Quantum coarse-grained entropy and thermalization in closed systems

Abstract

We investigate the detailed properties of Observational entropy, introduced by Šafránek et al. [Phys. Rev. A 99, 010101 (2019)] as a generalization of Boltzmann entropy to quantum mechanics. This quantity can involve multiple coarse-grainings, even those that do not commute with each other, without losing any of its properties. It is well-defined out of equilibrium, and for some coarse-grainings it generically rises to the correct thermodynamic value even in a genuinely isolated quantum system. The quantity contains several other entropy definitions as special cases, it has interesting information-theoretic interpretations, and mathematical properties -such as extensivity and upper and lower bounds -suitable for an entropy. Here we describe and provide proofs for many of its properties, discuss its interpretation and connection to other quantities, and provide numerous simulations and analytic arguments supporting the claims of its relationship to thermodynamic entropy. This quantity may thus provide a clear and well-defined foundation on which to build a satisfactory understanding of the second thermodynamical law in quantum mechanics.

Eigenstate thermalization hypothesis

“…To understand how this can increase in time, degrees of freedom are often coarse grained. It has been recently been shown [88,89] that this idea of coarse graining can be extended to quant um mechanics by making sequential observations of different observables. For example, one can observe coarse grained positional degrees of freedom and then energy.…”