February2012

= = flat = Feb 27 - Mar 2, Zixu Zhang =

Non-Abelian statistics of vortices with multiple Majorana fermions
[|Yuji Hirono], [|Shigehiro Yasui] , [|Kazunori Itakura] , [|Muneto Nitta] Comments: 23 pages Subjects: Superconductivity (cond-mat.supr-con) ; High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th) We consider the exchange statistics of vortices, each of which traps an odd number ($N$) of Majorana fermions. We assume that the fermions in a vortex transform in the vector representation of SO(N) group. Exchange of two vortices turns out to be non-Abelian, and the corresponding operator is further decomposed into two parts: a part that is essentially equivalent to the exchange operator of vortices having a single Majorana fermion in each vortex, and a part representing the Coxeter group. Similar decomposition was already found in the case with N=3, and the result shown here is a generalization to the case with an arbitrary odd $N$. We can obtain the matrix representation of the exchange operators in the Hilbert space that is constructed by using Dirac fermions non-locally defined by Majorana fermions trapped in separated vortices. We also show that the decomposition of the exchange operator implies tensor product structure in its matrix representation.

Tunable gauge potential for neutral and spinless particles in driven lattices
[|Julian Struck], [|Christoph Ölschläger] , [|Malte Weinberg] , [|Philipp Hauke] , [|Juliette Simonet] , [|André Eckardt] , [|Maciej Lewenstein] , [|Klaus Sengstock] , [|Patrick Windpassinger] Subjects: Quantum Gases (cond-mat.quant-gas) We present a universal method to create a tunable, artificial vector gauge potential for neutral particles trapped in an optical lattice. The necessary Peierls phase of the hopping parameters between neighboring lattice sites is generated by applying a suitable periodic inertial force such that the method does not rely on any internal structure of the particles. We experimentally demonstrate the realization of such artificial potentials, which generate ground state superfluids at arbitrary non-zero quasi-momentum. We furthermore investigate possible implementations of this scheme to create tuneable magnetic fluxes, going towards model systems for strong-field physics.

Mar 1
=== 1. [|arXiv:1202.6446] (cross-list from quant-ph) [ [|pdf], [|ps] , [|other] ]  ===

Control of Wannier orbitals for generating entanglement of ultracold atoms in an optical lattice
[|Kensuke Inaba], [|Yuuki Tokunaga] , [|Kiyoshi Tamaki] , [|Kazuhiro Igeta] , [|Makoto Yamashita] Comments: 5 pages, 5 figures Subjects: Quantum Physics (quant-ph) ; Quantum Gases (cond-mat.quant-gas) We propose a method for controlling the quantum state of ultracold atoms in an optical lattice by using current atom manipulation techniques, which can generate high-fidelity entanglement resources. We utilize higher Wannier orbitals as a controllable and accessible environment, and then realize a tunable Ising interaction between atoms in the lowest orbital to generate multipartite entangled cluster states. We can enhance the fidelity by employing a post-selection scheme based on spectroscopic measurements of states in the environment. Precise numerical simulations using realistic parameters suggest that our method offers significant advantages for high-fidelity entanglement generation toward scalable measurement-based quantum computation.

Exotic Quantum Criticality in One-Dimensional Coupled Dipolar Bosons Tubes
[|P. Lecheminant], [|H. Nonne] Comments: 18 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el) The competition between intertube hopping processes and density-density interactions is investigated in one-dimensional quantum dipolar bosons systems of N coupled tubes at zero temperature. Using a phenomenological bosonization approach, we show that the resulting competition leads to an exotic quantum phase transition described by a U(1) x Z_N conformal field theory with a fractional central charge. The emerging Z_N parafermionic critical degrees of freedom are highly nontrivial in terms of the original atoms or polar molecules of the model. We further determine the main physical properties of the quantum critical point a double-tube system which has central charge c=3/2. In triple-tube systems, we show that the competition between the two antagonistic processes is related to the physics of the two-dimensional Z_3 chiral Potts model. This work opens the possibility to study the exotic properties of the Z_N parafermions in the context of ultracold quantum Bose gases.

Unconventional Quantum Critical Points
[|Cenke Xu] Comments: 28 pages, 6 figures. Review article for Int. J. Mod. Phys. B. Please do email me if you find some important reference is missing in this article Subjects: Strongly Correlated Electrons (cond-mat.str-el) In this paper we review the theory of unconventional quantum critical points that are beyond the Landau's paradigm. Three types of unconventional quantum critical points will be discussed: (1). The transition between topological order and semiclassical spin ordered phase; (2). The transition between topological order and valence bond solid phase; (3). The direct second order transition between different competing orders. We focus on the field theory and universality class of these unconventional quantum critical points. Relation of these quantum critical points with recent numerical simulations and experiments on quantum frustrated magnets are also discussed.

Evaporation limited loading of an atom trap
[|Markus Falkenau], [|Valentin V. Volchkov] , [|Jahn Rührig] , [|Hannes Gorniaczyk] , [|Axel Griesmaier] Comments: 10 pages, 7 figures Journal-ref: Phys. Rev. A 85, 023412 (2012) Subjects: Atomic Physics (physics.atom-ph) ; Quantum Gases (cond-mat.quant-gas) Recently, we have experimentally demonstrated a continuous loading mechanism for an optical dipole trap from a guided atomic beam [1]. The observed evolution of the number of atoms and temperature in the trap are consequences of the unusual trap geometry. In the present paper, we develop a model based on a set of rate equations to describe the loading dynamics of such a mechanism. We consider the collision statistics in the non-uniform trap potential that leads to twodimensional evaporation. The comparison between the resulting computations and experimental data allows to identify the dominant loss process and suggests ways to enhance the achievable steady-state atom number. Concerning subsequent evaporative cooling, we ?nd that the possibility of controlling axial and radial confinement independently allows faster evaporation ramps compared to single beam optical dipole traps.

