Arxiv Selection Nov 2019
Nov 1-Nov 7 Zehan Li, Nov 8- Nov 14 Jiansong Pan, Nov 15- Nov 21 Ahmet Keles, Nov 22 - Nov 28 Haiping Hu
Nov 13
arXiv:1911.04858 [pdf, other]
Observation of two-dimensional Anderson localisation of ultracold atoms
Donald H. White, Thomas A. Haase, Dylan J. Brown, Maarten D. Hoogerland, Mojdeh S. Najafabadi, John L. Helm, Christopher Gies, Daniel Schumayer, David A. W. Hutchinson
Comments: 10 pages, 7 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Physics (quant-ph)
Anderson localisation -- the inhibition of wave propagation in disordered media -- is a surprising interference phenomenon which is particularly intriguing in two-dimensional (2D) systems. While an ideal, non-interacting 2D system of infinite size is always localised, the localisation length-scale may be too large to be unmistakably observed in an experiment. In this sense, 2D is a marginal dimension between one-dimension where all states are strongly localised, and three-dimensions where a well-defined localisation-delocalisation phase transition exists. Motivated by the goal of observing and closely studying the quantum interference leading to Anderson localisation in a 2D quantum system, we design a transmission experiment in which ultracold atoms propagate through a custom-shaped disordered channel connecting two reservoirs. This design overcomes many of the technical challenges that have hampered observation in previous works. Our precise control of disorder allows us to tune the localisation length to be shorter than the system size. We observe exponential localisation and demonstrate the presence of strong localisation in a 2D ultracold atom system.
arXiv:1911.05057 (cross-list from cond-mat.mes-hall) [pdf, other]
Designing effective lattices for two-body bound states: From interaction-induced flat bands to higher-order topological insulators
Grazia Salerno, Giandomenico Palumbo, Nathan Goldman, Marco Di Liberto
Comments: 8 pages, 6 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas)
Bound states of two interacting particles moving on a lattice can exhibit remarkable features that are not captured by the underlying single-particle picture. Inspired by this phenomenon, we introduce a novel framework by which genuine interaction-induced geometric and topological effects can be realized in quantum-engineered systems. Our approach builds on the design of effective lattices for the center-of-mass motion of two-body bound states, which can be created through long-range interactions. This general scenario is illustrated on several examples, where flat-band localization, topological pumps and higher-order topological corner modes emerge from genuine interaction effects. Our results pave the way for the exploration of interaction-induced topological effects in a variety of platforms, ranging from ultracold gases to interacting photonic devices.
Nov 12
arXiv:1911.04409 [pdf, other]
Anomalous transport in a topological Wannier-Stark ladder
Kun Woo Kim, Alexei Andreanov, Sergej Flach
Comments: 8 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Other Condensed Matter (cond-mat.other)
A dc (e.g. electric) field with commensurate lattice direction turns a single particle band structure in d=3 dimensions into an infinite set of equally spaced irreducible (d−1)=2-dimensional Wannier-Stark (WS) band structures that are spatially localized along the field direction. Particle transport is expected to be suppressed once the WS bands are gapped in energy. The topological character of the irreducible band structure leads to one-dimensional sets of boundary states which fill the energy gaps. As a result, eigenmodes are smoothly connected in energy and space and yield anomalous particle transport throughout the ladder. The number of chiral boundary modes can be tuned by the dc field strength and manifests through the distribution of dissipated energy and spatial motion, and the temperature dependence of angular momentum carried by particles.
arXiv:1911.04149 [pdf, other]
Fractional corner charges in a 2D super-lattice Bose-Hubbard model
Julian Bibo, Izabella Lovas, Yizhi You, Fabian Grusdt, Frank Pollmann
Subjects: Quantum Gases (cond-mat.quant-gas)
We study a two dimensional super-lattice Bose-Hubbard model with alternating hoppings in the limit of strong on-site interactions. We evaluate the phase diagram of the model around half-filling using the density matrix renormalization group method and find two gapped phases separated by a gapless superfluid region. We demonstrate that the gapped states realize two distinct higher order symmetry protected topological phases, which are protected by a combination of charge conservation and C4 lattice symmetry. The phases are distinguished in terms of a quantized fractional corner charge and a many-body topological invariant that is robust against arbitrary, symmetry preserving edge manipulations. We support our claims by numerically studying the full counting statistics of the corner charge, finding a sharp distribution peaked around the quantized values. These results are experimentally observable in ultracold atomic settings using state of the art quantum gas microscopy.
