Quantum chaos and ensemble inequivalence of quantum long-range Ising chains

TL;DR

This study investigates quantum chaos and ensemble inequivalence in long-range quantum Ising chains, revealing that quantum chaos persists across all decay exponents, but ensemble inequivalence arises due to non-overlapping multiplets for certain interaction ranges.

Contribution

It provides a detailed numerical and analytical analysis of quantum chaos and ensemble inequivalence in long-range Ising chains, highlighting the role of multiplet structures for b1<1.

Findings

Quantum chaos indicated by Wigner-Dyson statistics for all b1>0

Nonconvex microcanonical entropy for b1<1 due to multiplet organization

Potential persistence of ensemble inequivalence at low energies for b1b1b1<1

Abstract

We use large-scale exact diagonalization to study the quantum Ising chain in a transverse field with long-range power-law interactions decaying with exponent $\alpha$. We numerically study various probes for quantum chaos and eigenstate thermalization {on} the level of eigenvalues and eigenstates. The level-spacing statistics yields a clear sign towards a Wigner-Dyson distribution and therefore towards quantum chaos across all values of $\alpha>0$. Yet, for $\alpha<1$ we find that the microcanonical entropy is nonconvex. This is due to the fact that the spectrum is organized in energetically separated multiplets for $\alpha<1$. While quantum chaotic behaviour develops within the individual multiplets, many multiplets don't overlap and don't mix with each other, as we analytically and numerically argue. Our findings suggest that a small fraction of the multiplets could persist at low energies for $\alpha\ll 1$ even for large $N$, giving rise to ensemble inequivalence.