Pomeranchuk cooling of the SU($2N$) ultra-cold fermions in optical lattices
[|Zi Cai], [|Hsiang-hsuan Hung] , [|Lei Wang] , [|Dong Zheng] , [|Congjun Wu] Comments: 4.2 pages Subjects: Quantum Gases (cond-mat.quant-gas) We investigate the thermodynamic properties of a half-filled SU(2N) Fermi-Hubbard model in the two-dimensional square lattice using the determinantal quantum Monte Carlo simulation, which is free of the fermion "sign problem". The large number of hyperfine-spin components enhances spin fluctuations, which facilitates the Pomeranchuk cooling to temperatures comparable to the superexchange energy scale at the case of SU$(6)$. Various quantities including entropy, charge fluctuation, and spin correlations have been calculated.

Experimental realization of plaquette resonating valence bond states with ultracold atoms in optical superlattices
[|Sylvain Nascimbène], [|Yu-Ao Chen] , [|Macros Atala] , [|Monika Aidelsburger] , [|Stefan Trotzky] , [|Belén Paredes] , [|Immanuel Bloch] Comments: 7 page, 4 figures in main text, 3 figures in appendix Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) The concept of valence bond resonance plays a fundamental role in the theory of the chemical bond and is believed to lie at the heart of many-body quantum physical phenomena. Here we show direct experimental evidence of a time-resolved valence bond quantum resonance with ultracold bosonic atoms in an optical lattice. By means of a superlattice structure we create a three-dimensional array of independent four-site plaquettes, which we can fully control and manipulate in parallel. Moreover, we show how small-scale plaquette resonating valence bond states with s- and d-wave symmetry can be created and characterized. We anticipate our findings to open the path towards the creation and analysis of many-body RVB states in ultracold atomic gases.

Propagation front of correlations in an interacting Bose gas
[|Peter Barmettler], [|Dario Poletti] , [|Marc Cheneau] , [|Corinna Kollath] We analyze the quench dynamics of a one-dimensional bosonic Mott insulator and focus on the time evolution of density correlations. For these we identify a pronounced propagation front, the velocity of which, once correctly extrapolated at large distances, can serve as a quantitative characteristic of the many-body Hamiltonian. In particular, the velocity allows distinguishing the weakly interacting regime, which is qualitatively well described by free bosons, from the strongly interacting one, in which pairs of distinct quasiparticles dominate the dynamics. In order to describe the latter case analytically, we introduce a general approximation to solve the Bose--Hubbard Hamiltonian based on the Jordan--Wigner fermionization of auxiliary particles. This approach can also be used to determine the ground-state properties. Complementary to the fermionization approach, we derive explicitly the time-dependent many-body state in the non-interacting limit and compare our results to numerical simulations in the whole range of interactions of the Bose--Hubbard model.

Band mass anisotropy and the intrinsic metric of fractional quantum Hall systems
[|Bo Yang], [|Z. Papić] , [|E. H. Rezayi] , [|R. N. Bhatt] , [|F. D. M. Haldane] It was recently pointed out that topological liquid phases arising in the fractional quantum Hall effect (FQHE) are not required to be rotationally invariant, as most variational wavefunctions proposed to date have been. Instead, they possess a geometric degree of freedom corresponding to a shear deformation that acts like an intrinsic metric. We apply this idea to a system with an anisotropic band mass, as is intrinsically the case in many-valley semiconductors such as AlAs and Si, or in isotropic systems like GaAs in the presence of a tilted magnetic field, which breaks the rotational invariance. We perform exact diagonalization calculations with periodic boundary conditions (torus geometry) for various filling fractions in the lowest, first and second Landau levels. In the lowest Landau level, we demonstrate that FQHE states generally survive the breakdown of rotational invariance by moderate values of the band mass anisotropy. At 1/3 filling, we generate a variational family of Laughlin wavefunctions parametrized by the metric degree of freedom. We show that the intrinsic metric of the Laughlin state adjusts as the band mass anisotropy or the dielectric tensor are varied, while the phase remains robust. In the n=1 Landau level, mass anisotropy drives transitions between incompressible liquids and compressible states with charge density wave ordering. In n>=2 Landau levels, mass anisotropy selects and enhances stripe ordering with compatible wave vectors at partial 1/3 and 1/2 fillings.

Topological interface engineering and defect crossing in ultracold atomic gases
[|Magnus O. Borgh], [|Janne Ruostekoski] We propose an experimentally feasible scheme for topological interface engineering and show how it can be used for studies of dynamics of topologically nontrivial interfaces and perforation of defects and textures across such interfaces. The method makes use of the internal spin structure of the atoms together with locally applied control of interaction strengths to create many-particle states with highly complex topological properties. In particular, we consider a constructed coherent interface between topologically distinct phases of spinor Bose-Einstein condensates.

Searching for Non-Abelian Phases in the Bose-Einstein Condensate of Dysprosium
[|Biao Lian], [|Tin-Lun Ho] , [|Hui Zhai] The recently realized Bose-Einstein condensate of Dysprosium will become a spin-8 spinor condensate at ultralow magnetic fields. In such a high spin condensate, many phases with different symmetries can exist. Among them the most interesting ones are those with non-abelian point group symmetry. In this letter we discuss the variety of symmetry phases in a spin-8 condensate resulting from numerical solutions of the Hamiltonian. We show that these symmetries can be determined uniquely from the measurements of density population on each spin component in an ultralow magnetic field, together with the measurements of the collective modes in the zero-field limit. This method can also be applied to Bose-Einstein condensate of other magnetic atoms, such as Cr and Er.

Pseudogap phenomenon in an ultracold Fermi gas with a p-wave pairing interaction
[|D. Inotani], [|R. Watanabe] , [|M. Sigrist] , [|Y. Ohashi] We investigate single-particle properties of a one-component Fermi gas with a tunable p-wave interaction. Including pairing fluctuations associated with this anisotropic interaction within a $T$-matrix theory, we calculate the single-particle density of states, as well as the spectral weight, above the superfluid transition temperature $T_{\rm c}$. Starting from the weak-coupling regime, we show that the so-called pseudogap first develops in these quantities with increasing the interaction strength. However, when the interaction becomes strong to some extent, the pseudogap becomes obscure to eventually disappear in the strong-coupling regime. This non-monotonic interaction dependence is quite different from the case of an s-wave interaction, where the pseudogap simply develops with increasing the interaction strength. The difference between the two cases is shown to originate from the momentum dependence of the p-wave interaction, which vanishes in the low momentum limit. We also identify the pseudogap regime in the phase diagram with respect to the temperature and the p-wave interaction strength. Since the pseudogap is a precursor phenomenon of the superfluid phase transition, our results would be useful for the research toward the realization of p-wave superfluid Fermi gases.

