Quantum Hertz entropy increase in a quenched spin chain

TL;DR

This paper investigates the behavior of quantum Hertz entropy in a quenched spin chain, confirming it never decreases and thus supports its role as a proper entropy measure in isolated quantum systems.

Contribution

It provides the first analysis of quantum Hertz entropy in a quenched spin chain, demonstrating its non-decreasing nature and relation to thermal equilibrium.

Findings

Quantum Hertz entropy does not decrease after a quench.

The system's distance from thermal equilibrium is quantified.

Closest equilibrium temperature is estimated.

Abstract

The classical Hertz entropy is the logarithm of the volume of phase space bounded by the constant energy surface; its quantum counterpart, the quantum Hertz entropy, is $\hat S = k_B \ln \hat N$, where the quantum operator $\hat N$ specifies the number of states with energy below a given energy eigenstate. It has been recently proved that, when an isolated quantum mechanical system is driven out of equilibrium by an external driving, the change in the expectation of its quantum Hertz entropy cannot be negative, and is null for adiabatic driving. This is in full agreement with the Clausius principle. Here we test the behavior of the expectation of the quantum Hertz entropy in the case when two identical XY spin chains initially at different temperatures are quenched into a single XY chain. We observed no quantum Hertz entropy decrease. This finding further supports the statement that the quantum Hertz entropy is a proper entropy for isolated quantum systems. We further quantify how far the quenched chain is from thermal equilibrium and the temperature of the closest equilibrium.