Nov 11
arXiv:1911.02774 (cross-list from cond-mat.dis-nn) [pdf, ps, other]
Neural-network quantum states at finite temperature
Naoki Irikura, Hiroki Saito
Comments: 6 pages, 3 figures
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We propose a method to obtain the thermal-equilibrium density matrix of a many-body quantum system using artificial neural networks. The variational function of the many-body density matrix is represented by a convolutional neural network with two input channels. We first prepare an infinite-temperature state, and the temperature is lowered by imaginary-time evolution. We apply this method to the one-dimensional Bose-Hubbard model and compare the results with those obtained by exact diagonalization.
arXiv:1911.02575 (cross-list from cond-mat.stat-mech) [pdf, other]
Entanglement and classical fluctuations at finite-temperature critical points
Sascha Wald, Raul Arias, Vincenzo Alba
Comments: 28 pages, 19 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 investigate several entanglement-related quantities at finite-temperature criticality in the three-dimensional quantum spherical model, both as a function of temperature T and of the quantum parameter g, which measures the strength of quantum fluctuations. While the von Neumann and the Rényi entropies exhibit the volume-law for any g and T, the mutual information obeys an area law. The prefactors of the volume-law and of the area-law are regular across the transition, reflecting that universal singular terms vanish at the transition. This implies that the mutual information is dominated by nonuniversal contributions. This hampers its use as a witness of criticality, at least in the spherical model. We also study the logarithmic negativity. For any value of g,T, the negativity exhibits an area-law. The negativity vanishes deep in the paramagnetic phase, it is larger at small temperature, and it decreases upon increasing the temperature. For any g, it exhibits the so-called sudden death, i.e., it is exactly zero for large enough T. The vanishing of the negativity defines a "death line", which we characterise analytically at small g. Importantly, for any finite T the area-law prefactor is regular across the transition, whereas it develops a cusp-like singularity in the limit T→0. Finally, we consider the single-particle entanglement and negativity spectra. The vast majority of the levels are regular across the transition. Only the larger ones exhibit singularities. These are related to the presence of a zero mode, which reflects the symmetry breaking. This implies the presence of sub-leading singular terms in the entanglement entropies. Interestingly, since the larger levels do not contribute to the negativity, sub-leading singular corrections are expected to be suppressed for the negativity.
Nov 7
arXiv:1911.02384 [pdf, other]
Quantum droplets in a dipolar Bose gas at a dimensional crossover
Pawel Zin, Maciej Pylak, Tomasz Wasak, Krzysztof Jachymski, Zbigniew Idziaszek
Subjects: Quantum Gases (cond-mat.quant-gas)
We study the beyond-mean-field corrections to the energy of a dipolar Bose gas confined to two dimensions by a box potential with dipoles oriented in plane. At a critical strength of the dipolar interaction the system becomes unstable on the mean field level. We find that the ground state of the gas is strongly influenced by the corrections, leading to formation of a self-bound droplet, in analogy to the free space case. Properties of the droplet state can be found by minimizing the extended Gross-Pitaevskii energy functional. In the limit of strong confinement we show analytically that the correction can be interpreted as an effective three-body repulsion which stabilizes the gas at finite density.
arXiv:1911.02211 (cross-list from cond-mat.mes-hall) [pdf, other]
Dynamical topology of quantum quenches in two dimensions
Haiping Hu, Erhai Zhao
Comments: 6+5 pages, 3+1 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
The quench dynamics following a sudden change in the Hamiltonian of a quantum system can be very complex. For band insulators, the global properties of a quantum quench may be captured by its topological invariants over the space-time continuum, which are intimately related to the static band topology of the pre- and post-quench Hamiltonian. We introduce the concept of loop unitary Ul and its homotopy invariant W3 to fully characterize the quench dynamics of arbitrary two-band insulators in two dimensions, going beyond existing scheme based on Hopf invariant which is only valid for trivial initial states. The theory traces the origin of nontrivial dynamical topology to the emergence of π-defects in the phase band of Ul, and establishes that W3=Cf−Ci, i.e. the Chern number change across the quench. We further show that the dynamical singularity is also encoded in the winding of the eigenvectors of Ul along a lower dimensional curve where dynamical quantum phase transition occurs, if the pre- or post-quench Hamiltonian is trivial. The winding along this curve is related to the Hopf link, and gives rise to torus links and knots for quench to Hamiltonians with Dirac points. This framework can be generalized to multiband systems and other dimensions.