Ultra-cold bosons in zig-zag optical lattices
[|S. Greschner], [|L. Santos] , [|T. Vekua] Ultra-cold bosons in zig-zag optical lattices present a rich physics due to the interplay between frustration, induced by lattice geometry, two-body interaction and three-body constraint. Unconstrained bosons may develop chiral superfluidity and a Mott-insulator even at vanishingly small interactions. Bosons with a three-body constraint allow for a Haldane-insulator phase in non-polar gases, as well as pair-superfluidity and density wave phases for attractive interactions. These phases may be created and detected within the current state of the art techniques.

Skyrmions with quadratic band touching fermions: A way to achieve charge 4e superconductivity
[|Eun-Gook Moon] We study Skyrmion quantum numbers, charge and spin, in (2+1) dimension induced by quadratic band toucing(QBT) fermions. It is shown that induced charge of Skyrmions is twice bigger than corresponding Dirac particles' and their statistics are always bosonic. Applying to Bernal stacking bi-layer graphene, we show that Skyrmions of quantum spin Hall(QSH) are charge 4e bosons, so its condensation realizes charge $4e$ superconductivity(SC). One candidate for a critical theory of the phase transition is non linear sigma model with Wess-Zumino-Witten term, and we calculate renormalization group beta function of the model perturbatively and show its phase diagram. We also discuss how QBT fermion is different from two copies of Dirac particles.

= Feb 20 - Feb 24, Bin Wang =

Coherent negative mobility of ultracold atoms in an optical lattice
[|T. Salger], [|S. Kling], [|S. Denisov], [|A. V.Ponomarev], [|P. Hanggi], [|M. Weitz] The paradoxical response, when a system produces an output - current, flux or rotation - in the direction opposite to an applied bias or torque, seemingly contradicts Newton's second law. Nevertheless, such a response, termed absolute negative mobility, can occur in a system driven far out of thermal equilibrium. In earlier works, the presence of strong decoherence mechanisms was regarded as a prerequisite for negative mobility to appear. Here we report an experimental and theoretical study showing absolute negative mobility in a fully coherent quantum regime. Using a rubidium Bose-Einstein condensate loaded in a time-periodically modulated optical lattice, we observe a negative response from a coherent quantum system. The experimental results are explained in terms of eigenstates of a time-periodic Hamiltonian. Our findings open new possibilities for the control of quantum transport in the decoherence-free limit.

Doublon dynamics in the extended Fermi Hubbard model
[|Felix Hofmann], [|Michael Potthoff] Two fermions occupying the same site of a lattice model with strongly repulsive Hubbard-type interaction form a doublon, a long-living excitation the decay of which is suppressed because of energy conservation. By means of an exact-diagonalization approach based on the Krylov-space technique, we study the dynamics of a single doublon, of two doublons, and of a doublon in the presence of two additional fermions prepared locally in the initial state of the extended Hubbard model. The time dependence of the expectation value of the double occupancy at the different sites of a large one-dimensional lattice is analyzed by perturbative arguments. In this way the spatiotemporal evolution of the doublon, its decay on a short time scale and the long-time average of the total double occupancy can be understood. We demonstrate how the dynamics of a doublon in the initial state is related to the spectrum of two-fermion excitations obtained from linear-response theory, we work out the difference between doublons composed of fermions vs. doublons composed of bosons and show that despite the increase of phase space for inelastic decay processes, the stability of a doublon is enhanced by the presence of additional fermions on an intermediate time scale.

Nucleating a metastable quantum phase transition
[|Oleksandr Fialko], [|Marie-Coralie Delattre], [|Joachim Brand], [|Andrey R. Kolovsky] Finite topological quantum systems can undergo continuous metastable quantum phase transitions to change their topological nature. Here we show how to nucleate the transition between ring currents and dark soliton states in a toroidally trapped Bose-Einstein condensate. An adiabatic passage to wind and unwind its phase is achieved by explicit global breaking of the rotational symmetry. This could be realized with current experimental technology.

Topological $p_{x}+ip_{y}$ Superfluid Phase of a Dipolar Fermi Gas in a 2D Optical Lattice
[|Bo Liu], [|Lan Yin] In a dipolar Fermi gas, the anisotropic interaction between electric dipoles can be turned into an effectively attractive interaction in the presence of a rotating electric field. We show that the topological $p_{x}+ip_{y}$ superfluid phase can be realized in a single-component dipolar Fermi gas trapped in a 2D square optical lattice with this attractive interaction at low temperatures. The $p_{x}+ip_{y}$ superfluid state has potential applications for topological quantum computing. We obtain the phase diagram of this system at zero temperature. In the weak-coupling limit, the p-wave superfluid phase is stable for all filling factors. As the interaction strength increases, it is stable close to filling factors $n=0$ or $n=1$, and phase separation takes place in between. When the interaction strength is above a threshold, the system is phase separated for any $0<n<1$. The transition temperature of the $p_{x}+ip_{y}$ superfluid state is estimated and the implication for experiments is discussed.

Forming doublons by a quantum quench
[|Michael Schecter], [|Alex Kamenev] Repulsive interactions between particles on a lattice may lead to bound states, so called doublons. Such states may be created by dynamically tuning the interaction strength, e.g. using a Feshbach resonance, from attraction to repulsion. We study the doublon production efficiency as a function of the tuning rate at which the on-site interaction is varied. An expectation based on the Landau- Zener law suggests that exponentially few doublons are created in the adiabatic limit. Contrary to such an expectation, we found that the number of produced doublons scales as a power law of the tuning rate with the exponent dependent on the dimensionality of the lattice. The physical reason for this anomaly is the effective decoupling of doublons from the two-particle continuum for center of mass momenta close to the corners of the Brillouin zone. The study of doublon production may be a sensitive tool to extract detailed information about the band structure.

