208Pb targets show that, even in heavy neutron-rich nuclei, short-range interactions between the fermions form correlated high-momentum neutron-proton pairs. Thus, in neutron-rich nuclei, protons have a greater probability than neutrons to have momentum greater than the Fermi momentum. This finding has implications ranging from nuclear few body systems to neutron stars and may also be observable experimentally in two-spin state, ultra-cold atomic gas systems.

==<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">2. [|arXiv:1412.0571] (cross-list from cond-mat.mes-hall) [ [|pdf], [|other] ] == Topological Hofstadter Insulators in a Two-Dimensional Quasicrystal [|Duc-Thanh Tran], [|Alexandre Dauphin] , [|Nathan Goldman] , [|Pierre Gaspard]   Comments: 5 pages, 5 figures + Appendices (A,B,C) Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ; Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph) We investigate the properties of a two-dimensional quasicrystal in the presence of a uniform magnetic field. In this configuration, the density of states (DOS) displays a Hofstadter butterfly-like structure when it is represented as a function of the magnetic flux per tile. We show that the low-DOS regions of the energy spectrum are associated with chiral edge states, in direct analogy with the Chern insulators realized with periodic lattices. We establish the topological nature of the edge states by computing the topological Chern number associated with the bulk of the quasicrystal. This topological characterization of the non-periodic lattice is achieved through a local (real-space) topological marker. This work opens a route for the exploration of topological insulating materials in a wide range of non-periodic lattice systems, including photonic crystals and cold atoms in optical lattices.

3. [|arXiv:1412.0521] [ [|pdf], [|other] ]
Ferroelectric quantum phase transition with cold polar molecules [|Markus Klinsmann], [|David Peter] , [|Hans Peter Büchler]   Comments: 8 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas)

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We analyze a system of polar molecules in a one-dimensional optical lattice. By controlling the internal structure of the polar molecules with static electric and microwave fields, we demonstrate the appearance of a quantum phase transition into a ferroelectric phase via spontaneous breaking of a <span class="MathJax" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> U ( 1 ) <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> symmetry. The phase diagram is first analyzed within mean-field theory, while in a second step the results are verified by a mapping onto the Bose-Hubbard model for hard-core bosons. The latter is studied within the well-established bosonization procedure. We find that the ferroelectric phase is characterized by (quasi) long-range order for the electric dipole moments.