Jan 2018
Jan 1- Jan 4 Ahmet Keles, Jan 5-Jan 9 Max Arzamasovs, Jan 10-Jan 14 Xuguang Yue, Jan 15-Jan 19 Biao Huang, Jan 20-Jan 24 Haiyuan Zou, Jan 25-Jan 29 Zehan Li, Jan 30-Jan 31 Jiansong Pan
Jan 31
arXiv:1801.10077 [pdf, other]
Observation of interaction-induced gauge fields in a Bose-Einstein condensate based on micromotion control in a shaken two-dimensional lattice
Logan W. Clark, Brandon M. Anderson, Lei Feng, Kathy Levin, Cheng Chin
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
We demonstrate an interaction-induced gauge field for Bose-Einstein condensates in a modulated two-dimensional optical lattice. The gauge field results from the synchronous coupling between the interactions and micromotion of the atoms. As a first step, we show that a coherent shaking of the lattice in two directions can couple the momentum and interactions of atoms and break the four-fold symmetry of the lattice. We then create a full interaction-induced gauge field by modulating the interaction strength in synchrony with the lattice shaking. When a condensate is loaded into this shaken lattice, the gauge field acts to preferentially prepare the system in different quasimomentum ground states depending on the modulation phase. We envision that these interaction-induced fields, created by fine control of micromotion, will provide a stepping stone to model new quantum phenomena within and beyond condensed matter physics.
Jan 30
arXiv:1801.08942 [pdf, other]
Exact edge, bulk and bound states of finite topological systems
Callum W. Duncan, Patrik Öhberg, Manuel Valiente
Comments: 12 pages, 6 figures, comments welcome
Subjects: Quantum Gases (cond-mat.quant-gas)
Finite topologically non-trivial systems are often characterised by the presence of bound states at their physical edges. These topological edge modes can be distinguished from usual Shockley waves energetically, as their energies remain finite and in-gap. On a clean 1D or reducible 2D model, in either the commensurate or semi-infinite case, the edge modes can be obtained analytically, as shown in [PRL 71, 3697 (1993)] and [PRA 89, 023619 (2014)]. We put forward a method for obtaining the spectrum and wave functions of topological edge modes for arbitrary finite lattices, including the incommensurate case. A small number of parameters are easily determined numerically, with the form of the eigenstates remaining fully analytical. We also obtain the bulk modes in the finite system analytically and their eigenenergies, which lie within the infinite-size limit continuum. Our method is general and can be easily applied to obtain the properties of non-topological models and/or extended to include impurities. As an example, we consider the case of an impurity located next to one edge of a 1D system, equivalent to a softened boundary in a separable 2D model. We show that a localised impurity can have a drastic effect on the edge modes of the system. Using the periodic Harper and Hofstadter models to illustrate our method, we find that, on increasing the impurity strength, edge states can enter or exit the continuum, and a trivial Shockley state bound to the impurity may appear. The fate of the topological edge modes in the presence of impurities can be addressed by quenching the impurity strength. We find that at certain critical impurity strengths, the transition probability for a particle initially prepared in an edge mode to decay into the bulk exhibits discontinuities that mark the entry and exit points of edge modes from and into the bulk spectrum.
Jan 28
[4] arXiv:1801.08169 (cross-list from quant-ph) [pdf, other]
Spontaneous generation of entanglement in quantum dark-soliton qubits
Muzzamal I. Shaukat, Eduardo. V. Castro, Hugo Terças
Comments: 5 Pages, 5 Figures
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)
We show that entanglement between two solitary qubits in quasi one-dimensional Bose-Einstein condensates can be spontaneously generated due to quantum fluctuations. Recently, we have shown that dark solitons are an appealing platform for qubits due to their appreciably long lifetime. We explore the creation of entanglement between dark soliton qubits by using the superposition of two maximally entangled states in the dissipative process of spontaneous emission. By driving the qubits with the help of Raman lasers, we observe the formation of long distance steady-state concurrence. Our results suggest that dark-soliton qubits are a good candidates for quantum information protocols based purely on matter-wave phononics.