Feb 22
===1. [|arXiv:1202.4697] (cross-list from cond-mat.str-el) [[|pdf], [|ps], [|other]] ===

Comparison of the density-matrix renormalization group method applied to fractional quantum Hall systems in different geometries
[|Zi-Xiang Hu], [|Z. Papic], [|S. Johri], [|R. N. Bhatt], [|Peter Schmitteckert] We report a systematic study of the fractional quantum Hall effect (FQHE) using the density-matrix renormalization group (DMRG) method on two different geometries: the sphere and the cylinder. We provide convergence benchmarks based on model Hamiltonians known to possess exact zero-energy ground states, as well as an analysis of the number of sweeps and basis elements that need to be kept in order to achieve the desired accuracy.The ground state energies of the Coulomb Hamiltonian at $\nu=1/3$ and $\nu=5/2$ filling are extracted and compared with the results obtained by previous DMRG implementations in the literature. A remarkably rapid convergence in the cylinder geometry is noted and suggests that this boundary condition is particularly suited for the application of the DMRG method to the FQHE.

Quantum degenerate dipolar Fermi gas
[|Mingwu Lu], [|Nathaniel Q. Burdick], [|Benjamin L. Lev] The interplay between crystallinity and superfluidity is of great fundamental and technological interest in condensed matter settings. In particular, electronic quantum liquid crystallinity arises in the non-Fermi liquid, pseudogap regime neighboring a cuprate's unconventional superconducting phase. While the techniques of ultracold atomic physics and quantum optics have enabled explorations of the strongly correlated, many-body physics inherent in, e.g., the Hubbard model, lacking has been the ability to create a quantum degenerate Fermi gas with interparticle interactions---such as the strong dipole-dipole interaction---capable of inducing analogs to electronic quantum liquid crystals. We report the first quantum degenerate dipolar Fermi gas, the realization of which opens a new frontier for exploring strongly correlated physics and, in particular, the quantum melting of smectics in the pristine environment provided by the ultracold atomic physics setting. A quantum degenerate Fermi gas of the most magnetic atom 161Dy is produced by laser cooling to 10 uK before sympathetically cooling with ultracold, bosonic 162Dy. The temperature of the spin-polarized 161Dy is a factor T/TF=0.2 below the Fermi temperature TF=300 nK. The co-trapped 162Dy concomitantly cools to approximately Tc for Bose-Einstein condensation, thus realizing a novel, nearly quantum degenerate dipolar Bose-Fermi gas mixture.

Orbital physics of polar Fermi molecules
[|O. Dutta], [|T. Sowinski], [|M. Lewenstein] We study Fermi-Hubbard model for spinless dipolar fermions. We show that multi-band description is necessary to study such systems. By taking into account both on-site as well as long-range interactions between different bands, and occupation-dependent inter- and intra-band tunneling, we predict appearance of novel phases in the strongly-interacting limit.

Dipolar gases in one-dimensional coupled tubes
[|Marianne Bauer], [|Meera M. Parish] We consider dipolar bosons in two tubes of one-dimensional lattices, where the dipoles are aligned to be maximally repulsive and the particle filling fraction is the same in each tube. In the classical limit of zero inter-site hopping, the particles arrange themselves into an ordered crystal for any rational filling fraction, forming a complete devil's staircase like in the single tube case. Turning on hopping within each tube then gives rise to a competition between the crystalline Mott phases and a liquid of defects or solitons. However, for the two-tube case, we find that solitons from different tubes can bind into pairs for certain topologies of the filling fraction. This provides an intriguing example of pairing that is purely driven by correlations close to a Mott insulator.

Expansion of 1D polarized superfluids: The FFLO state reveals itself
[|Hong Lu], [|L. O. Baksmaty], [|C. J. Bolech], [|Han Pu] We study the expansion dynamics of a one dimensional polarized Fermi gas after sudden release from confinement using both the mean-field Bogoliubov-de Gennes and the numerically exact Time-Evolving Block Decimation methods. Our results show that experimentally observable spin density modulations directly related to the presence of a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state develop during the expansion of the cloud, providing incontrovertible evidence of this long-sought state.

= Feb 13- Feb 17, Johannes Schachenmayer =

Feb 17

 * 1. [|arXiv:1202.3574] [[|pdf], [|other]] **

Relative and center-of-mass motion in the attractive Bose-Hubbard model
[|Ole Søe Sørensen], [|Søren Gammelmark], [|Klaus Mølmer] We present first-principle numerical calculations for few particle solutions of the attractive Bose-Hubbard model with periodic boundary conditions. We show that the low-energy many-body states found by numerical diagonalization can be written as translational superposition states of compact composite systems of particles. These compact states break the translational symmetry of the problem and their center-of-mass and internal excitations offer simple explanations of the energy spectrum of the full model.


 * 2. [|arXiv:1202.3447] [[|pdf], [|other]] **

How does an interacting many-body system tunnel through a potential barrier to open space?
[|Axel U. J. Lode], [|Alexej I. Streltsov], [|Kaspar Sakmann], [|Ofir E. Alon], [|Lorenz S. Cederbaum] The tunneling process in a many-body system is a phenomenon which lies at the very heart of quantum mechanics. It appears in nature in the form of alpha-decay, fusion and fission in nuclear physics, photoassociation and photodissociation in biology and chemistry. A detailed theoretical description of the decay process in these systems is a very cumbersome problem, either because of very complicated or even unknown interparticle interactions or due to a large number of constitutent particles. In this work, we theoretically study the phenomenon of quantum many-body tunneling in a more transparent and controllable physical system, in an ultracold atomic gas. We analyze a full, numerically exact many-body solution of the Schr\"odinger equation of a one-dimensional system with repulsive interactions tunneling to open space. We show how the emitted particles dissociate or fragment from the trapped and coherent source of bosons: the overall many-particle decay process is a quantum interference of single-particle tunneling processes emerging from sources with different particle numbers taking place simultaneously. The close relation to atom lasers and ionization processes allows us to unveil the great relevance of many-body correlations between the emitted and trapped fractions of the wavefunction in the respective processes.


