The Question of Spontaneous Symmetry Breaking in Condensates

TL;DR

This paper reviews the debate on whether Bose-Einstein condensates involve spontaneous symmetry breaking, comparing theories and experimental perspectives, including fixed particle number treatments and measurement-induced coherence.

Contribution

It provides a comparative analysis of symmetry breaking theories in condensates and explores different viewpoints on their experimental and conceptual implications.

Findings

Symmetry breaking in condensates is controversial and debated.

Different theoretical approaches offer alternative interpretations.

Experimental evidence is interpreted variably depending on the viewpoint.

Abstract

The question of whether Bose-Einstein condensation involves spontaneous symmetry breaking is surprisingly controversial. We review the theory of spontaneous symmetry breaking in ferromagnets, compare it to the theory of symmetry breaking in condensates, and discuss the different viewpoints on the correspondence to experiments. These viewpoints include alternative perspectives in which we can treat condensates with fixed particle numbers, and where coherence arises from measurements. This question relates to whether condensates of quasiparticles such as polaritons can be viewed as "real" condensates.