Apr 29 - May 3, Stepan Langer
May 3
1. arXiv:1305.0504 [pdf, other]
Real time evolution at finite temperatures with operator space matrix product states
Iztok Pizorn, Viktor Eisler, Sabine Andergassen, Matthias Troyer
We propose a method to simulate the real time evolution of one dimensional quantum many-body systems at finite temperature by expressing both the density matrices and the observables as matrix product states. This allows the calculation of expectation values and correlation functions as scalar products in operator space. The simulations of density matrices in inverse temperature and the local operators in the Heisenberg picture are independent and result in a grid of expectation values for all intermediate temperatures and times. Simulations can be performed using real arithmetics with only polynomial growth of computational resources in inverse temperature and time for integrable systems. The method is illustrated for the XXZ model and the single impurity Anderson model.
2. arXiv:1305.0133 [pdf, ps, other]
Ballistic spin transport in exciton gases
A. V. Kavokin, M. Vladimirova, B. Jouault, T. C. H. Liew, J.R. Leonard, L.V. Butov
Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. This brings several complications: the Pauli principle prevents the carriers from moving with the same speed; Coulomb repulsion leads to rapid dephasing of electron flows. Spin-optronics is a valuable alternative to traditional spintronics. In spin-optronic devices the spin currents are carried by electrically neutral bosonic quasi-particles: excitons or exciton-polaritons. They can form highly coherent quantum liquids and carry spins over macroscopic distances. The price to pay is a finite life-time of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems where bosonic spin transport has been reported, in particular, to indirect excitons in coupled GaAs/AlGaAs quantum wells. We describe the effect of spin-orbit interaction of electrons and holes on the exciton spin, account for the Zeeman effect induced by external magnetic fields, long range and short range exchange splittings of the exciton resonances. We also consider exciton transport in the non-linear regime and discuss the definitions of exciton spin current, polarization current and spin conductivity.
May 2
1. arXiv:1305.0114 [pdf, ps, other]
Realizing Universal Majorana Fermionic Quantum Computation
Ya-Jie Wu, Jing He, Su-Peng Kou
Majorana-fermionic quantum computation (MFQC) was proposed by Bravyi and Kitaev (See Ref.\cite{Kitaev}), in which a fault-torrent (non-topological) quantum computer built from Majorana fermions may be more efficient than that built from distinguishable two-state systems. However, till now people don't know how to realize a MFQC in a physical system. In this paper we proposed a possible realization of MFQC. We find that the end of a line-defect of p-wave superconductor or superfluid on a honeycomb lattice will trap a Majorana zero mode, which will become the starting point of MFQC. Then we show how to manipulate Majorana fermions to do universal MFQC, which possesses unique possibilities for high-level local controllability, individual addressing, and readout of the quantum states of individual constituent elements by using timely cold-atom technology.
2. arXiv:1305.0081 [pdf, other]
Transport and the first passage time problem with application to cold atoms in optical traps
Eli Barkai, David A. Kessler
Measurements of spatial diffusion of cold atoms in optical lattices have revealed anomalous super-diffusion, which is controlled by the depth of the optical lattice. We use first passage time statistics to derive the diffusion front of the atoms. In particular, the distributions of areas swept under the first passage curve till its first arrival, and of areas under the Bessel excursion are shown to be powerful tools in the analysis of the atomic cloud. A rather general relation between first passage time statistics and diffusivity is discussed, showing that first passage time analysis is a useful tool in the calculation of transport coefficients. A brief introduction to the semi-classical description of Sisyphus cooling is provided which yields a rich phase diagram for the dynamics.
May 1
1. arXiv:1304.8096 [pdf, other]
Wigner crystallization of photons in cold Rydberg ensembles
Johannes Otterbach, Matthias Moos, Dominik Muth, Michael Fleischhauer
The coupling of weak light fields to Rydberg states of atoms under conditions of electromagnetically induced transparency (EIT) leads to the formation of Rydberg polaritons which are quasi-particles with tunable effective mass and long-range interactions. Confined to one spatial dimension their low energy physics is that of a moving-frame Luttinger liquid which due to the long-range character of the repulsive interaction can form a Wigner crystal. We calculate the Luttinger $K$ parameter using density-matrix renormalization group (DMRG) simulations and find that under typical slow-light conditions kinetic energy contributions are too strong for crystal formation. However, adiabatically increasing the polariton mass by turning a light pulse into stationary spin excitations allows to generate true crystalline order over a finite length. The dynamics of this process and asymptotic correlations are analyzed in terms of a time-dependent Luttinger theory.
2. arXiv:1304.7944 [pdf, ps, other]
Exterior integrability: Yang-Baxter form of nonequilibrium steady state density operator
Tomaz Prosen, Enej Ilievski, Vladislav Popkov
A new type of quantum transfer matrix, arising as a Cholesky factor for the steady state density matrix of a dissipative Markovian process associated with the boundary-driven Lindblad equation for the isotropic spin-1/2 Heisenberg (XXX) chain, is presented. The transfer matrix forms a commuting family of non-Hermitian operators depending on the spectral parameter which is essentially the strength of dissipative coupling at the boundaries. The intertwining of the corresponding Lax and monodromy matrices is performed by an infinitely dimensional Yang-Baxter R-matrix which we construct explicitly and which is essentially different from the standard XXX R-matrix. We also discuss a possibility to construct Bethe Ansatz for the spectrum and eigenstates of the non-equilibrium steady state density operator. Furthermore, we indicate the existence of a deformed R-matrix in the infinitely-dimensional auxiliary space for the anisotropic XXZ spin-1/2 chain which in general provides a sequence of new, possibly quasi-local, conserved quantities of the bulk XXZ dynamics.
3. arXiv:1304.7807 [pdf, ps, other]
Efficient Monte Carlo methods for simulating diffusion-reaction processes in complex systems
Denis Grebenkov
We briefly review the principles, mathematical bases, numerical shortcuts and applications of fast random walk (FRW) algorithms. This Monte Carlo technique allows one to simulate individual trajectories of diffusing particles in order to study various probabilistic characteristics (harmonic measure, first passage/exit time distribution, reaction rates, search times and strategies, etc.) and to solve the related partial differential equations. The adaptive character and flexibility of FRWs make them particularly efficient for simulating diffusive processes in porous, multiscale, heterogeneous, disordered or irregularly-shaped media.
Apr 30
1. arXiv:1304.7279 [pdf, ps, other]
Fluctuation-Dissipation Theorem in Isolated Quantum Dipolar Bosons After a Quench
Ehsan Khatami, Guido Pupillo, Mark Srednicki, Marcos Rigol
We examine the validity of fluctuation-dissipation relations in isolated quantum systems taken out of equilibrium by a sudden quench. We focus on the dynamics of trapped hard-core bosons in one-dimensional lattices with dipolar interactions whose strength is changed during the quench. We find that fluctuation-dissipation relations hold if the system is nonintegrable after the quench. They also hold if the system is integrable after the quench if the initial state is an equilibrium state of a nonintegrable Hamiltonian. However, they fail if the system is integrable both before and after quenching.
2. arXiv:1304.7636 [pdf, other]
Bose glass transition and spin-wave localization for 2D bosons in a random potential
Juan Pablo Álvarez Zúñiga, Nicolas Laflorencie
A spin-wave (SW) approach of the zero temperature superfluid - insulator transition for two dimensional hard-core bosons in a random potential mu=+/- W is developed. While at the classical level there is no intervening phase between the Bose-condensed superfluid (SF) and the gapped disordered insulator, the introduction of quantum fluctuations lead to a much richer physics. Upon increasing the disorder strength W, the Bose-condensed fraction disappears first, before the SF. Then a gapless Bose-glass (BG) phase emerges over a finite region, until the insulator appears. Furthermore, in the strongly disordered SF regime, a mobility edge in the SW excitation spectrum is found at a finite frequency Omega_c, decreasing with W, and presumably vanishing in the BG phase.