 * 3. [|arXiv:1202.3489] [[|pdf], [|other]] **

Condensate deformation and quantum depletion of Bose-Einstein condensates in external potentials
[|C. A. Müller], [|C. Gaul] The one-body density matrix of weakly interacting, condensed bosons in external potentials is calculated using inhomogeneous Bogoliubov theory. We determine the condensate deformation caused by weak external potentials on the mean-field level. The momentum distribution of quantum fluctuations around the deformed ground state is obtained analytically, and finally the resulting quantum depletion is calculated. The depletion due to the external potential, or potential depletion for short, is a small correction to the homogeneous depletion, validating our inhomogeneous Bogoliubov theory. Analytical results are derived for weak lattices and spatially correlated random potentials, with simple, universal results in the Thomas-Fermi limit of very smooth potentials.

Feb 16

 * 1. [|arXiv:1202.3400] [[|pdf], [|other]] **

Quantum Dynamics of Solitons in Strongly Interacting Systems on Optical Lattices
[|Chester P. Rubbo], [|Indubala I. Satija], [|William P. Reinhardt], [|Radha Balakrishnan], [|Ana Maria Rey], [|Salvatore R. Manmana] Mean-field dynamics of strongly interacting bosons has been shown to support two species of solitons: one of Gross-Pitaevski (GP)-type where the condensate fraction remains dark and a novel non-GP-type characterized by brightening of the condensate fraction. Here we study the effects of quantum fluctuations on these solitons using the adaptive time-dependent density matrix renormalization group method, which takes into account the effect of strong correlations. We use local observables as the density, condensate density and correlation functions as well as the entanglement entropy to characterize the stability of the initial states. We find both species of solitons to be stable under quantum evolution for a finite duration, their tolerance to quantum fluctuations being enhanced as the width of the soliton increases. We describe possible experimental realizations in atomic Bose Einstein Condensates, polarized degenerate Fermi gases, and in systems of polar molecules on optical lattices.


 * 2. [|arXiv:1202.3166] [[|pdf], [|ps], [|other]] **

Quantum resonant effects in the delta-kicked rotor revisited
[|A. Ullah], [|S. K. Ruddell], [|J-A. Currivan], [|M. D. Hoogerland] We review the theoretical model and experimental realization of the atom optics $\delta-$kicked rotor (AOKR), a paradigm of classical and quantum chaos. We have performed a number of experiments with an all-optical Bose-Einstein condensate (BEC) in a periodic standing wave potential in an AOKR system. We discuss results of the investigation of the phenomena of quantum resonances in the AOKR. An interesting feature of the momentum distribution of the atoms obtained as a result of short pulses of light, is the variance of the momentum distribution or the kinetic energy $/2m$ in units of the recoil energy $E_{rec} = \hbar \omega_{rec}$. The energy of the system is examined as a function of pulse period for a range of kicks that allow the observation of quantum resonances. In particular we study the behavior of these resonances for a large number of kicks. Higher order quantum resonant effects corresponding to the fractional Talbot time of (1/4)$T_{T}$ and (1/5)$T_{T}$ for five and ten kicks have been observed. Moreover, we describe the effect of the initial momentum of the atoms on quantum resonances in the AOKR.


 * 3. [|arXiv:1111.1537] [[|pdf], [|ps], [|other]] **

Finite-temperature phase diagram of two-component bosons in a cubic optical lattice: Three-dimensional t-J model of hard-core bosons
[|Y. Nakano], [|T. Ishima], [|N. Kobayashi], [|T. Yamamoto], [|I. Ichinose], [|T. Matsui] We study the three-dimensional bosonic t-J model, i.e., the t-J model of "bosonic electrons", at finite temperatures. This model describes the $s={1 \over 2}$ Heisenberg spin model with the anisotropic exchange coupling $J_{\bot}=-\alpha J_z$ and doped {\it bosonic} holes, which is an effective system of the Bose-Hubbard model with strong repulsions. The bosonic "electron" operator $B_{r\sigma}$ at the site $r$ with a two-component (pseudo-)spin $\sigma (=1,2)$ is treated as a hard-core boson operator, and represented by a composite of two slave particles; a "spinon" described by a Schwinger boson (CP$^1$ boson) $z_{r\sigma}$ and a "holon" described by a hard-core-boson field $\phi_r$ as $B_{r\sigma}=\phi^\dag_r z_{r\sigma}$. By means of Monte Carlo simulations, we study its finite-temperature phase structure including the $\alpha$ dependence, the possible phenomena like appearance of checkerboard long-range order, super-counterflow, superfluid, and phase separation, etc. The obtained results may be taken as predictions about experiments of two-component cold bosonic atoms in the cubic optical lattice.


 * 4. [|arXiv:1202.3348] [[|pdf], [|ps], [|other]] **

Mass imbalance effect in resonant Bose-Fermi mixtures
[|E. Fratini], [|P. Pieri] We consider a homogeneous Bose-Fermi mixture, with the boson-fermion interaction tuned by a Fano-Feshbach resonance, in the presence of mass and density imbalance between the two species. We first study the finite temperature phase diagram for the specific case of the mass-imbalanced mixture 87Rb-40K for different values of the density imbalance. We then analyse the quantum phase transition associated with the disappearance at zero temperature of the boson condensate above a critical boson-fermion coupling. We find a pronounced dependence of the critical coupling on the mass ratio and a weak dependence on the density imbalance. For a vanishingly small boson density, we derive the asymptotic expressions for the critical coupling in the limits of small and large mass ratios. These expressions are relevant also for the polaron-molecule transition in a Fermi mixture at small and large mass ratios. The analysis of the momentum distribution functions at sufficiently large density imbalances shows an interesting effect in the bosonic momentum distribution due to the simultaneous presence of composite fermions and unpaired fermions.

Feb 15

 * 1. [|arXiv:1202.3054] [[|pdf], [|other]] **

First order SF-MI transition in the Bose-Hubbard model with tunable three-body onsite interaction
[|Arghavan Safavi-Naini], [|Javier von Stecher], [|Barbara Capogrosso-Sansone], [|Seth T. Rittenhouse] We study the two-dimensional Bose-Hubbard model in the presence of a 3-body interaction term, both at a mean field level and via quantum Monte Carlo simulations. The 3-body term is tuned by coupling the triply occupied states to a trapped universal trimer. We find that, for sufficiently attractive 3-body interaction the n = 2 Mott lobe disappears and the system displays first order phase transitions separating the n = 1 from the n = 3 lobes, and the n = 1 and n = 3 Mott insulator from the superfluid. We have also analyzed the effect of finite temperature and found that transitions are still of first order at temperatures T ~ J.


