Apr 2015
Apr 6-Apr 10 Zhifang Xu & Xuguang Yue, Apr 13-Apr 17 Bo Liu & Max, Apr 20-Apr 24, Zhenyu Zhou & Jinlong Yu, Apr 27-May 1, Jiyao chen & Jianhui zhou
Fri, 17 1. arXiv:1504.04288 [pdf, other] Does an isolated quantum system relax? B. Rauer, T. Schweigler, T. Langen, J. Schmiedmayer Statistical mechanics is one of the most comprehensive theories in physics. From a boiling pot of water to the complex dynamics of quantum many-body systems it provides a successful connection between the microscopic dynamics of atoms and molecules to the macroscopic properties of matter. However, statistical mechanics only describes the thermal equilibrium situation of a system, and there is no general framework to describe how equilibrium is reached or under which circumstances it can be reached at all. This problem is particularly challenging in quantum mechanics, where unitarity appears to render the very concept of thermalization counterintuitive. With the rapid experimental progress in the control and probing of ultracold quantum gases this question has become within reach of detailed experimental investigations. In these notes we present a series of experiments with ultracold one-dimensional Bose gases, which provide novel insights into this fundamental question. 2. arXiv:1504.04260 (cross-list from quant-ph) [pdf, other] Robust quantum correlations in out-of-equilibrium matter-light systems O. L. Acevedo, L. Quiroga, F. J. Rodríguez, N. F. Johnson High precision macroscopic quantum control in interacting light-matter systems remains a signi?cant goal toward novel information processing and ultra-precise metrology. We show that the out-of-equilibrium behavior of a paradigmatic light-matter system (Dicke model) reveals two successive stages of enhanced quantum correlations beyond the traditional schemes of near-adiabatic and sudden quenches. The fi?rst stage features magni?cation of matter-only and light-only entanglement and squeezing due to e?ective non linear self-interactions. The second stage results from a highly entangled light-matter state, with enhanced superradiance and signatures of chaotic and highly quantum states. We show that these new e?ects scale up consistently with matter system size, and are reliable even in dissipative environments. Thu, 16 1. arXiv:1504.03925 [pdf, other] Exact numerical methods for a many-body Wannier Stark system Carlos A. Parra-Murillo, Javier Madroñero, Sandro Wimberger We present exact methods for the numerical integration of the Wannier-Stark system in a many-body scenario including two Bloch bands. Our ab initio approaches allow for the treatment of a few-body problem with bosonic statistics and strong interparticle interaction. The numerical implementation is based on the Lanczos algorithm for the diagonalization of large, but sparse symmetric Floquet matrices. We analyze the scheme efficiency in terms of the computational time, which is shown to scale polynomially with the size of the system. The numerically computed eigensystem is applied to the analysis of the Floquet Hamiltonian describing our problem. We show that this allows, for instance, for the efficient detection and characterization of avoided crossings and their statistical analysis. We finally compare the efficiency of our Lanczos diagonalization for computing the temporal evolution of our many-body system with an explicit fourth order Runge-Kutta integration. Both implementations heavily exploit efficient matrix-vector multiplication schemes. Our results should permit an extrapolation of the applicability of exact methods to increasing sizes of generic many-body quantum problems with bosonic statistics. 2. arXiv:1504.03635 (cross-list from physics.atom-ph) [pdf, other] Direct observation of ultrafast many-body electron dynamics in a strongly-correlated ultracold Rydberg gas Nobuyuki Takei, Christian Sommer, Claudiu Genes, Guido Pupillo, Haruka Goto, Kuniaki Koyasu, Hisashi Chiba, Matthias Weidemüller, Kenji Ohmori Many-body interactions govern a variety of important quantum phenomena ranging from superconductivity and magnetism in condensed matter to solvent effects in chemistry. Understanding those interactions beyond mean field is a holy grail of modern sciences. AMO physics with advanced laser technologies has recently emerged as a new platform to study quantum many-body systems. One of its latest developments is the study of long-range interactions among ultracold particles to reveal the effects of many-body correlations. Rydberg atoms distinguish themselves by their large dipole moments