3. arXiv:1304.7348 [pdf, other]
Engineering entanglement for metrology with rotating matter waves
L.M. Rico-Gutierrez, T.P. Spiller, J.A. Dunningham
Entangled states of rotating, trapped ultracold bosons form a very promising scenario for quantum metrology. In order to employ such states for metrology, it is vital to understand their detailed form and the enhanced accuracy with which they could measure phase, in this case generated through rotation. In this work we study the rotation of ultracold bosons in an asymmetric trapping potential beyond the lowest Landau level (LLL) approximation. We demonstrate that whilst the LLL can identify reasonably the critical frequency for a quantum phase transition and entangled state generation, it is vital to go beyond the LLL to identify the details of the state and quantify the quantum Fisher information (which bounds the accuracy of the phase measurement). We thus identify a new parameter regime for useful entangled state generation, amenable to experimental investigation.
Apr 29
1. arXiv:1304.7022 [pdf, ps, other]
Geometric Stability Spectra of Dipolar Bose Gases in Tunable Optical Lattices
John P. Corson, Ryan M. Wilson, John L. Bohn
We examine the stability of quasi-two-dimensional dipolar Bose-Einstein condensates in the presence of weak optical lattices of various geometries. We find that when the condensate possesses a roton-maxon quasiparticle dispersion, the conditions for stability exhibit a strong dependence both on the lattice geometry and the polarization tilt. This results in rich structures in the system's stability diagram akin to spectroscopic signatures. We show how these structures originate from the mode matching of rotons to the perturbing lattice. In the case of a one-dimensional lattice, some of the features emerge only when the polarization axis is tilted into the plane of the condensate. Our results suggest that the stability diagram may be used as a novel means to spectroscopically measure rotons in dipolar condensates.
2. arXiv:1304.7036 [pdf, other]
Excitations of a trapped two component Bose Einstein Condensate
Christopher Ticknor
We present analysis of the excitation spectrum for a 2 component quasi2D Bose Einstein Condensate. We study how the character of the excitations change across the miscible to immiscible phase transition. We find that the bulk excitations are typical of a single-component BEC with the addition of interface bending excitations. We study how these excitations change as a function of the interaction strength.
3. arXiv:1304.7196 [pdf, other]
Bose-Hubbard models with photon pairing in circuit-QED
Benjamin Villalonga Correa, Andreas Kurcz, Juan jose Garcia-Ripoll
In this work we study a family of bosonic lattice models that combine an on-site repulsion term with a nearest-neighbor pairing term, $\sum_{< i,j>} a^\dagger_i a^\dagger_j + \mathrm{H.c.}$ Like the original Bose-Hubbard model, the nearest-neighbor term is responsible for the mobility of bosons and it competes with the local interaction, inducing two-mode squeezing. However, unlike a trivial hopping, the counter-rotating terms form pairing cannot be studied with a simple mean-field theory and does not present a quantum phase transition in phase space. Instead, we show that there is a cross-over from a pure insulator to long-range correlations that start up as soon as the two-mode squeezing is switched on. We also show how this model can be naturally implemented using coupled microwave resonators and superconducting qubits.
Apr 22 - Apr 26, Bo Liu
Apr 26
1. arXiv:1304.6980 [pdf]
Berezinskii-Kosterlitz-Thouless transition in a photonic lattice
Guohai Situ, Stefan Muenzel, Jason W. Fleischer
Phase transitions give crucial insight into many-body systems, as crossovers between different regimes of order are determined by the underlying dynamics. These dynamics, in turn, are often constrained by dimensionality and geometry. For example, in one- and two-dimensional systems with continuous symmetry, thermal fluctuations prevent the formation of long-range order[1,2]. Two-dimensional systems are particularly significant, as vortices can form in the plane but cannot tilt out of it. At high temperatures, random motion of these vortices destroys large-scale coherence. At low temperatures, vortices with opposite spin can pair together, cancelling their circulation and allowing quasi-long-range order to appear. This Berezenskii-Kosterlitz-Thouless (BKT) transition[3,4] is essentially classical, arising for example in the traditional XY model for spins, but to date experimental evidence has been obtained only in cold quantum systems. Measurements of superfluid sound speed[5] and critical velocity[6] have been consistent with scaling predictions, and vortices have been observed directly in cold atom experiments[7,8]. However, the presence of trapping potentials restricts measurement to vortex density, rather than number, and obscures the process of vortex unbinding. Further, atom and fluid experiments suffer from parasitic heating and difficulties in phase recording, leading to results that differ from theory in many quantitative aspects. Here, we use a nonlinear optical system to directly observe the ideal BKT transition, including vortex pair dynamics and the correlation properties of the wavefunction, for both repulsive and attractiveinteractions (the photonic equivalent of ferromagnetic and antiferromagnetic conditions[9]). The results confirm the thermodynamics of the BKT transition and expose outstanding issues in the crossovers to superfluidity and Bose-Einstein condensation.
2. arXiv:1304.6831 [pdf, ps, other]
Dynamic structure function of a cold Fermi gas at unitarity
G. E. Astrakharchik, J. Boronat, E. Krotscheck, T. Lichtenegger
We present a theoretical study of the dynamic structure function of a resonantly interacting two-component Fermi gas at zero temperature. Our approach is based on dynamic many-body theory able to describe excitations in strongly correlated Fermi systems. The fixed-node diffusion Monte Carlo method is used to produce the ground-state correlation functions which are used as an input for the excitation theory. Our approach reproduces recent Bragg scattering data in both the density and the spin channel. In the BCS regime, the response is close to that of the ideal Fermi gas. On the BEC side, the Bose peak associated with the formation of dimers dominates the density channel of the dynamic response. When the fraction of dimers is large our theory departs from the experimental data, mainly in the spin channel.
Apr 25
1. arXiv:1304.6675 [pdf, ps, other]
Asymptotic evaluation of the matrix permanents giving the probability amplitudes in linear bosonic networks
V. S. Shchesnovich
The probability amplitudes of N bosons unitarily transformed from the input $M$ modes to the output $M$ modes of a unitary linear network, given by the matrix permanents, are expressed as an average over the lattice of contingency tables with margins equal to the distributions of bosons in the input and output modes. In the limit $N\gg M$ the finite sum over the contingency tables is converted to a multidimensional integral with the integrand containing a large parameter (N) in the exponent. The integral representation allows an asymptotic estimate for the bosonic matrix permanents up to a small multiplicative error of order 1/N. The estimate depends on the solution of the scaling problem of the unitary $M\times M$-dimensional network matrix: to find the left and right diagonal matrices which scale the unitary matrix to a matrix which has specified rows and columns sums (equal to the distributions of bosons in the input and output modes). Such scaled matrices give the saddle points of the integral. In the case of simple saddle points an explicit formula giving an asymptotic estimate for the bosonic matrix permanents is given. It is compared with the exact result in the simplest case of just two-mode network (the beam-splitter) where the saddle-points are the roots of a quadratic.