Jan 27
arXiv:1801.07129 [pdf, ps, other]
Pressure induced BEC-BCS crossover in multi-band superconductors
D. Reyes, M. A. Continentino, F. Deus, C. Thomas
Comments: 10 pages, 8 figures
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el) Abstract. Superconductivity in strongly correlated systems is a remarkable phenomenon that attracts a huge interest. The study of this problem is relevant for materials as the high Tc oxides, pnictides and heavy fermions. These systems also have in common the existence of electrons of several orbitals that coexist at a common Fermisurface. In this paper we study the effect of pressure, chemical or applied on multi-band superconductivity. Pressure varies the atomic distances and consequently the overlap of the wave-functions in the crystal. This rearranges the electronic structure that we model including a pressure dependent hybridization between the bands. We consider the case of two-dimensional systems in a square lattice with inverted bands. We study the conditions for obtaining a pressure induced superconductor quantum critical point and show that hybridization, i.e., pressure can induce a BCS-BEC crossover in multiband systems even for moderate interactions. We briefly discuss the influence of the symmetry of the order parameter in the results.
Jan 24
arXiv:1801.07376 [pdf, ps, other]
Title: Emergent interlayer nodal superfluidity of a dipolar fermi gas in bilayer optical lattices
Authors: Bo Liu, Peng Zhang, Ren Zhang, Hong Gao, Fuli Li
Comments: 5 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
Understanding the interplay between magnetism and superconductivity is one of the central issues in condensed matter physics. Such interplay induced nodal structure of superconducting gap is widely believed to be a signature of exotic pairing mechanism (not phonon mediated) to achieve unconventional superconductivity, such as in heavy fermion, high Tc , and organic superconductors. Here we report a new mechanism to drive the interplay between magnetism and superfluidity via the spatially anisotropic interaction. This scheme frees up the usual requirement of suppressing long-range magnetic order to access unconventional superconductivity like through doping or adding pressure in solids. Surprisingly, even for the half-filling case, such scheme can lead the coexistence of superfluidity and antiferromagnetism and interestingly an unexpected interlayer nodal superfluid emerges, which will be demonstrated through a cold atom system composed of a pseudospin-1/2 dipolar fermi gas in bilayer optical lattices. Our mechanism should pave an alternative way to unveil exotic pairing scheme resulting from the interplay between magnetism and superconductivity or superfluidity.
Jan 23
arXiv:1801.07031 [pdf, ps, other]
Title: Real-Time Dynamics of Typical and Untypical States in Non-Integrable Systems
Authors: Jonas Richter, Fengping Jin, Hans De Raedt, Kristel Michielsen, Jochen Gemmer, Robin Steinigeweg
Comments: 15 pages, 15 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph) For a class of typical states, the real-time and real-space dynamics of non-equilibrium density profiles has been recently studied for integrable models, i.e. the spin-1/2 XXZ chain [PRB 95, 035155 (2017)] and the Fermi-Hubbard chain [PRE 96, 020105 (2017)]. It has been found that the non-equilibrium dynamics agrees with linear response theory. Moreover, in the regime of strong interactions, clear signatures of diffusion have been observed. However, this diffusive behavior strongly depends on the choice of the initial state and disappears for untypical states without internal randomness. In the present work, we address the question whether or not the above findings persist for non-integrable models. As a first step, we study the spin-1/2 XXZ chain, where integrability can be broken due to an additional next-nearest neighbor interaction. Furthermore, we analyze the differences of typical and untypical initial states on the basis of their entanglement and their local density of states.