and tunability of dipolar interactions. Most of ultracold Rydberg experiments have been performed with narrow-band lasers in the Rydberg blockade regime. Here we demonstrate an ultracold Rydberg gas in a complementary regime, where electronic coherence is created using a broadband picosecond laser pulse, thus circumventing the Rydberg blockade to induce strong many-body correlations. The effects of long-range Rydberg interactions have been investigated by time-domain Ramsey interferometry with attosecond precision. This approach allows for the real-time observation of coherent and ultrafast many-body dynamics in which the electronic coherence is modulated by the interaction-induced correlations. The modulation evolves more rapidly than expected for two-body correlations by several orders of magnitude. We have actively controlled such ultrafast many-body dynamics by tuning the principal quantum number and the population of the Rydberg state. The observed Ramsey interferograms are well reproduced by a theoretical model beyond mean-field approximation, which can be relevant to other similar many-body phenomena in condensed matter physics and chemistry. Our new approach opens a new avenue to observe and manipulate nonequilibrium dynamics of strongly-correlated quantum many-body systems on the ultrafast timescale. Wed, 15 1. arXiv:1504.03583 [pdf, other] The role of real-space micromotion for bosonic and fermionic Floquet fractional Chern insulators Egidijus Anisimovas, Giedrius Žlabys, Brandon M. Anderson, Gediminas Juzeliūnas, André Eckardt Fractional Chern insulators are the proposed phases of matter mimicking the physics of fractional quantum Hall states on a lattice without an overall magnetic field. The notion of Floquet fractional Chern insulators refers to the potential possibilities to generate the underlying topological bandstructure by means of Floquet engineering. In these schemes, a highly controllable and strongly interacting system is periodically driven by an external force at a frequency such that double tunneling events during one forcing period become important and contribute to shaping the required effective energy bands. We show that in the described circumstances it is necessary to take into account also third order processes combining two tunneling events with interactions. Referring to the obtained contributions as micromotion-induced interactions, we find that those interactions tend to have a negative impact on the stability of of fractional Chern insulating phases and discuss implications for future experiments. 2. arXiv:1504.03434 [pdf, ps, other] Stability of Bose Gases Near Resonance: The scale-dependent interactions and thermal effects Shao-Jian Jiang, Fei Zhou The stability of Bose gases near resonance has been a puzzling problem in recent years. In this Letter, we demonstrate that while the scale-dependent interactions lead to a zero temperature instability at a positive scattering length, thermal excitations can effectively stabilize Bose gases by enhancing the repulsiveness of the interatomic interactions and generating thermal pressure. We further find that near the instability, the density profile of a Bose gas in a harmonic trap develops a flat top near the center due to a rapid drop of the compressibility, a precursor of instability. Tue, 14 1. arXiv:1504.03223 [pdf, ps, other] Topological Superfluid and Majorana Zero Modes in Synthetic Dimension Zhongbo Yan, Shaolong Wan, Zhong Wang Recently it has been shown that multicomponent spin-orbit-coupled fermions in one-dimensional optical lattices can be viewed as spinless fermions moving in two-dimensional synthetic lattices with synthetic magnetic flux. The quantum Hall edge states in these systems have been observed in recent experiments. In this paper we study the effect of an attractive Hubbard interaction. Since the Hubbard interaction is long-range in the synthetic dimension, it is able to efficiently induce Cooper pairing between the counterpropagating chiral edge states. The topological class of the resultant one-dimensional superfluid is determined by the parity (even/odd) of the Chern number in the two-dimensional synthetic lattice. We explicitly solve the Majorana zero modes with various boundary conditions to establish this picture. 