Apr 24
1. arXiv:1304.6299 [pdf, ps, other]
Dynamical restoration of symmetry breaking in realistic optical lattices
Tomasz Sowiński, Mateusz Łacki, Omjyoti Dutta, Joanna Pietraszewicz, Piotr Sierant,Mariusz Gajda, Jakub Zakrzewski, Maciej Lewenstein
We show that properties of bosons loaded into the $p$-band of a rectangular optical lattice are substantially affected by the anharmonicity of the lattice potential. Even for very deep lattices, the optical confinement treated as a harmonic potential is highly oversimplified and in the low tunneling limit it leads incorrectly to a degeneracy of the ground state. We show that in this limit the degeneracy is lifted by the anharmonicity. For finite range of larger tunnelings the degeneracy is dynamically restored and the system gains an additional symmetry. This symmetry is, however, spontaneously broken in the thermodynamic limit and the state with non vanishing staggered angular momentum becomes the true ground state of the system.
2. arXiv:1304.6201 [pdf, other]
The study of random vorticity in quantum fluids through interference fluctuations
Michiel Wouters
We study the vortex dynamics of a quantum degenerate Bose gas through the intensity fluctuations of the interference from particles extracted at two different positions. It is shown numerically with classical field simulations that an interacting Bose gas with proliferating vortices exhibits long correlation times for these intensity fluctuations. This behavior is contrasted with the case of a noninteracting gas, that we describe analytically, and with the case of a well condensed Bose gas without vortices. We discuss the observability of our predictions in quantum fluids of exciton-polaritons.
Apr 23
1. arXiv:1304.5716 [pdf, ps, other]
Integer Quantum Hall State in Two-Component Bose Gases in a Synthetic Magnetic Field
Shunsuke Furukawa, Masahito Ueda
We study two-component (or pseudospin-1/2) Bose gases in a strong synthetic magnetic field. Using exact diagonalization, we show that a bosonic analogue of an integer quantum Hall state (without intrinsic topological order) appears at the total filling factor \nu=1+1 when the strengths of intracomponent and intercomponent interactions are comparable with each other. This provides a prime example of a symmetry-protected topological phase in a controlled setting of quantum gases. The real-space entanglement spectrum of this state is found to be comprised of counter-propagating chiral modes consistent with the predicted edge theory.
2. arXiv:1304.5889 [pdf, ps, other]
Luttinger liquid properties of the steady state after a quantum quench
D.M. Kennes, V. Meden
We study two-component (or pseudospin-1/2) Bose gases in a strong synthetic magnetic field. Using exact diagonalization, we show that a bosonic analogue of an integer quantum Hall state (without intrinsic topological order) appears at the total filling factor \nu=1+1 when the strengths of intracomponent and intercomponent interactions are comparable with each other. This provides a prime example of a symmetry-protected topological phase in a controlled setting of quantum gases. The real-space entanglement spectrum of this state is found to be comprised of counter-propagating chiral modes consistent with the predicted edge theory.
Apr 22
1. arXiv:1304.5520 [pdf, other]
Engineering Ising-XY spin models in a triangular lattice via tunable artificial gauge fields
Julian Struck, Malte Weinberg, Christoph Ölschläger, Patrick Windpassinger,Juliette Simonet, Klaus Sengstock, Robert Höppner, Philipp Hauke, André Eckardt, Maciej Lewenstein, Ludwig Mathey
Emulation of gauge fields for ultracold atoms provides access to a class of exotic states arising in strong magnetic fields. Here we report on the experimental realisation of tunable staggered gauge fields in a periodically driven triangular lattice. For maximal staggered magnetic fluxes, the doubly degenerate superfluid ground state breaks both a discrete Z2 (Ising) symmetry and a continuous U(1) symmetry. By measuring an Ising order parameter, we observe a thermally driven phase transition from an ordered antiferromagnetic to an unordered paramagnetic state and textbook-like magnetisation curves. Both the experimental and theoretical analysis of the coherence properties of the ultracold gas demonstrate the strong influence of the Z2 symmetry onto the condensed phase.
2. arXiv:1304.5454 [pdf, other]
Universal behaviour of four-boson systems from a functional renormalisation group
Benjamin Jaramillo Avila, Michael C. Birse
We apply a functional renormalisation group to systems of four bosonic atoms close to the unitary limit. We work with a local effective action that includes a trimer field to describe energy dependence of three-body subsystems. We also use this field to eliminate structures that do not correspond to the Faddeev-Yakubovsky equations. In the physical limit, we find three four-body bound states below the shallowest three-body state. The values of the scattering lengths at which two of these states become bound are in good agreement with exact solutions of the four-body equations and experimental observations. The third state is extremely shallow. During the evolution we find an infinite number of four-body states based on each three-body state which follow a double-exponential pattern in the running scale. None of the four-body states shows any evidence of dependence on a four-body parameter.
Apr 15 - Apr 19, Xiaopeng Li
Apr 19
1. arXiv:1304.5196 [pdf, other]
Dicke Quantum Spin and Photon Glass in Optical Cavities: Non-equilibrium theory and experimental signatures
Michael Buchhold, Philipp Strack, Subir Sachdev, Sebastian Diehl
In the context of ultracold atoms in multimode optical cavities, the appearance of a quantum-critical glass phase of atomic spins has been predicted recently. Due to the long-range nature of the cavity-mediated interactions, but also the presence of a driving laser and dissipative processes such as cavity photon loss, the quantum optical realization of glassy physics has no analog in condensed matter, and could evolve into a "cavity glass microscope" for frustrated quantum systems out-of-equilibrium. Here we develop the non-equilibrium theory of the multimode Dicke model with quenched disorder and Markovian dissipation. Using a unified Keldysh path integral approach, we show that the defining features of a low temperature glass, representing a critical phase of matter with algebraically decaying temporal correlation functions, are seen to be robust against the presence of dissipation due to cavity loss. The universality class however is modified due to the Markovian bath. The presence of strong disorder leads to an enhanced equilibration of atomic and photonic degrees of freedom, including the emergence of a common low-frequency effective temperature. The imprint of the atomic spin glass physics onto a "photon glass" makes it possible to detect the glass state by standard experimental techniques of quantum optics. We provide an unambiguous characterization of the superradiant and glassy phases in terms of fluorescence spectroscopy, homodyne detection, and the temporal photon correlation function $g^{(2)}(\tau)$.
2. arXiv:1304.4959 [pdf, ps, other]
Unconventional magnetism via optical pumping of interacting spin systems
Tony E. Lee, Sarang Gopalakrishnan, Mikhail D. Lukin
We consider strongly interacting systems of effective spins, subject to dissipative spin-flip processes associated with optical pumping. We predict the existence of novel magnetic phases in the steady-state of this system, which emerge due to the competition between coherent and dissipative processes. Specifically, for strongly anisotropic spin-spin interactions, we find ferromagnetic, antiferromagnetic, spin-density-wave, and staggered-XY steady states, which are separated by nonequilibrium phase transitions meeting at a Lifshitz point. These transitions are accompanied by quantum correlations, resulting in spin squeezing. Experimental implementations in ultracold atoms and trapped ions are discussed.
3. arXiv:1304.4938 [pdf, other]Deconfined Quantum Criticality and Conformal Phase Transition in Two-Dimensional Antiferromagnets
Flavio S. Nogueira, Asle Sudbo
Deconfined quantum criticality of two-dimensional SU(2) quantum antiferromagnets featuring a transition from an antiferromagnetically ordered ground state to a so-called valence-bond solid state, is governed by a non-compact CP^1 model with a Maxwell term in 2+1 spacetime dimensions. We introduce a new perspective on deconfined quantum criticality within a field-theoretic framework based on an expansion in powers of \epsilon=4-d for fixed number N of complex matter fields. Namely, we show that in the allegedly weak first-order transition regime, a so-called conformal phase transition leads to a genuine deconfined quantum critical point. In a conformal phase transition, the gap vanishes when the critical point is approached from above and diverges when it is approached from below. We also find that the spin stiffness has a universal jump at the critical point.