Jan 22
arXiv:1801.06256 [pdf, other]
Title: Quantum Landau damping in dipolar Bose-Einstein condensates
Authors: J. T. Mendonça, H. Terças, A. Gammal
Comments: Quantum fluctuations, LHY correction, Landau damping, roton minimum, dipolar Bose gases
Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph) We consider Landau damping of elementary excitations in Bose-Einstein condensates (BECs) with dipolar interactions. We discuss quantum and quasi-classical regimes of Landau damping. We use a generalized wave-kinetic description of BECs which, apart from the long range dipolar interactions, also takes into account the quantum fluctuations and the finite energy corrections to short-range interactions. Such a description is therefore more general than the usual mean field approximation. The present wave-kinetic approach is well suited for the study of kinetic effects in BECs, such as those associated with Landau damping, atom trapping and turbulent diffusion. The inclusion of quantum fluctuations and energy corrections change the dispersion relation and the damping rates, leading to possible experimental signatures of these effects. Quantum Landau damping is described with generality, and particular examples of dipole condensates in two and three dimensions are studied. The occurrence of roton-maxon configurations, and their relevance to Landau damping is also considered in detail, as well as the changes introduced by the three different processes, associated with dipolar interactions, quantum fluctuations and finite energy range collisions. The present approach is mainly based on a linear perturbative procedure, but the nonlinear regime of Landau damping, which includes atom trapping and atom diffusion, is also briefly discussed.
Jan 18
arXiv:1801.05658 (cross-list from physics.atom-ph) [pdf, other]
Accurate Determination of the Dynamical Polarizability of Dysprosium
C. Ravensbergen, V. Corre, E. Soave, M. Kreyer, S. Tzanova, E. Kirilov, R. Grimm
Comments: 4 pages and 4 figures
Subjects: Atomic Physics (physics.atom-ph); Quantum Gases (cond-mat.quant-gas)
We report a measurement of the dynamical polarizability of dysprosium atoms in their electronic ground state at the optical wavelength of 1064 nm, which is of particular interest for laser trapping experiments. Our method is based on collective oscillations in an optical dipole trap, and reaches unprecedented accuracy and precision by comparison with an alkali atom (potassium) as a reference species. We obtain values of 184.4(2.4) a.u. and 1.7(6) a.u. for the scalar and tensor polarizability, respectively. Our experiments have reached a level that permits meaningful tests of current theo- retical descriptions and provides valuable information for future experiments utilizing the intriguing properties of heavy lanthanide atoms.
Jan 16
arXiv:1801.05182 [pdf, other]
From Nodal Ring Topological Superfluids to Spiral Majorana Modes in Cold Atomic Systems
Wen-Yu He, Dong-Hui Xu, Benjamin T. Zhou, Qi Zhou, K. T. Law
Comments: 5 pages, 4 figures. Comments are welcome
Subjects: Quantum Gases (cond-mat.quant-gas)
In this work, we consider a 3D cubic optical lattice composed of coupled 1D wires with 1D spin-orbit coupling. When the s-wave pairing is induced through Feshbach resonance, the system becomes a topological superfluid with ring nodes, which are the ring nodal degeneracies in the bulk, and supports a large number of surface Majorana zero energy modes. The large number of surface Majorana modes remain at zero energy even in the presence of disorder due to the protection from a chiral symmetry. When the chiral symmetry is broken, the system becomes a Weyl topological superfluid with Majorana arcs. With 3D spin-orbit coupling, the Weyl superfluid becomes a novel gapless phase with spiral Majorana modes on the surface. The spatial resolved radio frequency spectroscopy is suggested to detect this novel nodal ring topological superfluid phase.
Jan 12
1. arXiv:1801.03782 [pdf, other]
16-qubit IBM universal quantum computer can be fully entangled
Yuanhao Wang, Ying Li, Zhang-qi Yin, Bei Zeng
Comments: 6 pages, 11 figures
Subjects: Quantum Physics (quant-ph)
Multipartite entanglement is an important evidence that a quantum device can potentially solve problems intractable for classical computers. In this paper, we report on genuine multipartite entanglement of up to 16 qubits on ibmqx5, a 16-qubit superconducting quantum processor accessible via IBM cloud. Connected graph states involving 8 to 16 qubits are prepared on ibmqx5 using low-depth circuits. We demonstrate that any pair of qubits connected on the graph are entangled, therefore for any cut on the graph two parts are entangled, i.e. the entanglement is genuine. Our results set a new record for the number of genuinely entangled qubits for both superconducting circuits and trapped ions systems.
