Classical nature of ordered quantum phases and origin of spontaneous symmetry breaking

TL;DR

This paper investigates the classical nature of ordered quantum phases and the role of spontaneous symmetry breaking, showing that maximally symmetry-breaking ground states are the most classical and globally ordered, with unique entanglement properties.

Contribution

It provides a quantitative analysis linking symmetry-breaking ground states to classicality and global order, demonstrating their unique entanglement and convertibility features.

Findings

Maximally symmetry-breaking ground states are the most classical.

These states are locally convertible from all other ground states.

They minimize macroscopic bipartite entanglement monogamy inequality.

Abstract

We analyse the nature of spontaneous symmetry breaking in complex quantum systems by investigating the long-standing conjecture that the maximally symmetry-breaking quantum ground states are the most classical ones corresponding to a globally ordered phase. We make this argument quantitatively precise by comparing different local and global indicators of classicality and quantumness, respectively in symmetry-breaking and symmetry-preserving quantum ground states. We first discuss how naively comparing local, pairwise entanglement and discord apparently leads to the opposite conclusion. Indeed, we show that in symmetry-preserving ground states the two-body entanglement captures only a modest portion of the total two-body quantum correlations, while, on the contrary, in maximally symmetry-breaking ground states it contributes the largest amount to the total two-body quantum correlations. We then put to test the conjecture by looking at the global, macroscopic correlation properties of quantum ground states. We prove that the ground states which realize the maximum breaking of the Hamiltonian symmetries, associated to a globally ordered phase, are the only ones that: I) are always locally convertible, i.e. can be obtained from all other ground states by only applying LOCC transformations (local operations and classical communication), while the reverse is never possible; II) minimize the monogamy inequality on the globally shared, macroscopic bipartite entanglement.