4. arXiv:1304.4939 (cross-list from quant-ph) [pdf, other]
Real-time observation of fluctuations at the driven-dissipative Dicke phase transitionFerdinand Brennecke, Rafael Mottl, Kristian Baumann, Renate Landig, Tobias Donner, Tilman Esslinger
We experimentally study the influence of dissipation on the driven Dicke quantum phase transition, realized by coupling external degrees of freedom of a Bose-Einstein condensate to the light field of a high-finesse optical cavity. The cavity provides a natural dissipation channel, which gives rise to vacuum-induced fluctuations and allows us to observe density fluctuations of the gas in real-time. We monitor the divergence of these fluctuations over two orders of magnitude while approaching the phase transition and observe a behavior which significantly deviates from that expected for a closed system. A correlation analysis of the fluctuations reveals the diverging time scale of the atomic dynamics and allows us to extract a damping rate for the external degree of freedom of the atoms. We find good agreement with our theoretical model including both dissipation via the cavity field and via the atomic field. Utilizing a dissipation channel to non-destructively gain information about a quantum many-body system provides a unique path to study the physics of driven-dissipative systems.
Apr 18
1. Thermodynamics of Strongly Correlated One-Dimensional Bose Gases
Andreas Vogler, Ralf Labouvie, Felix Stubenrauch, Giovanni Barontini, Vera Guarrera, Herwig Ott
We investigate the thermodynamics of one-dimensional Bose gases in the strongly correlated regime. To this end, we prepare ensembles of independent 1D Bose gases in a two-dimensional optical lattice and perform high-resolution in situ imaging of the column-integrated density distribution. Using an inverse Abel transformation we derive e?ective one-dimensional line-density pro?les and compare them to exact theoretical models. The high resolution allows for a direct thermometry of the trapped ensembles. The knowledge about the temperature enables us to extract thermodynamic equations of state such as the phase-space density, the entropy per particle and the local pair correlation function.
2. arXiv:1304.4615 (cross-list from quant-ph) [pdf, other]
Flux qubits with neutral currents in optical lattices
Luigi Amico, Davit Aghamalyan, H. Crepaz, F. Auksztol, R. Dumke, L.-C. Kwek
We study an experimentally feasible qubit system employing neutral currents. Our system is based on bosonic cold atoms trapped in ring-shaped optical lattice potentials. The lattice makes the system strictly one dimensional and it provides the infrastructure to realize a tunable ring-ring interaction. By breaking the Galilean invariance we demonstrate how atomic currents trough the lattice provide a realization of a qubit. We break Galilean invariance either by artificially creating a phase slip in a single ring, or by considering two homogeneous ring lattices, coupled by tunneling interaction. The Hamiltonian of the system effectively leads to a washboard potential in the phase representation, tilted by the applied 'flux'. The single qubit infrastructure is experimentally investigated with tailored optical potentials. An experimentally feasible scheme of the two-ring-qubit is discussed. In this case, the dynamics is demonstrated to show macroscopic quantum self trapping. Time-of-flight expansion maps the pattern of atomic currents into detectable atomic density distributions. Based on our analysis, we provide viable protocols to initialize, address, and read-out the qubit.
Apr 17
1. arXiv:1304.4569 [pdf, other]
The effect of interactions on 2D fermionic symmetry-protected topological phases with Z2 symmetry
Zheng-Cheng Gu, Michael Levin
We study the effect of interactions on 2D fermionic symmetry-protected topological (SPT) phases using the recently proposed braiding statistics approach. We focus on a simple class of examples: superconductors with a Z2 Ising symmetry. Although these systems are classified by Z in the noninteracting limit, our results suggest that the classification collapses to Z8 in the presence of interactions -- consistent with previous work that analyzed the stability of the edge. Specifically, we show that there are at least 8 different types of Ising superconductors that cannot be adiabatically connected to one another, even in the presence of strong interactions. In addition, we prove that each of the 7 nontrivial superconductors have protected edge modes.
2.arXiv:1304.4366 [pdf, other]Unraveling of the fractional topological phase in one-dimensional flatbands with nontrivial topology
Jan Carl Budich, Eddy Ardonne
We consider a topologically non-trivial flat band structure in one spatial dimension in the presence of nearest and next nearest neighbor Hubbard interaction. The non-interacting band structure is characterized by a symmetry protected topologically quantized Berry phase. At certain fractional fillings, a gapped phase with a filling-dependent ground state degeneracy, and fractionally charged quasi-particles emerges. At filling 1/3, the ground states carry a fractional Berry phase in the momentum basis. These features at first glance suggest a certain analogy to the fractional quantum Hall scenario in two dimensions. We solve the interacting model analytically in the physically relevant limit of a large band gap in the underlying band structure, the analog of a lowest Landau level projection. Our solution affords a simple physical understanding of the properties of the gapped interacting phase. We pinpoint crucial differences to the fractional quantum Hall case by studying the Berry phase and the entanglement entropy associated with the degenerate ground states. Finally, the symmetry protected nature of the interacting phase is demonstrated by explicitly constructing a gapped interpolation to a state with a trivial Berry phase.
3.arXiv:1304.4268 [pdf, ps, other]
Spontaneous time-reversal symmetry breaking in the boundary Majorana flat bandsYi Li, Da Wang, Congjun Wu
We study the boundary Majorana modes for the single component p-wave weak topological superconductors or superfluids, which form zero energy flat bands protected by time-reversal symmetry. However, due to the divergence of density of states, the band flatness is unstable under spatial variations of Cooper pairing phases, which spontaneously breaks time-reversal symmetry and lifts the degeneracy. Staggered current loops are generated near the boundary forming non-quantized vortices. This effect can appear in both condensed matter and ultra-cold atom systems.
Apr 16
1. arXiv:1304.4037 [pdf, ps, other]
Many Topological Insulators Fail the Surface Conduction Test
Sourabh Barua, K. P. Rajeev
In this report, we scrutinize the thickness dependent resistivity data from the recent literature on electrical transport measurements in topological insulators. A linear increase in resistivity with increase in thickness is expected in the case of these materials since they have an insulating bulk and conducting surface. However, such a trend is not seen in the resistivity versus thickness data for all the cases examined, except for some samples, where it holds for a narrow range of thickness.
2. arXiv:1304.3926 [pdf, ps, other]
FFLO or Majorana superfluids: Quantum phases of fermionic cold atoms in spin-orbit coupled optical lattices
Chunlei Qu, Ming Gong, Chuanwei Zhang
The recent experimental realization of spin-orbit coupling (SOC) for ultra-cold atoms opens a completely new avenue for exploring new quantum matter. In experiments, the SOC is implemented simultaneously with a Zeeman field. Such spin-orbit coupled Fermi gases are predicted to support Majorana fermions with non-Abelian exchange statistics in one dimension (1D). However, as shown in recent theory and experiments for 1D spin-imbalanced Fermi gases, the Zeeman field can lead to the long-sought Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluids with non-zero momentum Cooper pairings, in contrast to the zero momentum pairing in Majorana superfluids. Therefore a natural question to ask is which phase, FFLO or Majorana superfluids, will survive in spin-orbit coupled Fermi gases in the presence of a large Zeeman field. In this paper, we address this question by studying the mean field quantum phases of 1D spin-orbit coupled fermionic cold atom optical lattices.