 * 2. [|arXiv:1202.3036] [[|pdf], [|ps], [|other]] **

Quantum tri-criticality and phase transitions in spin-orbit coupled Bose-Einstein condensates
[|Yun Li], [|Lev P. Pitaevskii], [|Sandro Stringari] We consider a spin-orbit coupled configuration of spin-1/2 interacting bosons with equal Rashba and Dresselhaus couplings. The phase diagram of the system is discussed with special emphasis to the role of the interaction treated in the mean-field approximation. For a critical value of the density and of the Raman coupling we predict the occurrence of a characteristic tri-critical point separating the spin mixed, the phase separated and the single minimum states of the Bose gas. The corresponding quantum phases are investigated analyzing the momentum distribution, the longitudinal and transverse spin-polarization and the emergence of density fringes. The effect of harmonic trapping as well as the role of the breaking of spin symmetry in the interaction Hamiltonian are also discussed.


 * 3. [|arXiv:1202.2971] [[|pdf], [|ps], [|other]] **

Macroscopic amplification of electroweak effects in molecular Bose-Einstein condensates
[|Pedro Bargueno], [|Fernando Sols] We investigate the possible use of Bose-Einstein condensates of diatomic molecules to measure nuclear spin-dependent parity violation effects, outlining a detection method based on the internal Josephson effect between molecular states of opposite parity. When applied to molecular condensates, the fine experimental control achieved in atomic bosonic Josephson junctions could provide data on anapole moments and neutral weak couplings.


 * 4. [|arXiv:1202.2939] [[|pdf], [|other]] **

The Contact in the BCS-BEC crossover for finite range interatomic potentials
[|Santiago F. Caballero-Benitez], [|Rosario Paredes], [|Victor Romero-Rochin] Using mean-field theory for the Bardeen-Cooper-Schriefer (BCS) to the Bose-Einstein condensate (BEC) crossover we investigate the ground state thermodynamic properties of an interacting homo- geneous Fermi gas. The interatomic interactions modeled through a finite range potential allows us to explore the entire region from weak to strong interacting regimes with no approximations. To exhibit the thermodynamic behavior as a function of the potential parameters in the whole crossover region, we concentrate in studying the contact variable, the thermodynamic conjugate of the inverse of the s-wave scattering length. Our analysis allows us to validate the correctness of the contact approximation in the strong interacting regime where the scattering length diverges in the limit of zero potential range. It also leads to predict a quantum phase transition-like in the opposite case when the potential range becomes large. These findings are direct consequence of the k-dependent energy gap for finite interaction range potentials.

Feb 14

 * 1. [|arXiv:1202.2777] [[|pdf], [|other]] **

**Beating dark-dark solitons in Bose-Einstein condensates**
[|D. Yan], [|J. J. Chang], [|C. Hamner], [|M. Hoefer], [|P. G. Kevrekidis], [|P. Engels], [|V. Achilleos], [|D. J. Frantzeskakis], [|J. Cuevas] Motivated by recent experimental results, we study beating dark-dark solitons as a prototypical coherent structure that emerges in two-component Bose-Einstein condensates. We showcase their connection to dark- bright solitons via SO(2) rotation, and infer from it both their intrinsic beating frequency and their frequency of oscillation inside a parabolic trap. We identify them as exact periodic orbits in the Manakov limit of equal inter- and intra-species nonlinearity strengths with and without the trap and showcase the persistence of such states upon weak deviations from this limit. We also consider large deviations from the Manakov limit illustrating that this breathing state may be broken apart into dark-antidark soliton states. Finally, we consider the dynamics and interactions of two beating dark-dark solitons in the absence and in the presence of the trap, inferring their typically repulsive interaction.


 * 2. [|arXiv:1202.2649] (cross-list from cond-mat.mes-hall) [[|pdf], [|other]] **

Resonantly Tunable Majorana Polariton in a Microwave Cavity
[|Mircea Trif], [|Yaroslav Tserkovnyak] We study the spectrum of a one-dimensional Kitaev chain placed in a microwave cavity. In the off-resonant regime, the frequency shift of the cavity can be used to identify the topological phase transition of the coupled system. In the resonant regime, the topology of the system can be controlled via the microwave cavity occupation and, moreover, for a large number of photons (classical limit), the physics becomes similar to that of periodically-driven systems (Floquet insulators). We also analyze numerically a finite chain and show the existence of a degenerate subspace in the presence of the cavity that can be interpreted as a Majorana polariton.

Feb 13

 * 1.** ** [|arXiv:1202.2298] [[|pdf], [|ps], [|other]] **

Analytical treatment of bosonic d-wave scattering in isotropic harmonic waveguides
[|P. Giannakeas], [|V. S. Melezhik], [|P. Schmelcher] We analyze d-wave resonances in atom-atom scattering in the presence of harmonic confinement by employing a higher partial wave pseudopotential. Analytical results for the scattering amplitude and transmission are obtained and compared to corresponding numerical ones, which employ the Lennard-Jones potential. Qualitative agreement is observed for weak confinement. For strong confinement the pseudopotential does not capture the s- and d-wave interference phenomena yielding an asymmetric Fano profile for the transmission resonance.


 * 2.** ** [|arXiv:1202.2182] [[|pdf], [|ps], [|other]] **

Effect of photoions on the line shapes of the Förster resonance and microwave transitions in cold rubidium Rydberg atoms
[|D. B. Tretyakov], [|I. I. Beterov], [|V. M. Entin], [|E. A. Yakshina], [|I. I. Ryabtsev], [|S. F. Dyubko], [|E. A. Alekseev], [|N. L. Pogrebnyak], [|N. N. Bezuglov], [|E. Arimondo] Experiments on the spectroscopy of the F\"orster resonance Rb(37P)+Rb(37P) -> Rb(37S)+Rb(38S) and microwave transitions nP -> n'S, n'D between Rydberg states of cold Rb atoms in a magneto-optical trap have been performed. Under ordinary conditions, all spectra exhibited a 2-3 MHz line width independently of the interaction time of atoms with each other or with microwave radiation, although the ultimate resonance width should be defined by the inverse interaction time. Analysis of the experimental conditions has shown that the main source of the line broadening was the inhomogeneous electric field of cold photoions appeared at the excitation of initial Rydberg nP states by broadband pulsed laser radiation. Using an additional pulse of the electric field, which rapidly removed the photoions after the laser pulse, lead to a substantial narrowing of the microwave and F\"orster resonances. An analysis of various sources of the line broadening in cold Rydberg atoms has been conducted.