Nov 5
arXiv:1911.01345 [pdf, other]
High-precision numerical solution of the Fermi polaron problem and large-order behavior of its diagrammatic series
K. Van Houcke, F. Werner, R. Rossi
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We introduce a simple determinant diagrammatic Monte Carlo algorithm to compute the groundstate properties of a particle interacting with a Fermi sea through a zero-range interaction. The fermionic sign does not cause any fundamental problem when going to high diagram orders, and we reach order N = 30. The data reveal that the diagrammatic series diverges exponentially as (−1/R) N with a radius of convergence R < 1. Furthermore, on the polaron side of the polarondimeron transition, the value of R is determined by a special class of three-body diagrams, corresponding to repeated scattering of the impurity between two particles of the Fermi sea. A powercounting argument explains why finite R is possible for zero-range interactions in three dimensions. Resumming the divergent series through a conformal mapping yields the polaron energy with record accuracy
Nov 4
arXiv:1911.00003 (cross-list from quant-ph) [pdf, other]
Simulating Lattice Gauge Theories within Quantum Technologies
M.C. Bañuls, R. Blatt, J. Catani, A. Celi, J.I. Cirac, M. Dalmonte, L. Fallani, K. Jansen, M. Lewenstein, S. Montangero, C.A. Muschik, B. Reznik, E. Rico, L. Tagliacozzo, K. Van Acoleyen, F. Verstraete, U.-J. Wiese, M. Wingate, J. Zakrzewski, P. Zoller
Comments: 45 pages, 38 figures, review article
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); High Energy Physics - Lattice (hep-lat); High Energy Physics - Theory (hep-th)
Lattice gauge theories, which originated from particle physics in the context of Quantum Chromodynamics (QCD), provide an important intellectual stimulus to further develop quantum information technologies. While one long-term goal is the reliable quantum simulation of currently intractable aspects of QCD itself, lattice gauge theories also play an important role in condensed matter physics and in quantum information science. In this way, lattice gauge theories provide both motivation and a framework for interdisciplinary research towards the development of special purpose digital and analog quantum simulators, and ultimately of scalable universal quantum computers. In this manuscript, recent results and new tools from a quantum science approach to study lattice gauge theories are reviewed. Two new complementary approaches are discussed: first, tensor network methods are presented - a classical simulation approach - applied to the study of lattice gauge theories together with some results on Abelian and non-Abelian lattice gauge theories. Then, recent proposals for the implementation of lattice gauge theory quantum simulators in different quantum hardware are reported, e.g., trapped ions, Rydberg atoms, and superconducting circuits. Finally, the first proof-of-principle trapped ions experimental quantum simulations of the Schwinger model are reviewed.
Nov 1
arXiv:1910.14606 (cross-list from cond-mat.mes-hall) [pdf, other]
Non-Hermitian topological phase transitions for quantum spin Hall insulators
Junpeng Hou, Ya-Jie Wu, Chuanwei Zhang
Comments: 10 pages, 9 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Optics (physics.optics)
The interplay between non-Hermiticity and topology opens an exciting avenue for engineering novel topological matter with unprecedented properties. While previous studies have mainly focused on one-dimensional systems or Chern insulators, here we investigate topological phase transitions to/from quantum spin Hall (QSH) insulators driven by non-Hermiticity. We show that a trivial to QSH insulator phase transition can be induced by solely varying non-Hermitian terms, and there exists exceptional edge arcs in QSH phases. We establish two topological invariants for characterizing the non-Hermitian phase transitions: i) with time-reversal symmetry, the biorthogonal Z2 invariant based on non-Hermitian Wilson loops, and ii) without time-reversal symmetry, a biorthogonal spin Chern number through biorthogonal decompositions of the Bloch bundle of the occupied bands. These topological invariants can be applied to a wide class of non-Hermitian topological phases beyond Chern classes, and provides a powerful tool for exploring novel non-Hermitian topological matter and their device applications.