2. arXiv:1504.02864 [pdf, other] Orbital Feshbach Resonance in Alkali-Earth Atoms Ren Zhang, Yanting Cheng, Hui Zhai, Peng Zhang For a mixture of alkali-earth atomic gas in the long-lived excited state 3P0 and ground state 1S0, in addition to nuclear spin, another "orbital" index is introduced to distinguish these two internal states. In this letter we propose a mechanism to induce Feshbach resonance between two atoms with different orbital and nuclear spin quantum numbers. Two essential ingredients are inter-orbital spin-exchanging scattering and orbital dependence of the Land\'e g-factors. Here the orbital degrees of freedom plays similar role as electron spin degree of freedom in magnetic Feshbach resonance in alkali-metal atoms. This resonance is particularly accessible for 173Yb system. The BCS-BEC crossover in this system requires two fermion pairing order parameters, and displays significant difference comparing to that in alkali-metal system Mon, 13 1. arXiv:1504.02624 (cross-list from quant-ph) [pdf, other] Sub-Poissonian Statistics of Jamming Limits in Ultracold Rydberg Gases Jaron Sanders, Matthieu Jonckheere, Servaas Kokkelmans Several recent experiments have established by measuring the Mandel Q parameter that the number of Rydberg excitations in ultracold gases exhibits sub-Poissonian statistics. This effect is attributed to the Rydberg blockade that occurs due to the strong interatomic interactions between highly-excited atoms. Because of this blockade effect, the system can end up in a state in which all particles are either excited or blocked: a jamming limit. We analyze appropriately constructed random-graph models that capture the blockade effect, and derive formulae for the mean and variance of the number of Rydberg excitations in jamming limits. This yields an explicit relationship between the Mandel Q parameter and the blockade effect, and comparison to measurement data shows strong agreement between theory and experiment. Apr 10 1. arXiv:1504.02263 [pdf, other] Charge Pumping of Interacting Fermion Atoms in the Synthetic Dimension Tian-Sheng Zeng, Ce Wang, Hui Zhai Comments: 5 pages, 6 figures Subjects: Quantum Gases (cond-mat.quant-gas) Recently it has been proposed and experimentally demonstrated that a spin-orbit coupled multi-component gas in 1d lattice can be viewed as spinless gas in a synthetic 2d lattice with a magnetic flux. In this letter we consider interaction effect of such a Fermi gas, and propose signatures in charge pumping experiment, which can be easily realized in this setting. Using 1/3 filling of the lowest 2d band as an example, in strongly interacting regime, we show that the charge pumping value gradually approaches a universal fractional value for large spin component and low filling of 1d lattice, indicating a fractional quantum Hall type behavior; while the charge pumping value is zero if the 1d lattice filling is commensurate, indicating a Mott insulator behavior. The charge-density-wave order is also discussed. 2. Time-of-flight images of the Mott insulators in the Hofstadter-Bose-Hubbard model M. Iskin Comments: A brief report with 4 pages and a figure Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el) We analyze the momentum distribution function and its artificial-gauge-field dependence for the Mott insulator phases of the Hofstadter-Bose-Hubbard model. By benchmarking the results of the random-phase approximation (RPA) approach against those of the strong-coupling expansion (SCE) for the Landau and symmetric gauges, we find pronounced corrections to the former results in two dimensions. Apr 9 1. arXiv:1504.01729 [pdf, other] Photon-Induced Spin-Orbit Coupling in Ultracold Atoms inside Optical Cavity Lin Dong, Chuanzhou Zhu, Han Pu Comments: 14 pages, 5 figures, and comments are welcome Subjects: Quantum Gases (cond-mat.quant-gas) We consider an atom inside a ring cavity, where a plane-wave cavity field together with an external coherent laser beam induces a two-photon Raman transition between two hyperfine ground states of the atom. This cavity-assisted Raman transition induces effective coupling between atom's internal degrees of freedom and its center-of-mass motion. In the meantime, atomic dynamics exerts a back-action to cavity photons. We investigate the properties of this system by adopting a mean-field and a full quantum approach, and show that the interplay between the atomic dynamics and the cavity field gives rise to intriguing nonlinear phenomena. 