Spontaneous Symmetry Breaking via Measurement: From Bose-Einstein Condensates to Josephson Effect

TL;DR

This paper proposes that quantum measurement can induce spontaneous symmetry breaking, explaining phenomena like Bose-Einstein condensate interference and the Josephson effect without assuming initial symmetry-breaking states.

Contribution

It introduces a measurement-based framework for understanding spontaneous symmetry breaking, applying the projection postulate to explain key quantum phenomena.

Findings

Measurement can select symmetry-breaking states in quantum systems.

The approach accurately describes Bose-Einstein condensate interference.

Superconducting states with definite phase are obtained via measurement of current.

Abstract

Why does spontaneous symmetry breaking occur? Why is a state breaking symmetry realized? We explore an idea that measurement selects such a state even if a system is given in a state respecting the symmetry of the system. We point out that the spectrum of the relevant observable is important, and simply apply the projection postulate for quantum measurement. We first show that this approach correctly describes the well-known interference of Bose-Einstein condensates. We then examine a fermionic system and prove that superconducting states with a definite relative phase are selected by the measurement of the current flowing between two superconductors, eliminating the need to assume the presence of an a priori phase to explain the Josephson effect.