= Feb 6 - Feb 10, Saubhik Sarkar =

Feb 10
1. [|arXiv:1202.1961] [ [|pdf], [|ps] , [|other] ] Non-equilibrium dynamics of coupled qubit-cavity arrays [|Felix Nissen], [|Sebastian Schmidt] , [|Matteo Biondi] , [|Gianni Blatter] , [|Hakan E. Türeci] , [|Jonathan Keeling] We study the coherence and fluorescence properties of the coherently pumped and dissipative Jaynes-Cummings-Hubbard model describing polaritons in a coupled-cavity array. At weak hopping we find strong signatures of photon blockade similar to single-cavity systems. At strong hopping the state of the photons in the array depends on its size. While the photon blockade persists in a dimer consisting of two coupled cavities, a coherent state forms on an extended lattice, which can be described in terms of a semi-classical model.

2. [|arXiv:1202.1911] [ [|pdf], [|ps] , [|other] ] Fractionalized skyrmion in rotating and rapidly quenched spin-1 Bose-Einstein condensates with spin-orbit coupling [|Chao-Fei Liu], [|Wu-Ming Liu] We investigate the fractionalized skyrmion in rotating and rapidly quenched spin-1 Bose-Einstein condensates with spin-orbit coupling. Our results show that the fractionalized skyrmion excitation needs the association of spin-orbit coupling and rotation, and it occurs even when the ferromagnetism/antiferromagnetism is enhanced. The fractionalized skyrmion originates from a dipole which is always embedded in three vortices constructed by each condensate respectively. The fractionalized skyrmions form a radial lattice where the fractionalized skyrmions encircle the center skyrmion. We can adjust the lattice with both the spin-orbit coupling and the rotation.

3. [|arXiv:1202.1965] (cross-list from cond-mat.stat-mech) [ [|pdf], [|ps] , [|other] ] Finite-Size Scaling Analysis of the Eigenstate Thermalization Hypothesis in a One-Dimensional Interacting Bose gas [|Tatsuhiko N. Ikeda], [|Yu Watanabe] , [|Masahito Ueda] By calculating correlation functions of the Lieb-Liniger model based on the algebraic Bethe ansatz method, we conduct a finite-size scaling analysis of the eigenstate thermalization hypothesis (ETH) which is considered to be a possible mechanism of thermalization for isolated quantum systems, and find the ETH holds in the thermodynamic limit even for an integrable system. However, we show that the ETH gives only a logarithmic correction to other thermalization mechanisms such as the eigenstate randomization hypothesis and the typicality argument for the microcanonical ensemble.

4. [|arXiv:1202.1857] (cross-list from physics.atom-ph) [ [|pdf], [|other] ] Sensitive imaging of electromagnetic fields with paramagnetic polar molecules [|Sergey V. Alyabyshev], [|Mikhail Lemeshko] , [|Roman V. Krems] We propose a method for sensitive parallel detection of low-frequency electromagnetic fields based on the fine structure interactions in paramagnetic polar molecules. Compared to the recently implemented scheme employing ultracold $^{87}$Rb atoms [B{\"o}hi \textit{et al.}, Appl. Phys. Lett. \textbf{97}, 051101 (2010)], the technique based on molecules offers a 100-fold higher sensitivity, the possibility to measure both the electric and magnetic field components, and a probe of a wide range of frequencies from the dc limit to the THz regime.

Feb 9
1. [|arXiv:1202.1689] [ [|pdf], [|ps] , [|other] ] Creating two-dimensional bright solitons in dipolar Bose-Einstein condensates [|Patrick Köberle], [|Damir Zajec] , [|Günter Wunner] , [|Boris Malomed] We propose a realistic experimental setup for creating quasi-two-dimensional (2D) bright solitons in dipolar Bose-Einstein condensates (BECs), the existence of which was proposed in Phys. Rev. Lett. 100, 090406 (2008). A challenging feature of the expected solitons is their strong inherent anisotropy, due to the necessary in-plane orientation of the local moments in the dipolar gas. This may be the first chance of making multidimensional matter-wave solitons, as well as solitons featuring the anistropy due to their intrinsic dynamics. Our analysis is based on the extended Gross-Pitaevskii equation, which includes three-body losses and noise in the scattering length, induced by fluctuations of currents inducing the necessary magnetic fields, which are factors crucial to the adequate description of experimental conditions. By means of systematic 3D simulations, we find a ramping scenario for the change of the scattering length and trap frequencies which results in the creation of robust solitons, that readily withstand the concomitant excitation of the condensate.

Feb 8
1. [|arXiv:1202.1492] [ [|pdf], [|ps] , [|other] ] BCS-BEC crossover in 3D Fermi gases with spherical spin-orbit coupling [|Lianyi He], [|Xu-Guang Huang] We present a systematic theoretical study of the BCS-BEC crossover problem in 3D atomic Fermi gases at zero temperature with a spherical spin-orbit coupling which can be realized by a synthetic non-Abelian gauge field coupled to fermions. Our investigations are based on the path integral formalism which is a powerful theoretical scheme for the study of the properties of bound state, the superfluid ground state, and the collective excitations in the BCS-BEC crossover. At large spin-orbit coupling, the system enters the BEC state of a novel type of bound state (referred to as rashbon) which possesses a non-trivial effective mass. Analytical results and interesting universal behaviors for various physical quantities at large spin-orbit coupling are obtained. Our theoretical predictions can be tested in future experiments of cold Fermi gases with 3D spherical spin-orbit coupling.

