Nov 2016
Nov 1-Nov 5 Max, Nov 8-Nov 12 Biao Huang, Nov 15-Nov 19 Haiyuan Zou, Nov 22-Nov 26 Ahmet Kel
Nov 19 arXiv:1611.05450 (cross-list from quant-ph) [pdf, other] Symmetry protected topological order at nonzero temperature Sam Roberts, Beni Yoshida, Aleksander Kubica, Stephen D. Bartlett Comments: 42 pages, 10 figures, comments welcome Subjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el) We address the question of whether symmetry-protected topological (SPT) order can persist at nonzero temperature, with a focus on understanding the thermal stability of several models studied in the theory of quantum computation. We present three results in this direction. First, we prove that nontrivial SPT order protected by a global on-site symmetry cannot persist at nonzero temperature, demonstrating that several quantum computational structures protected by such on-site symmetries are not thermally stable. Second, we prove that the 3D cluster state model used in the formulation of topological measurement-based quantum computation possesses a nontrivial SPT-ordered thermal phase when protected by a global generalized (1-form) symmetry. The SPT order in this model is detected by long-range localizable entanglement in the thermal state, which compares with related results characterizing SPT order at zero temperature in spin chains using localizable entanglement as an order parameter. Our third result is to demonstrate that the high error tolerance of this 3D cluster state model for quantum computation, even without a protecting symmetry, can be understood as an application of quantum error correction to effectively enforce a 1-form symmetry. Nov 17 arXiv:1611.05016 [pdf, other] Probing the conformal Calabrese-Cardy scaling with cold atoms J. Unmuth-Yockey, Jin Zhang, P.M. Preiss, Li-Ping Yang, S.-W. Tsai, Y. Meurice Comments: 5 pages, 4 figures, uses revote Subjects: Quantum Gases (cond-mat.quant-gas); High Energy Physics - Lattice (hep-lat); Quantum Physics (quant-ph) We demonstrate that current experiments using cold bosonic atoms trapped in one-dimensional optical lattices and designed to measure the second-order Renyi entanglement entropy S_2, can be used to verify detailed predictions of conformal field theory (CFT) and estimate the central charge c. We discuss the adiabatic preparation of the ground state at half-filling where we expect a CFT with c=1. This can be accomplished with a very small hoping parameter J, in contrast to existing studies with density one where a much larger J is needed. We provide two complementary methods to estimate and subtract the classical entropy generated by the experimental preparation and imaging processes. We compare numerical calculations for the classical O(2) model with a chemical potential on a 1+1 dimensional lattice, and the quantum Bose-Hubbard Hamiltonian implemented in the experiments. S_2 is very similar for the two models and follows closely the Calabrese-Cardy scaling, (c/8)\ln(N_s), for N_s sites with open boundary conditions, provided that the large subleading corrections are taken into account. Nov 15 arXiv:1611.03611 [pdf, other] Density correlations and transport in models of many-body localization P. Prelovšek, M. Mierzejewski, O. Barišić, J. Herbrych Comments: Submitted as "Feature Article" to the special issue of Annalen der Physik: "Many-Body Localization" Subjects: Strongly Correlated Electrons (cond-mat.str-el) We present a review of recent theoretical results concerning the many-body localization (MBL) phenomenon, with the emphasis on dynamical density correlations and transport quantities. They are shown to be closely related, providing a comprehensive description of the ergodic-to-nonergodic transition, consistent with experimental findings. While the focus is set mostly on the one-dimensional model of interacting spinless fermions, we also present evidence for the absence of full MBL in the one-dimensional Hubbard model and for the density-wave decay induced by the interchain coupling. Nov. 10 arXiv:1611.03304 [pdf, other] Measuring Topological Number of a Chern-Insulator from Quench Dynamics Ce Wang, Pengfei Zhang, Xin Chen, Jinlong Yu, Hui Zhai Comments: 5 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el) In this letter we address the issue that whether the topological number of a static Hamiltonian can be measured from a dynamical quench process. We focus on a two-band Chern insulator in two-dimension, for instance, the Haldane model, whose quantum state is a pseudo-spin on the Bloch sphere. The dynamical process is described by a mapping from the [kx,ky,t] space to the Bloch sphere, characterized by the Hopf invariant. We show that, taking any two constant vectors on the Bloch sphere, the linking number of their inverse images of this mapping (i.e. two trajectories in the [kx,ky,t] space) exactly equals to the Chern number of the static Hamiltonian. Applying this result to a recent experiment from Hamburg group, we show that it is not the appearance of the phase vortices, but the linking number of the trajectories of these phase vortices that determines the phase boundary of the static Hamiltonian.
