title: 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 rho(ss) can be represented by a weighted ensemble of pure states rho(ss) = [equation: see text] 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 /psi(t)>, stochastically evolving such that the proportion of time for which /psi(t)> = /psi(k)> equals Weierstrass p(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.
authors.affiliation: Centre for Quantum Dynamics, School of Science, Griffith University, Brisbane, Queensland 4111, Australia.
authors.forename: H M
authors.initials: HM
authors.lastname: Wiseman
authors.forename: J A
authors.initials: JA
authors.lastname: Vaccaro