2. arXiv:1801.03922 [pdf, other]
Quantum algorithm for simulating real time evolution of lattice Hamiltonians
Jeongwan Haah, Matthew B. Hastings, Robin Kothari, Guang Hao Low
Comments: 30 pages, 3 figures
Subjects: Quantum Physics (quant-ph)
We present a decomposition of the real time evolution operator e−iTH of any local Hamiltonian H on lattices Λ⊆ℤD into local unitaries based on Lieb-Robinson bounds. Combining this with recent quantum simulation algorithms for real time evolution, we find that the resulting quantum simulation algorithm has gate count (Tn polylog(Tn/ϵ)) and depth (T polylog(Tn/ϵ)), where n is the space volume or the number of qubits, T is the time of evolution, and ϵ is the accuracy of the simulation in operator norm. In contrast to this, the previous best quantum algorithms have gate count (Tn2 polylog(Tn/ϵ)). Our approach readily generalizes to time-dependent Hamiltonians as well, and yields an algorithm with similar gate count for any piecewise slowly varying time-dependent bounded local Hamiltonian. Finally, we also prove a matching lower bound on the gate count of such a simulation, showing that any quantum algorithm that can simulate a piecewise time-independent bounded local Hamiltonian in one dimension requires Ω(Tn/polylog(Tn)) gates in the worst case. In the appendix, we prove a Lieb-Robinson bound tailored to Hamiltonians with small commutators between local terms. Unlike previous Lieb-Robinson bounds, our version gives zero Lieb-Robinson velocity in the limit of commuting Hamiltonians. This improves the performance of our algorithm when the Hamiltonian is close to commuting.
Jan 11
1. arXiv:1801.03446 [pdf, ps, other]
Spin dynamics and Andreev-Bashkin effect in mixtures of one-dimensional Bose gases
Luca Parisi, G.E. Astrakharchik, Stefano Giorgini
Subjects: Quantum Gases (cond-mat.quant-gas)
We investigate the propagation of spin waves in two-component mixtures of one-dimensional Bose gases interacting through repulsive contact potentials. By using quantum Monte Carlo methods we calculate static ground-state properties, such as the spin susceptibility and the spin structure factor, as a function of both the intra-species and inter-species coupling strength and we determine the critical parameters for phase separation. In homogeneous mixtures, results of the velocity of spin waves and of its softening close to the critical point of phase separation are obtained by means of a sum-rule approach. We quantify the non-dissipative drag effect, resulting from the Andreev-Bashkin current-current interaction between the two components of the gas, and we show that in the regime of strong coupling it causes a significant suppression of the spin-wave velocity.
2. arXiv:1801.03118 (cross-list from cond-mat.stat-mech) [pdf, other]
A nonperturbative renormalization-group approach preserving the momentum dependence of correlation functions
Félix Rose, Nicolas Dupuis
Comments: 19 pages, 10 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th)
We present an approximation scheme of the nonperturbative renormalization group that preserves the momentum dependence of correlation functions. This approximation scheme can be seen as a simple improvement of the local potential approximation (LPA) where the derivative terms in the effective action are promoted to arbitrary momentum-dependent functions. As in the LPA the only field dependence comes from the effective potential, which allows us to solve the renormalization group equations at a relatively modest numerical cost (as compared, e.g., to the Blaizot--Mend\'ez-Galain--Wschebor approximation scheme). As an application we consider the two-dimensional quantum O(N) model at zero temperature. We discuss not only the two-point correlation function but also higher-order correlation functions such as the scalar susceptibility (which allows for an investigation of the "Higgs" amplitude mode) and the conductivity. In particular we show how, using Pad\'e approximants to perform the analytic continuation iωn→ω+i0+ of imaginary frequency correlation functions χ(iωn) computed numerically from the renormalization-group equations, one can obtain spectral functions in the real-frequency domain.