2. arXiv:1504.01910 (cross-list from cond-mat.str-el) [pdf, ps, other] Quantum Quenches in Chern Insulators M. D. Caio, N. R. Cooper, M. J. Bhaseen Comments: 9 pages, 13 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas) We explore the non-equilibrium response of Chern insulators. Focusing on the Haldane model, we study the dynamics induced by quantum quenches between topological and non-topological phases. A notable feature is that the Chern number, calculated for an infinite system, is unchanged under the dynamics following such a quench. However, in finite geometries, the initial and final Hamiltonians are distinguished by the presence or absence of edge modes. We study the edge excitations and describe their impact on the experimentally-observable edge currents and magnetization. We show that, following a quantum quench, the edge currents relax towards new equilibrium values, and that there is light-cone spreading of the currents into the interior of the sample. Apr 8 1. arXiv:1504.01474 [pdf, other] Hexagonal Plaquette Spin-spin Interactions and Quantum Magnetism in a Two-dimensional Ion Crystal Rejish Nath, Marcello Dalmonte, Alexander W Glaetzle, Peter Zoller, Ferdinand Schmidt-Kaler, Rene Gerritsma Comments: 26 pages, 11 figures Subjects: Quantum Gases (cond-mat.quant-gas) We propose a trapped ion scheme en route to realize spin Hamiltonians on a Kagome lattice which, at low energies, are described by emergent Z2 gauge fields, and support a topological quantum spin liquid ground state. The enabling element in our scheme is the hexagonal plaquette spin-spin interactions in a 2D ion crystal. For this, the phonon-mode spectrum of the crystal is engineered by standing-wave optical potentials or by using Rydberg excited ions, thus generating localized phonon-modes around a hexagon of ions selected out of the entire two-dimensional crystal. These tailored modes can mediate spin-spin interactions between ion-qubits on a hexagonal plaquette when subject to state-dependent optical dipole forces. We discuss how these interactions can be employed to emulate a generalized Balents-Fisher-Girvin model in minimal instances of one and two plaquettes. This model is an archetypical Hamiltonian in which gauge fields are the emergent degrees of freedom on top of the classical ground state manifold. Under realistic situations, we show the emergence of a discrete Gauss's law as well as the dynamics of a deconfined charge excitation on a gauge-invariant background using the two-plaquettes trapped ions spin-system. The proposed scheme in principle allows further scaling in a future trapped ion quantum simulator, and we conclude that our work will pave the way towards the simulation of emergent gauge theories and quantum spin liquids in trapped ion systems. 2. arXiv:1504.01620 (cross-list from quant-ph) [pdf, other] Non-Exponential Quantum Decay of a Many-Particle System Adolfo del Campo Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas) The exact quantum decay of a many-body system equivalent to a gas of particles obeying generalized exclusion statistics is presented. The survival probability of the initial state exhibits early on a quadratic dependence on time that turns into a power-law decay, during the course of the evolution. Its is shown that the particle number and the strength of interactions determine the power-law exponent in the latter regime, as recently conjectured. The non-exponential character of the decay is linked to the many-particle reconstruction of the initial state from the decaying products. Apr 7 1. arXiv:1504.01243 (cross-list from math-ph) [pdf, ps, other] Topological Current in Fractional Chern Insulators Tohru Koma Comments: 52 pages. no figure Subjects: Mathematical Physics (math-ph); Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph) We consider interacting fermions in a magnetic field on a two-dimensional lattice with the periodic boundary conditions. In order to measure the Hall current, we apply an electric potential with a compact support. Then, due to the Lorentz force, the Hall current appears along the equipotential line. Introducing a local current operator at the edge of the potential, we derive the Hall conductance as a linear response coefficient. For a wide class of the models, we prove that if there exists a spectral gap above the degenerate ground state, then the Hall conductance of the ground state is fractionally quantized without averaging over the fluxes. This is an extension of the topological argument for the integrally quantized Hall conductance in noninteracting fermion systems on lattices. 