3. arXiv:1304.3765 [pdf, other]Non-Abelian Majorana Doublets in Time-Reversal Invariant Topological Superconductor
Xiong-Jun Liu, Chris L. M. Wong, K. T. Law
The study of non-Abelian Majorana zero modes advances our understanding of the fundamental physics in quantum matter, and pushes their potential applications to topological quantum computation. Recent investigations have shown that in the two-dimensional (2D) and 1D chiral superconductors, braiding isolated Majorana zero modes leads to noncommutative transformation of the quantum states \cite{Ivanov,Alicea1}. However, Majorana fermions in a $Z_2$ time-reversal invariant (TRI) topological superconductor come in pairs due to Kramers' theorem \cite{Ryu}. Therefore, braiding operations in TRI superconductors always exchange two pairs of Majorana fermions. In this work, we show surprisingly that, due to the protection of time-reversal symmetry, non-Abelian statistics can be obtained in the 1D TRI topological superconductor and may have advantages in applying to topological quantum computation. Furthermore, we predict a novel phenomenon in the Josephson effect for 1D TRI topological superconductors, which measures the topological qubit states in such systems.
Apr 15
1. arXiv:1304.3628 [pdf, other]
Dynamical probing of a topological phase of bosons in one dimension
Emanuele G. Dalla Torre
We study the linear response to time-dependent probes of a symmetry-protected topological phase of bosons in one-dimension, the Haldane insulator (HI). This phase is separated from the ordinary Mott insulator (MI) and density-wave (DW) phases by continuous transitions, whose field theoretical description is here reviewed. Using this technique, we compute the absorption spectrum to two types of periodic perturbations and relate the findings to the nature of the critical excitations at the transition between the different phases. The HI-MI phase transition is topological and the critical excitations possess trivial quantum numbers: they correspond to particles and holes at zero momentum. Our findings are corroborated by a non-local mean-field approach, which allows us to directly relate the predicted spectrum to the known microscopic theory.
2. arXiv:1304.3476 [pdf, other]
Intrinsic Spin Hall Effect at Oxide Interfaces: a Simple Model
Lorien H. Hayden, R. Raimondi, M. E. Flatte', G. Vignale
An asymmetric triangular potential well provides the simplest model for the confinement of mobile electrons at the interface between two insulating oxides, such as LaAlO_3 and SrTiO_3 (LAO/STO). These electrons have been recently shown to exhibit a large spin-orbit coupling of the Rashba type, i.e., linear in the in-plane momentum. In this paper we study the intrinsic spin Hall effect due to Rashba coupling in an asymmetric triangular potential well. Besides splitting each subband into two branches of opposite helicity, the spin-orbit interaction causes the wave function in the direction perpendicular to the plane of the quantum well (the z direction) to depend on the in-plane wave vector k. In contrast to the extreme asymmetric case, i.e., the wedge-shaped quantum well, for which the intrinsic spin Hall effect vanishes due to vertex corrections, we find that the asymmetric well supports a non-vanishing intrinsic spin Hall conductivity, which increases in magnitude as the well becomes more symmetric. The spin Hall conductivity is found to be proportional to the square of the spin-orbit coupling constant and, in the limit of low carrier density, depends only on the effective mass renormalization associated with the k-dependence of the wave functions in the z direction. Its origin lies in the non-vanishing matrix elements of the spin current between subbands corresponding to different states of quantized motion perpendicular to the plane of the well.
April 8 - Apr 12, Saubhik Sarkar
Apr 12
1. arXiv:1304.3323 [pdf, other]
Exploring exchange mechanisms with a cold atom gas
P.O. Bugnion, G.J. ConduitFermionic atoms trapped in a double well potential are an ideal setting to study fundamental exchange mechanisms. We use exact diagonalization and complementary analytic calculations to demonstrate that two trapped fermions deliver a minimal model of the direct exchange mechanism. This is an ideal quantum simulator of the Heisenberg antiferromagnet, exposes the competition between covalent and ionic bonding, and can create, manipulate, and detect quantum entanglement. Three trapped atoms form a faithful simulator of the double exchange mechanism that is the fundamental building block behind many Heisenberg ferromagnets.
2. arXiv:1304.3299 [pdf, other]
Ferromagnetic spin correlations in a few-fermion system
P.O. Bugnion, G.J. Conduit
We study the spin correlations of a few two-component fermions in a quasi one-dimensional trap. Exact diagonalization and complementary variational Monte Carlo calculations demonstrate that repulsive interactions between the two species drives ferromagnetic correlations. We show how the ejection probability of an atom provides an experimental probe of the magnetic spin correlations. Finally, studying the energy levels of the excited states reveals that losses to Feshbach molecules are suppressed by the discretization of energy levels when fewer than seven atoms are trapped.
3. arXiv:1304.3178 [pdf, ps, other]
XYZ quantum Heisenberg models with p-orbital bosons
Fernanda Pinheiro, Georg M. Bruun, Jani-Petri Martikainen, Jonas Larson
We demonstrate how the spin-1/2 XYZ quantum Heisenberg model can be realized with bosonic atoms loaded in the p-band of an optical lattice in the Mott regime. The sign and relative strength of the couplings characterizing the model are demonstrated to be experimentally tuneable. We discuss the phase diagram in the one dimensional case, and show that finite size effects relevant for trapped systems lead to devil's staircase structure. Finally, we discuss experimental issuesrelated to preparation, manipulation and detection.
Apr 11
1. arXiv:1304.3012 [pdf, other]
Cluster Luttinger liquids of Rydberg-dressed gases in optical lattices
Marco Mattioli, Marcello Dalmonte, Wolfgang Lechner, Guido Pupillo
We investigate the zero-temperature phases of hard-core bosonic and fermionic gases confined to one dimension and interacting via a class of finite-range soft-core potentials. Using a combination of analytical and numerical methods, we demonstrate the stabilization of critical quantum liquids with qualitatively new features with respect to the Tomonaga-Luttinger liquid paradigm. These features result from frustration and cluster formation in the corresponding classical ground state. Characteristic signatures of these liquids are accessible in state-of-the-art experimental setups with Rydberg-dressed ground state atoms trapped in optical lattices.
2.arXiv:1304.2992 [pdf, other]
Fermionization of two-component few-fermion systems in a one-dimensional harmonic trap
E. J. Lindgren, J. Rotureau, C. Forssén, A. G. Volosniev, N. T. Zinner
We study two-component fermionic few-body systems in a one dimensional harmonic trap. Different component fermions interact via a short-range interaction of arbitrary strength. Using a new effective interaction technique we solve in particular the simplest imbalanced case of two spin-up and one spin-down fermion (2+1) and study its properties. In the so-called fermionized limit of infinitely strong repulsive interactions, we solve the problem analytically and find close agreement with the numerical results. Our findings indicate that the spin-up and spin-down fermions tend to phase separate in the trap, which we interpret as a microscopic precursor of ferromagnetism. We confirm this conclusion by performing calculations for (3+1) and (6+1) systems for strong repulsive interactions. Our predictions are directly addressable in current experiments on ultracold atomic few-body systems.
3. arXiv:1304.2778 [pdf, ps, other]
Single-particle and many-body analyses of a quasi-disordered integrable system after a quench
Kai He, Lea F. Santos, Tod M. Wright, Marcos Rigol
In general, isolated integrable quantum systems relax to an apparent equilibrium state in which the expectation values of few-body observables are described by the generalized Gibbs ensemble. However, recent work has shown that relaxation to such a generalized statistical ensemble can be precluded by localization in the presence of (quasi-)disorder. Here we undertake complementary single-particle and many-body analyses of noninteracting spinless fermions and hard-core bosons within the Aubry-Andre model to gain insight into this phenomenon. Our investigations span both the localized and delocalized regimes of the quasi-disordered system, as well as the critical point separating the two. Considering first the case of spinless fermions, we study the dynamics of the momentum distribution function and characterize the effects of real-space and momentum-space localization on the relevant single-particle wavefunctions and correlation functions. We show that although some observables do not relax in the delocalized and localized regimes, the observables that do relax in these regimes do so in a manner consistent with a recently proposed Gaussian equilibration scenario, whereas relaxation at the critical point has a more exotic character. We also construct various statistical ensembles from the many-body eigenstates of the fermionic and bosonic Hamiltonians and study the effect of localization on their properties.