2. [|arXiv:1202.1341] [ [|pdf], [|other] ] Quantum rainbows and ergodicity in many-body dynamics [|D. H. J. O'Dell] We consider rainbows in Fock space that form following a quench in a Bose Josephson junction. In the Gross-Pitaevskii mean-field theory the rainbows are singular caustics, but in the second-quantized theory they are described by well behaved Airy functions. We discuss the analyticity of the macroscopic limit and also show that the long-time dynamics are ergodic. Our results are relevant to the question posed by Berry [M.V. Berry, Nonlinearity 21, T19 (2008)]: are there circumstances when it is necessary to second-quantize wave theory in order to avoid singularities?

3. **  [|arXiv:1111.0919] (cross-list from quant-ph) [ [|pdf], [|ps] , [|other] ]  ** Topologically protected measurement-based quantum computation on the thermal state of a nearest-neighbor two-body Hamiltonian with spin-3/2 particles [|Keisuke Fujii], [|Tomoyuki Morimae] Recently, Li {\it et al.} [Phys. Rev. Lett. {\bf 107}, 060501 (2011)] have demonstrated that topologically protected measurement-based quantum computation can be implemented on the thermal state of a nearest-neighbor two-body Hamiltonian with spin-2 and spin-3/2 particles provided that the temperature is smaller than a critical value, namely, threshold temperature. Here we show that the thermal state of a nearest-neighbor two-body Hamiltonian, which consists of only spin-3/2 particles, allows us to perform topologically protected measurement-based quantum computation. The threshold temperature is calculated and turns out to be comparable to that with the spin-2 and spin-3/2 system. Furthermore, we generally show that a cluster state of high connectivity can be efficiently generated from the thermal state of the spin-3/2 system without severe thermal noise accumulation.

** Feb 7 **
1. [|arXiv:1202.1199] [ [|pdf], [|ps] , [|other] ] Feshbach spectroscopy and scattering properties of ultracold Li+Na mixtures [|T. Schuster], [|R. Scelle] , [|A. Trautmann] , [|S. Knoop] , [|M.K. Oberthaler] , [|M. M. Haverhals] , [|M. R. Goosen] , [|S. J. J. M. F. Kokkelmans] , [|E. Tiemann] We have observed 26 interspecies Feshbach resonances at fields up to 2050 G in ultracold $^6$Li+$^{23}$Na mixtures for different spin-state combinations. Applying the asymptotic bound-state model to assign the resonances, we have found that most resonances have d-wave character. This analysis serves as guidance for a coupled-channels calculation, which uses modified interaction potentials to describe the positions of the Feshbach resonances well within the experimental uncertainty and to calculate their widths. The scattering length derived from the improved interaction potentials is experimentally confirmed and deviates from previously reported values in sign and magnitude. We give prospects for $^7$Li+$^{23}$Na and predict broad Feshbach resonances suitable for tuning.

2. [|arXiv:1202.0956] [ [|pdf], [|ps] , [|other] ] Vortex-like superfluid of Bose-Einstein condensate in the honeycomb optical lattice with a sublattice of dissipative sites [|V. S. Shchesnovich]

We introduce the concept of the dissipative periodic lattice as an optical lattice with periodically distributed dissipative sites. In particular, the quantum state of Bose-Einstein condensate in the dissipative honeycomb lattice is considered. For weak nonlinearity, in the case when each dissipative lattice site neighbors three non-dissipated sites the condensate is driven to the coherent superposition of the four-site discrete vortex states with alternating vorticity, where the vortex centers are located in the dissipative sites. This result demonstrates utility of the dissipative periodic lattices for engineering of the quantum superfluid states with non-trivial complex order parameters.

3. [|arXiv:1202.0955] [ [|pdf], [|ps] , [|other] ] State diagram and the phase transition of $p$-bosons in a square bi-partite optical lattice [|V. S. Shchesnovich]

It is shown that, in a reasonable approximation, the quantum state of $p$-bosons in a bi-partite square two-dimensional optical lattice is governed by the nonlinear boson model describing tunneling of \textit{boson pairs} between two orthogonal degenerate quasi momenta on the edge of the first Brillouin zone. The interplay between the lattice anisotropy and the atomic interactions leads to the second-order phase transition between the number-squeezed and coherent phase states of the $p$-bosons. In the isotropic case of the recent experiment, Nature Physicis 7, 147 (2011), the $p$-bosons are in the coherent phase state, where the relative global phase between the two quasi momenta is defined only up to mod($\pi$): $\phi=\pm\pi/2$. The quantum phase diagram of the nonlinear boson model is given.

**Feb 6**
1. [|arXiv:1202.0599] [ [|pdf], [|ps] , [|other] ] Nonperiodic oscillation of bright solitons in the condensates with a periodically oscillating harmonic potential [|Z. M. He], [|D. L. Wang] , [|Y. C. She] , [|J. W. Ding] Considering a periodically oscillating harmonic potential, we explore the dynamics properties of bright solitons in a Bose-Einstein condensate. It is found that under a slower oscillating potential, soliton movement exhibits a nonperiodic oscillation while it is hardly affected under a fast oscillating potential. Furthermore, the head-on and/or "chase" collisions of two solitons have been obtained, which can be controlled by the oscillating frequency of potential.

2. [|arXiv:1202.0584] [ [|pdf], [|ps] , [|other] ] On a scale-invariant Fermi gas in a time-dependent harmonic potential [|Sergej Moroz]

We investigate a scale-invariant two-component Fermi gas in a time-dependent isotropic harmonic potential. The exact time evolution of the density distribution in position space in any spatial dimension is obtained. Two experimentally relevant examples, an abrupt change and a periodic modulation of the trapping frequency are solved. Small deviations from scale invariance and isotropy of the confinement are considered. We discuss the consequences for experiments with ultracold quantum gases such as the excitation of a tower of undamped breathing modes and a new alternative for measuring the Tan contact.

3. [|arXiv:1202.0573] (cross-list from physics.chem-ph) [ [|pdf], [|other] ] Harmonically trapped jellium [|Pierre-François Loos], [|Peter M. W. Gill]  We discuss the model of a $D$-dimensional confined electron gas in which the particles are trapped by a harmonic potential. In particular, we study the non-interacting kinetic and exchange energies of finite-size inhomogeneous systems, and compare the resulting Thomas-Fermi and Dirac coefficients with various uniform electron gas paradigms. We show that, in the thermodynamic limit, the properties of this model are identical to those of the $D$-dimensional Fermi gas.