2. arXiv:1504.00992 [pdf, other] Improved scaling of Time-Evolving Block-Decimation algorithm through Reduced-Rank Randomized Singular Value Decomposition D. Tamascelli, R. Rosenbach, M.B. Plenio Comments: 14 pages, 5 figures Subjects: Quantum Physics (quant-ph); Computational Physics (physics.comp-ph) When the amount of entanglement in a quantum system is limited, the relevant dynamics of the system is restricted to a very small part of the state space. When restricted to this subspace the description of the system becomes efficient in the system size. A class of algorithms, exemplified by the Time-Evolving Block-Decimation (TEBD) algorithm, make use of this observation by selecting the relevant subspace through a decimation technique relying on the Singular Value Decomposition (SVD). In these algorithms, the complexity of each time-evolution step is dominated by the SVD. Here we show that, by applying a randomized version of the SVD routine (RRSVD), the power law governing the computational complexity of TEBD is lowered by one degree, resulting in a considerable speed-up. We exemplify the potential gains in efficiency at the hand of some real world examples to which TEBD can be successfully applied to and demonstrate that for those system RRSVD delivers results as accurate as state-of-the-art deterministic SVD routines. 3. arXiv:1504.01187 [pdf, other] Distinguishing Quantum and Classical Many-Body Systems Dvir Kafri, Jacob Taylor Comments: 5 pages, 2 figuresSubjects: Quantum Physics (quant-ph) Controllable systems relying on quantum behavior to simulate distinctly quantum models so far rely on increasingly challenging classical computing to verify their results. We develop a general protocol for confirming that an arbitrary many-body system, such as a quantum simulator, can entangle distant objects. The protocol verifies that distant qubits interacting separately with the system can become mutually entangled, and therefore serves as a local test that excitations of the system can create non-local quantum correlations. We derive an inequality analogous to Bell's inequality which can only be violated through entanglement between distant sites of the many-body system. Although our protocol is applicable to general many-body systems, it requires finding system-dependent local operations to violate the inequality. A specific example in quantum magnetism is presented. 4. arXiv:1504.01222 [pdf] Photon-counting Brillouin optical time-domain reflectometry based on up-conversion detector and fiber Fabry-Perot scanning interferometer Haiyun Xia, Mingjia Shangguan, Guoliang Shentu, Chong Wang, Jiawei Qiu, Xiuxiu Xia, Chao Chen, Mingyang Zheng, Xiuping Xie, Qiang Zhang, Xiankang Dou, Jianwei Pan Comments: 4 pages, 5 figures Subjects: Quantum Physics (quant-ph); Optics (physics.optics) A direct-detection Brillouin optical time-domain reflectometry (BOTDR) is proposed and demonstrated by using an up-conversion single-photon detector and a fiber Fabry-Perot scanning interferometer (FFP-SI). Taking advantage of high signal-to-noise ratio of the detector and high spectrum resolution of the FFP-SI, the Brillouin spectrum along a polarization maintaining fiber (PMF) is recorded on a multiscaler with a small data size directly. In contrast with conventional BOTDR adopting coherent detection, photon-counting BOTDR is simpler in structure and easier in data processing. In the demonstration experiment, characteristic parameters of the Brillouin spectrum including its power, spectral width and frequency center are analyzed simultaneously along a 10 km PMF at different temperature and stain conditions. 5. arXiv:1504.01314 [pdf, other] Vortex Dynamics in a Spin-Orbit Coupled Bose-Einstein Condensate Alexander L. Fetter Comments: Conference presentation at Quantum Gases, Fluids, and Solids. Accepted for Journal of Low Temperature Physics Subjects: Quantum Gases (cond-mat.quant-gas) Vortices in a one-component dilute atomic ultracold Bose-Einstein condensate (BEC) usually arise as a response to externally driven rotation. Apart from a few special situations, these vortices are singly quantized with unit circulation. Recently, the NIST group has constructed a two-component BEC with a spin-orbit coupled Hamiltonian involving Pauli matrices, and I here study the dynamics of a two-component vortex in such a spin-orbit coupled condensate. These spin-orbit coupled BECs use an applied magnetic field to split the hyperfine levels. Hence they rely on a focused laser beam to trap the atoms. In addition, two Raman laser beams create an effective (or synthetic) gauge potential. The resulting spin-orbit Hamiltonian is discussed in some detail. The various laser beams are fixed in the laboratory, so that it is not feasible to nucleate a vortex by an applied rotation that would need to rotate all the laser beams and