3. arXiv:1304.2025 (cross-list from quant-ph) [pdf, other]
Sequential quantum-enhanced measurement with an atomic ensemble
A. V. Lebedev, P. Treutlein, G. BlatterWe propose a quantum-enhanced iterative (with $K$ steps) measurement scheme based on an ensemble of $N$ two-level probes which asymptotically approaches the Heisenberg limit $\delta_K \propto R^{-K/(K+1)}$, $R$ the number of quantum resources. The protocol is inspired by Kitaev's phase estimation algorithm and involves only collective manipulation and measurement of the ensemble. The iterative procedure takes the shot-noise limited primary measurement with precision $\delta_1\propto N^{-1/2}$ to increasingly precise results $\delta_K\propto N^{-K/2}$. A straightforward implementation of the algorithm makes use of a two-component atomic cloud of Bosons in the precision measurement of a magnetic field.
Apr 10
1.arXiv:1304.2690 [pdf, other]
Ground state phase diagram and critical temperature of two component Bose gases with Rashba spin-orbit coupling
Zeng-Qiang YuIn this work, we present ground state phase diagram of two component Bose gases with Rashba spin-orbit coupling. In addition to plane-wave condensate and spin density wave condensate which have been studied previously, a new phase that the fully polarized condensate occupies zero momentum is identified. This zero momentum phase competes with spin density wave phase when inter-species interaction is stronger than intra-species interaction, and the former one is always the ground state for weak enough spin-orbit coupling. When the energies of these two phases are close, there is a phase separation between them. At finite temperature, such a zero momentum condensation can be induced by a ferromagnetic phase transition in normal state. The spontaneous spin polarization generates an effective internal Zeeman field, and removes the degeneracy of quasi-particles energy minima. Consequently, the modified density-of-state accommodate a Bose condensation to appear below a critical temperature.
2. arXiv:1304.2634 [pdf, ps, other]
Generalized Effective Potential Landau Theory for Bosonic Quadratic SuperlatticesTao Wang, Xue-Feng Zhang, Sebastian Eggert, Axel PelsterWe study the properties of the Bose-Hubbard model for a quadratic optical superlattice. To this end we generalize a recently established effective potential Landau theory for a single component to the case of multi components and find not only the characteristic incompressible solid phases with fractional filling, but also obtain the underlying quantum phase diagram in the whole parameter region at zero temperature. Comparing our analytic results with corresponding ones from quantum Monte Carlo simulations demonstrates the high accuracy of the generalized effective potential Landau theory (GEPLT). Finally, we comment on the advantages and disadvantages of the GEPLT in view of a direct comparison with a corresponding decoupled mean-field theory.
3. arXiv:1304.2628 [pdf, other]
Many-body quantum quench in an atomic one-dimensional Ising chainFlorian Meinert, Manfred J. Mark, Emil Kirilov, Katharina Lauber, Philipp Weinmann, Andrew J. Daley, Hanns-Christoph NägerlWe study non-equilibrium dynamics for an ensemble of one-dimensional atomic Bose-Hubbard chains after a sudden quench to the vicinity of the transition point of the Ising paramagnetic to anti-ferromagnetic quantum phase transition. The quench results in coherent oscillations for the orientation of Ising spins, detected via oscillations in the number of doubly-occupied lattice sites. We characterize the quench by varying the system parameters. We find clear evidence for few-body and many-body effects in the oscillatory response.
Apr 9
1. arXiv:1304.2261 [pdf, other]
Stability of trapped degenerate dipolar Bose and Fermi gases
S. K. AdhikariTrapped degenerate dipolar Bose and Fermi gases of cylindrical symmetry with the polarization vector along the symmetry axis are only stable for the strength of dipolar interaction below a critical value. In the case of bosons, the stability of such a dipolar Bose-Einstein condensate (BEC) is investigated for di?erent strengths of contact and dipolar interactions using variational approximation and numerical solution of a mean-?eld model. In the disk shape, with the polarization vector perpendicular to the plane of the disk, the atoms experience an overall dipolar repulsion and this fact should contribute to the stability. However, a complete numerical solution of the dynamics leads to the collapse of a strongly disk-shaped dipolar BEC due to the long-range anisotropic dipolar interaction. In the case of fermions, the stability of a trapped single-component degenerate dipolar Fermi gas is studied including the Hartree-Fock exchange and Brueckner-Goldstone correlation energies in the local density approximation valid for a large number of atoms. Estimates for the maximum allowed number of polar Bose and Fermi molecules in BEC and degenerate Fermi gas are given.
2.arXiv:1304.2178 [pdf, other]
Phase diagrams and Thomas-Fermi estimates for spin-orbit coupled Bose-Einstein Condensates under rotationAmandine Aftalion (LM-Versailles), Peter MasonWe provide complete phase diagrams describing the ground state of a trapped spinor BEC under the combined effects of rotation and a Rashba spin orbit coupling. The interplay between the different parameters (magnitude of rotation, strength of the spin orbit coupling and interaction) leads to a rich ground state physics that we classify. We explain some features analytically in the Thomas Fermi approximation, writing the problem in terms of the total density, total phase and spin. In some regions of the phase diagrams, we relate the patterns to a ferromagnetic energy.
Apr 8
1. arXiv:1304.1742 [pdf, ps, other]
Matter-wave dark solitons and their excitation spectra in spin-orbit coupled Bose-Einstein condensates
V. Achilleos, J. Stockhofe, P. G. Kevrekidis, D. J. Frantzeskakis, P. SchmelcherWe present three types of dark solitons in quasi-one-dimensional spin-orbit coupled repulsive Bose-Einstein condensates. Among these families, two are always stable, while the third one is only stable sufficiently close to the linear regime. The solitons' excitation spectra reveal the potential existence of a second anomalous mode. While the first such mode describes the soliton oscillatory motion in a parabolic trap, the second, when present, reflects the double well structure of the underlying single-particle spectrum. This novel mode results in moving density stripes in the vicinity of the soliton core, or in an out-of-phase oscillation of the constituent components, with little effect on the nearly stationary striped total density of the composite soliton.
2.arXiv:1304.1573 [pdf, ps, other]
Entanglement entropy and macroscopic quantum states with dipolar bosons in a triple-well potential
L. Dell'Anna, G. Mazzarella, V. Penna, L. Salasnich
We study interacting dipolar atomic bosons in a triple-well potential within a ring geometry. This system is shown to be equivalent to a three-site Bose-Hubbard model. We analyze the ground state of dipolar bosons by varying the effective on-site interaction. This analysis is performed both numerically and analytically by using suitable coherent-state representations of the ground state. The latter exhibits a variety of forms ranging from the su(3) coherent state in the delocalization regime to a macroscopic cat-like state with fully localized populations, passing for a coexistence regime where the ground state displays a mixed character. We characterize the quantum correlations of the ground state from the bi-partition perspective. We calculate both numerically and analytically (within the previous coherent-state representation) the single-site entanglement entropy which, among various interesting properties, exhibits a maximum value in correspondence to the transition from the cat-like to the coexistence regime. In the latter case, we show that the ground-state mixed form corresponds, semiclassically, to an energy exhibiting two almost-degenerate minima.
