Bell correlations at finite temperature

TL;DR

This paper demonstrates that certain spin systems with infinite-range interactions can exhibit Bell nonlocality at finite temperatures, with implications for understanding quantum correlations in many-body systems.

Contribution

It introduces a framework to identify Bell inequality violations in thermal states of spin systems and shows the low-energy spectrum can be approximated by a quantum harmonic oscillator.

Findings

Bell violations occur up to a finite critical temperature

Low-energy spectrum approximates a quantum harmonic oscillator

Spin-squeezed states optimize Bell correlations

Abstract

We show that spin systems with infinite-range interactions can violate at thermal equilibrium a multipartite Bell inequality, up to a finite critical temperature $T_c$. Our framework can be applied to a wide class of spin systems and Bell inequalities, to study whether nonlocality occurs naturally in quantum many-body systems close to the ground state. Moreover, we also show that the low-energy spectrum of the Bell operator associated to such systems can be well approximated by the one of a quantum harmonic oscillator, and that spin-squeezed states are optimal in displaying Bell correlations for such Bell inequalities.