the magnetic field. In a one-component BEC, a vortex can also be created by a thermal quench, starting from the normal state and suddenly cooling deep into the condensed state. I propose that a similar method would work for a vortex in a spin-orbit coupled BEC. Such a vortex has two components, and each has its own circulation quantum number. If both components have the same circulation, I find that the composite vortex should execute uniform precession, like that observed in a single-component BEC. In contrast, if one component has unit circulation and the other has zero circulation, then some fraction of the dynamical vortex trajectories should eventually leave the condensate, providing clear experimental evidence for this unusual vortex structure. In the context of exciton-polariton condensates, such a vortex is known as a "half-quantum vortex". 6. arXiv:1504.01303 [pdf, ps, other] Strongly interacting few-fermion systems in a trap C. Forssén, R. Lundmark, J. Rotureau, J. Larsson, D. Lidberg Comments: 8 pages, 4 figures, contribution to the conference proceedings of the 7th International Workshop on the "Dynamics of Critically Stable Quantum Few-Body Systems"Subjects: Quantum Gases (cond-mat.quant-gas) Few- and many-fermion systems on the verge of stability, and consisting of strongly interacting particles, appear in many areas of physics. The theoretical modeling of such systems is a very difficult problem. In this work we present a theoretical framework that is based on the rigged Hilbert space formulation. The few-body problem is solved by exact diagonalization using a basis in which bound, resonant, and non-resonant scattering states are included on an equal footing. Current experiments with ultracold atoms offer a fascinating opportunity to study universal properties of few-body systems with a high degree of control over parameters such as the external trap geometry, the number of particles, and even the interaction strength. In particular, particles can be allowed to tunnel out of the trap by applying a magnetic-field gradient that effectively lowers the potential barrier. The result is a tunable open quantum system that allows detailed studies of the tunneling mechanism. In this Contribution we introduce our method and present results for the decay rate of two distinguishable fermions in a one-dimensional trap as a function of the interaction strength. We also study the numerical convergence. Many of these results have been previously published (R. Lundmark, C. Forss\'en, and J. Rotureau, arXiv: 1412.7175). However, in this Contribution we present several technical and numerical details of our approach for the first time. 7. arXiv:1504.00925 [pdf, ps, other] Exact Ground-State Properties of Strongly Interacting Fermi Gases in Two Dimensions Hao Shi, Simone Chiesa, Shiwei Zhang Comments: 6 pages, 4 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el) Exact calculations are performed on the two-dimensional strongly interacting, unpolarized, uniform Fermi gas with a zero-range attractive interaction. Two auxiliary-field approaches are employed which accelerate the sampling of imaginary-time paths using BCS trial wave functions and a force bias technique. Their combination enables calculations on large enough lattices to reliably compute ground-state properties in the thermodynamic limit. A new equation of state is obtained, with a parametrization provided, which can serve as a benchmark and allow accurate comparisons with experiments. The pressure, contact parameter, and condensate fraction are determined systematically vs.~kFa. The momentum distribution, pairing correlation, and the structure of the condensate wave function are computed. Apr 6 arXiv:1504.00818 [pdf, ps, other] Hong-Ou-Mandel interference between triggered and heralded single photons from separate atomic systems Victor Leong, Sandoko Kosen, Bharath Srivathsan, Gurpreet Kaur Gulati, Alessandro Ceré, Christian Kurtsiefer Comments: 5 pages, 5 figures Subjects: Quantum Physics (quant-ph) We present Hong-Ou-Mandel interference of single photons generated via two different physical processes by two independent atomic systems: scattering by a single atom, and parametric generation via four-wave mixing in a cloud of cold atoms. Without any spectral filtering, we observe a visibility of V=62±4%. After correcting for accidental coincidences, we obtain V=93±6%. The observed interference demonstrates the compatibility of the two sources, forming the basis for an efficient quantum interface between different physical systems.
