Spontaneous Symmetry Breaking via Measurement: From Bose-Einstein   Condensates to Josephson Effect
  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. 
Takaaki Monnai, Mauro Iazzi, Kazuya Yuasa
quant-ph cond-mat.quant-gas cond-mat.stat-mech cond-mat.supr-con