Typical and extreme entropies of long-lived isolated quantum systems

TL;DR

This paper compares bipartite entanglement entropy and observational entropy in long-lived isolated quantum systems, revealing their different behaviors and fluctuations in a one-dimensional lattice model.

Contribution

It provides a detailed numerical analysis of both entropies, highlighting fundamental differences and their dependence on system parameters in quantum dynamics.

Findings

Observational entropy aligns with classical Boltzmann entropy behavior.

Entanglement entropy shows distinct minimal and maximal configurations.

Both entropies exhibit characteristic fluctuations depending on temperature, particle number, and size.

Abstract

In this paper, we investigate and compare two well-developed definitions of entropy relevant for describing the dynamics of isolated quantum systems: bipartite entanglement entropy and observational entropy. In a model system of interacting particles in a one-dimensional lattice, we numerically solve for the full quantum behavior of the system. We characterize the fluctuations, and find the maximal, minimal, and typical entropy of each type that the system can eventually attain through its evolution. While both entropies are low for some "special" configurations and high for more "generic" ones, there are several fundamental differences in their behavior. Observational entropy behaves in accord with classical Boltzmann entropy (e.g. equilibrium is a condition of near-maximal entropy and uniformly distributed particles, and minimal entropy is a very compact configuration). Entanglement entropy is rather different: minimal entropy "empties out" one partition while maximal entropy apportions the particles between the partitions, and neither is typical. Beyond these qualitative results, we characterize both entropies and their fluctuations in some detail as they depend on temperature, particle number, and box size.