Theory Seminar

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About the Theory Seminar

New Rules (since May 2012), Old rules

Date Seminars 2017   (Go to Seminars 2016, Seminars 2015, Seminars 2014, and earlier therein)

09.01.2017 Bipartite charge fluctuations in Z_2 topological insulators and superconductors

Invited speaker: Loïc Herviou (Ecole Polytechnique, ENS)

Bipartite charge fluctuations (BCF) have been introduced to provide an experimental indication of many-body entanglement.They are a very efficient and useful tool to characterize phase transitions in a large variety of charge-conserving models in one and two dimensions In this seminar, we study the BCF in generic one- and two-dimensional Z_2 (topological) models such as the Kitaev chain, spin-orbit insulators, the graphene and the Haldane model, where the charge we observe is no longer conserved. In one-dimension, we demonstrate that at phase transitions characterized by a linear dispersion, the BCF probe the change in a winding number that allows to pinpoint the transition and corresponds to the topological invariant for standard models. Additionally, we prove that a sub-dominant logarithmic contribution is still present at the exact critical point. Its quantized coefficient is universal and a characteristic of the critical model. In two dimensions, a similar structure appears. While the area term no longer reveal directly the phase transition, a subdominant logarithmic term is still present. Similarly to the entanglement entropy, it depends on the exact shape of the considered region, with contributions of the corner of the regions only.

21.02.2017 at 14:00 Dissipation induced topological states: A recipe

Invited speaker: Moshe Goldstein (Tel-Aviv University)

It has recently been realized that driven-dissipative dynamics, which usually tends to destroy subtle quantum interference and correlation effects, could actually be used as a resource. By proper engineering of the reservoirs and their couplings, one may drive a system towards a desired quantum-correlated steady state, even in the absence of internal Hamiltonian dynamics.

An intriguing class of quantum phases is characterized by topology, including the quantum Hall effect and topological insulators and superconductors. Which of these noninteracting topological states can be achieved as the result of purely dissipative Lindblad-type dynamics? Recent studies have only provided partial answers to this question.

In this talk I will present a general recipe for the creation, classification, and detection of states of the integer quantum Hall and 2D topological insulator type as the outcomes of coupling a system to reservoirs, and show how the recipe can be realized with ultracold atoms and other quantum simulators. The mixed states so created can be made arbitrarily close to pure states. I will discuss ways to extend this construction to other topological phases, including non-Gaussian ones, such as fractional quantum Hall state