Non-classicalities in quantum many-body systems out of equilibrium
* SPEAKERS
Name
Affiliation
E-mail
Chae-Yeun Park
Seoul National University
kaeri17(at)gmail.com
* HOST(Applicant)
Name
Affiliation
E-mail
Jaeyoon Cho
APCTP
jaeyoon.cho(at)apctp.org
* DATE / TIME
2017-02-27, 16:00
* PLACE
Seminar Room #512, APCTP HQ (Hokil Kim Memorial Bldg., POSTECH)
* ABSTRACT
The entropy of entanglement, which measures entanglement in bipartite settings, have been widely used to investigate many-body spin systems both in and out of equilibrium. Especially, the dynamics of entanglement entropy of a subsystem has been used to examine whether a many-body system thermalizes or not. When a system thermalizes, an expectation value of (local) observables approaches to the thermal equilibrate value and entanglement entropy increases linearly with time. In contrast, for a many-body localized system, which does not thermalize, entanglement entropy increases logarithmically with time. Although, after a sufficiently long time evolution, saturated values of entanglement entropy increases with the size of the system (volume law) both for thermalizing and many-body localized systems. This extensive entanglement usually accepted as an indication of a classicality as the entropy of a subsystem resembles that of a thermal ensemble. Nonetheless, at the same time, one may ask that it is also an evidence of a non-classicality because entanglement quantifies quantum non-locality.
In this talk, using the suitable assumptions of a classical measurement, we explicitly show that a large value of entanglement entropy does not necessarily mean non-locality. Rather, non-locality by the classical measurements is more related to a well-established measure of macroscopic superpositions, which is usually called quantum macroscopicity. Using the eigenstate thermalization hypothesis and central limit theorem, we show that quantum macroscopicity is small after thermalization for general initial states which means locality of a quantum state. On the other hands, local integrals of motion which arise in a many-body localized system may preserve the initial values of quantum macroscopicity. Using these results, we numerically show that quantum macroscopicity can give discriminating values for thermalizing and many-body localized systems even for equally large values of entanglement. Our study, therefore, shows that quantum macroscopicity can be another meaningful measure for investigating many-body systems.