Apr 1 - Apr 5, Johannes Schachenmayer
Apr 5
1. arXiv:1304.1473 [pdf, ps, other]
Spin-orbit coupling induced polarization textures in trapped Fermi gases
M. Iskin
We use the Bogoliubov-de Gennes (BdG) theory to study the interplay between the Zeeman field, spin-orbit coupling (SOC) and resultant skyrmion-like polarization textures in harmonically trapped two-dimensional Fermi gases. Our primary finding is that, in sharp contrast to the no-SOC case where only an easy-axis polarization is possible beyond a threshold Zeeman field (depending on the interaction strength), any non-zero combination of the Zeeman field and Rashba SOC induces not only an easy-axis polarization everywhere in the system but also a spatially-modulated transverse one near the edges. Our secondary finding is that the ground states of Rashba spin-orbit coupled systems are revealed to have stronger Fulde-Ferrell type non-uniform superfluid characters (i.e. phase modulations) but not a Larkin-Ovchinnikov one (i.e. amplitude modulations) under the in-plane Zeeman field, for which the phases of the superfluid order parameters modulate near the edges and most prominently along the transverse direction. The effects of the interaction, temperature, SOC anisotropy and Zeeman field anisotropy on the intricate polarization textures and superfluid order parameter are also discussed.
2. arXiv:1304.1328 [pdf, ps, other]
Density-functional theory for the spin-1 bosons in a one-dimensional harmonic trap
Hongmei Wang, Yunbo Zhang
We propose the density-functional theory for one-dimensional harmonically trapped spin-1 bosons in the ground state with repulsive density-density interaction and anti-ferromagnetic spin-exchange interaction. The density distributions of spin singlet paired bosons and polarized bosons with different total polarization for various interaction parameters are obtained by solving the Kohn-Sham equations which are derived based on the local density approximation and the Bethe ansatz exact results for homogeneous system. Non-monotonicity of the central densities is attributed to the competition between the density interaction and spin-exchange. The results reveal the phase separation of the paired and polarized bosons, the density profiles of which respectively approach the Tonks-Girardeau gases of Bose-Bose pairs and scalar bosons in the case of strong interaction. We give the R-P phase diagram at strong interaction and find the critical polarization, which paves the way to direct observe the exotic singlet pairing in spinor gas experimentally.
3. arXiv:1304.1176 [pdf, other]
Time-of-flight patterns of ultra-cold bosons in optical lattices in various Abelian artificial magnetic field gauges
T. P. Polak, T. A. Zaleski
We calculate the time-of-flight patterns of strongly interacting bosons confined in two-dimensional square lattice in the presence of an artificial magnetic field using quantum rotor model that is inherently combined with the Bogolyubov approach. We consider various geometries of the magnetic flux, which are expected to be realizable, or have already been implemented in experimental settings. The flexibility of the method let us to study cases of the artificial magnetic field being uniform, staggered or forming a checkerboard configuration. Effects of additional temporal modulation of the optical potential that results from application of Raman lasers driving particle transitions between lattice sites are also included. The presented time-of-flight patterns may serve as a verification of chosen gauge in experiments, but also provide important hints on unconventional, non-zero momentum condensates, or possibility of observing graphene-like physics resulting from occurrence of Dirac cones in artificial magnetic fields in systems of ultra-cold bosons in optical lattices. Also, we elucidate on differences between effects of magnetic field in solids and the artificial magnetic field in optical lattices, which can be controlled on much higher level leading to effects not possible in condensed matter physics.
Apr 4
1. arXiv:1304.0918 [pdf, ps, other]
Energy, contact and universal thermodynamics of the one-dimensional Fermi polaron at finite temperature
E. V. H. Doggen, J. J. Kinnunen
We use the T-matrix approach for studying highly polarized homogeneous Fermi gases in one dimension with repulsive or attractive contact interactions. Using this approach, we compute ground state energies and values for the contact parameter that show excellent agreement with exact and other numerical methods at zero temperature, even in the strongly interacting regime. Furthermore, we derive an exact expression for the value of the contact parameter in one dimension. The model is then extended and used for studying the temperature dependence of ground state energies and the contact parameter.
2. arXiv:1304.0889 [pdf, ps, other]
Dirac fields in curved spacetime as Fermi-Hubbard model with non unitary tunnelings
Jiří Minář, Benoît Grémaud
In this letter we show that a Dirac Hamiltonian in a curved background spacetime can be interpreted, when discretized, as a tight binding Fermi-Hubbard model with non unitary tunnelings. We find the form of the nonunitary tunneling matrices in terms of the metric tensor. In a simple case of a static diagonal metric, the tunnelings become unitary. Alternative interpretation of the Fermi-Hubbard Hamiltonian is that of a Pauli Hamiltonian, i.e. a non relativistic limit of the Dirac Hamiltonian. In this case the tunnelings remain, in general, non unitary even for the static diagonal metric. We discuss a possibility of synthesizing such Hamiltonians by means of laser assisted tunnelings in cold atomic experiments.
Apr 3
1. arXiv:1304.0554 [pdf, ps, other]
Exotic phases of interacting p-band bosons
F. Hébert, Zi Cai, V. G. Rousseau, Congjun Wu, R. T. Scalettar, G. G. Batrouni
We study a model of interacting bosons that occupy the first excited p-band states of a two-dimensional optical lattice. In contrast to the much studied single band Bose-Hubbard Hamiltonian, this more complex model allows for non-trivial superfluid phases associated with condensation at non-zero momentum and staggered order of the orbital angular momentum in addition to the superfluid-Mott insulator transition. More specifically, we observe staggered orbital angular momentum order in the Mott phase at commensurate filling and superfluidity at all densities. We also observe a transition between the staggered angular momentum superfluid phase and a striped superfluid, with an alternation of the phase of the superfluid along one direction. The transition between these two phases was observed in a recent experiment, which is then qualitatively well described by our model.
2. arXiv:1304.0506 [pdf, ps, other]
An exact formalism for quench dynamics
Deepak Iyer, Huijie Guan, Natan Andrei
We describe a formulation for studying the quench dynamics of integrable systems generalizing an approach by Yudson. We study the evolution of the Lieb-Liniger model, a gas of interacting bosons moving on the continuous infinite line and interacting via a short range potential. The formalism allows us to quench the system from any initial state. We find that for any value of repulsive coupling independently of the initial state the system asymptotes towards a strongly repulsive gas, while for any value of attractive coupling, the system forms a maximal bound state that dominates at longer times. In either case the system equilibrates but does not thermalize. We compare this to quenches in a Bose-Hubbard lattice and show that there, initial states determine long-time dynamics independent of the sign of the coupling.
Apr 2
1. arXiv:1304.0387 [pdf, ps, other]
Fulde-Ferrell superfluidity in ultracold Fermi gases with Rashba spin-orbit coupling
Hui Hu, Xia-Ji Liu
We theoretically investigate the inhomogeneous Fulde-Ferrell (FF) superfluidity in a three dimensional atomic Fermi gas with Rashba spin-orbit coupling near a broad Feshbach resonance. We show that within mean-field theory the FF superfluid state is always more favorable than the standard Bardeen-Cooper-Schrieffer (BCS) superfluid state when an in-plane Zeeman field is applied. We present a qualitative finite-temperature phase diagram near resonance and argue that the predicted FF superfluid is observable with experimentally attainable temperatures (i.e., $T\sim0.2T_{F}$, where $T_{F}$ is the characteristic Fermi degenerate temperature).