3. arXiv:1801.03101 (cross-list from cond-mat.str-el) [pdf, other]
Many-Body Spectral Reflection Symmetry and Protected Infinite-Temperature Degeneracy
Michael Schecter, Thomas Iadecola
Comments: 5 pages, 3 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
Protected zero modes in quantum physics traditionally arise in the context of ground states of many-body Hamiltonians. Here we study the case where zero-modes exist in the center of a reflection-symmetric many-body spectrum, giving rise to the notion of a protected "infinite-temperature" degeneracy. For a certain class of nonintegrable spin chains, we show that the number of zero modes is determined by a chiral index that grows exponentially with system size. We propose a dynamical protocol, feasible in ongoing experiments in Rydberg atom quantum simulators, to detect these many-body zero modes and their protecting spectral reflection symmetry.
Jan 10
1. arXiv:1801.03052 [pdf, other]
Superfluid drag in the two-component Bose-Hubbard model
Karl Sellin, Egor Babaev
Comments: 10 pages, 11 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
In multicomponent superfluids and superconductors, co- and counter-flows of components have in general different properties. It was discussed in 1975 by Andreev and Bashkin, in the context of He3/He4 superfluid mixtures, that inter-particle interactions produce a dissipationless drag. The drag can be understood as a superflow of one component induced by phase gradients of the other component. Importantly the drag can be both positive (entrainment) and negative (counter-flow). The effect is known to be of crucial importance for many properties of diverse physical systems ranging from the dynamics of neutron stars, rotational responses of Bose mixtures of ultra-cold atoms to magnetic responses of multicomponent superconductors. Although there exists a substantial literature that includes the drag interaction phenomenologically, much fewer regimes are covered by quantitative studies of the microscopic origin of the drag and its dependence on microscopic parameters. Here we study the microscopic origin and strength of the drag interaction in a quantum system of two-component bosons on a lattice with short-range interaction. By performing quantum Monte-Carlo simulations of a two-component Bose-Hubbard model we obtain dependencies of the drag strength on the boson-boson interactions and properties of the optical lattice. Of particular interest are the strongly-correlated regimes where the ratio of co-flow and counter-flow superfluid stiffnesses can diverge, corresponding to the case of saturated drag.
2. arXiv:1801.02639 [pdf, other]
Robust Fulde-Ferrell-Larkin-Ovchinikov superfluidity in laser-assisted bilayer Fermi gases
Qing Sun, Liang-Liang Wang, G. Juzeliūnas, An-Chun Ji
Comments: 8 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)
Inspired by a recent observation of the supersolidity for bosonic atoms in a laser-assisted superlattice potential [J. Li, et. al., Nature 543, 91 (2017)], we consider an attractive two-component degenerate Fermi gas in a bilayer structure with two atomic hyperfine ground states involved. We find that the synthetic spin-orbit coupling (SOC) induced by the laser-assisted interlayer tunneling has a dramatical effect both on the two-body and many-body physics for atomic fermions. A bound molecular state appears, and the molecular dispersion can contain two minima at finite momenta even for vanishing intra- and inter-layer detunings. This is responsible for the pairing instability of the underlying attractive Fermi gas. As a result, the ground state of the system is found to be a Fulde-Ferrell-Larkin-Ovchinikov (FFLO) superfluid with a spontaneous density-modulation of the order parameter. Due to the specific pairing mechanism in this system, the FFLO state is robust and appears in a wide parameter regime with the characteristic pairing momentum on the order of the SOC strength. The unique features of the two-body and many-body state can be observed in current experiments with ultracold atoms.
