Inequivalence of Pure State Ensembles for Open Quantum Systems: The Preferred Ensembles are Those That Are Physically Realizable

Abstract

An open quantum system in steady state ρ-^ ss can be represented by a weighted ensemble of pure states ρ-^ ss = Σk℘k|ψk〉 〈ψk| in infinitely many ways. A physically realizable (PR) ensemble is one for which some continuous measurement of the environment will collapse the system into a pure state |ψ(t)〉, stochastically evolving such that the proportion of time for which |ψ(t)〉 = |ψk〉 equals ℘k. Some, but not all, ensembles are PR. This constitutes the preferred ensemble fact. We present the necessary and sufficient conditions for a given ensemble to be PR, and illustrate the method by showing that the coherent state ensemble is not PR for an atom laser.