2. arXiv:1304.0335 [pdf, ps, other]
Composite pairing and superfluidity in a one-dimensional resonant Bose-Fermi mixture
Shimul Akhanjee, Masahisa Tsuchiizu, Akira Furusaki
We study the ground-state properties of one-dimensional mixtures of bosonic and fermionic atoms resonantly coupled to fermionic Feshbach molecules. When the particle densities of fermionic atoms and Feshbach molecules are different, the system undergoes various depletion transitions between binary and ternary mixtures, as a function of the detuning parameter. However, when the particle densities of fermionic atoms and Feshbach molecules are identical, the molecular conversion/disassociation term induces a gap in a sector of low-energy excitations, and the remaining system can be described by a two-component Tomonaga-Luttinger liquid. Using a bosonization scheme, we derive the effective low-energy Hamiltonian for the system, which has a similar form as that of the two-chain problem of coupled Tomonaga-Luttinger liquid. With the help of improved perturbative renormalization group analysis of the latter problem, we determine the ground-state phase diagram and find that it contains a phase dominated by composite superfluid or pairing correlations between the open and closed resonant channels.
3. arXiv:1304.0358 [pdf, ps, other]
Probing non-Abelian anyonic statistics with cold atom in optical lattice
Sheng Liu, Zheng-Yuan Xue
We propose a scheme to probe the non-Abelian statistics of the collective anyonic excitation in Kitaev's honeycomb model with cold atoms in an optical lattice. The generation of the anyonic excitation can be realized by simple rotating operation acting on an effective spin-1/2 system, which is encoded in the atomic hyperfine energy levels. The non-Abelian nature of the anyonic excitation is manifested by the braiding of four vortices, which leads to different operations on the subspace of degenerate ground states, and thus results in different final states. Here, by introducing an ancilla atom, the effective control over the lattice atoms can be realized and the final different states can also be imprinted on the ancilla and further distinguished by measurement.
3. arXiv:1304.0297 [pdf, ps, other]
Sensitivity to thermal noise of atomic Einstein-Podolsky-Rosen entanglement
R. J. Lewis-Swan, K. V. Kheruntsyan
We examine the prospect of demonstrating Einstein-Podolsky-Rosen (EPR) entanglement for massive particles using spin-changing collisions in a spinor Bose-Einstein condensate. Such a demonstration has recently been attempted by Gross et al. [Nature 480, 219 (2011)] using a condensate of Rb-87 atoms trapped in an optical lattice potential. For the condensate initially prepared in the (F,m_{F})=(2,0) hyperfine state, with no population in the m_{F}=+-1 states, we predict a significant suppression of the product of inferred quadrature variances below the Heisenberg uncertainty limit, implying strong EPR entanglement. However, such EPR entanglement is lost when the collisions are initiated in the presence of a small (currently undetectable) thermal population n_{th} in the m_{F}=+-1 states. For condensates containing 150 to 200 atoms, we predict an upper bound of n_{th}~1 that can be tolerated in this experiment before EPR entanglement is lost.
4. arXiv:1304.0291 [pdf, other]
Realization of 2D Spin-orbit Interaction and Exotic Topological Orders in Cold Atoms
Xiong-Jun Liu, K. T. Law, T. K. Ng
Majorana zero bound mode exists in the vortex core of a chiral $p+ip$ superconductor or superfluid, which can be driven from an s-wave pairing state by two-dimensional (2D) spin-orbit (SO) coupling. However, a 2D Rashba-type SO interaction is not experimentally realistic in cold atom gases. We propose here a novel scheme based on realistic cold atom platforms to study exotic topological phases in a blue-detuned square optical lattice, and predict both the quantum anomalous Hall effect and chiral topological superfluid phase in the experimentally accessible parameter regimes. This work opens a new direction with remarkable experimental feasibility to observe non-Abelian topological orders in cold atom systems.
Apr 1
1. arXiv:1303.7362 [pdf, ps, other]
Exact analytical soliton solutions in dipolar BEC
P. A. Andreev, L. S. Kuz'menkov
The bright, dark and grey solitons are well-known soliton solutions of the Gross-Pitaevskii equation for the attractive and repulsive BEC. We consider solitons in the dipolar BEC of the fully polarized particles, speaking of the dipolar BEC we mean both the magnetized BEC and the electrically polarized BEC. We show that these two types of the dipolar BEC reveal different behavior of the collective excitations. This is related to the fact that the electric and the magnetic fields satisfy to the different pairs of the Maxwell equation set. Thus we consider them independently. We obtain exact analytical solutions for the bright, dark, and grey solitons in the magnetized (electrically polarized) BEC when they propagate parallel and perpendicular to an external magnetic (electric) field. Comparison of spectrum of the linear collective excitations for the two kinds of the dipolar BEC is presented as well.
2. arXiv:1303.7272 [pdf, ps, other]
Absence of Bose condensation in certain frustrated lattices
Tigran A. Sedrakyan, Leonid I. Glazman, Alex Kamenev
We study hard-core bosons on a class of frustrated lattices with the lowest Bloch band having a degenerate minimum along a closed contour in the reciprocal space. We suggest that the ground state of the system is given by non-condensed state, which may be viewed as a state of fermions subject to Chern-Simons gauge field. At fixed density of bosons, such a state exhibits domains of incompressible liquids. Their fixed densities are given by fractions of the reciprocal area enclosed by the minimal energy contour.
3. arXiv:1303.7241 [pdf, ps, other]
Superfluidity breakdown of periodic matter waves in quasi one-dimensional annular traps via resonant scattering with moving defects
A.V. Yulin, Yu. V. Bludov, V. V. Konotop, V. Kuzmiak, M. Salerno
We investigate, both analytically and numerically, the quasi-superfluidity properties of periodic Bose-Einstein condensates (BECs) in a quasi-one-dimensional (1D) ring with optical lattices (OL) of different kinds (linear and nonlinear) and with a moving defect of an infinite mass inside. To study the dynamics of the condensate we used a mean-field approximation describing the condensate by use of the Gross-Pitaevskii equation for the order parameter. We show that the resonant scattering of sound Bloch waves with the defect profoundly affect BEC superfluidity. In particular, a moving defect always leads to the breakdown of superfluidity independently of the value of its velocity. For weak periodic potentials the superfluidity breakdown may occur on a very long time scale (quasisuperfluidity) but the breakdown process can be accelerated by increasing the strength of the OL. Quite remarkably, we find that when the length of the ring is small enough to imply the discreteness of the reciprocal space, it becomes possible to avoid the resonant scattering and to restore quasi-superfluidity.
4. arXiv:1303.7359 [pdf, other]
Light induced crystallization of cold atoms in a thin 1D optical tube
Tobias Grießer, Helmut Ritsch
Collective off resonant scattering of coherent light by a cold gas induces long-range interactions via interference of light scattered by different particles. In a 1D configuration these interactions grow particularly strong for particles trapped along an optical nanofiber. We show that there exists a threshold pump laser intensity, above which the gas can be found in a crystalline, selfsustained order. In the nonabsorbing regime we determine the critical condition for the onset of order as well as the forms of particle density and scattered field patterns along the fiber above threshold. Surprisingly, there can coexist multiple stationary solutions with distinct density and field profiles.
5. arXiv:1303.7249 [pdf, other]
Multiple scattering of light in cold atomic clouds with a magnetic field
Olivier Sigwarth, Guillaume Labeyrie, Dominique Delande, Christian Miniatura
Starting from a microscopic theory for atomic scatterers, we describe the scattering of light by a single atom and study the coherent propagation of light in a cold atomic cloud in the presence of a magnetic field B in the mesoscopic regime. Non-pertubative expressions in B are given for the magneto-optical effects and optical anisotropy. We then consider the multiple scattering regime and address the fate of the coherent backscattering (CBS) effect. We show that, for atoms with nonzero spin in their ground state, the CBS interference contrast can be increased compared to its value when B=0, a result at variance with classical samples. We validate our theoretical results by a quantitative comparison with experimental data.
