Inequivalence of Pure State Ensembles for Open Quantum Systems: The Preferred Ensembles are Those That Are Physically Realizable
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An open quantum system in steady state ?-^ ss can be represented by a weighted ensemble of pure states ?-^ ss = SkPk|?k> <?k| 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 |?(t)>, stochastically evolving such that the proportion of time for which |?(t)> = |?k> equals Pk. 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.
Physical Review Letters
Copyright 2001 American Physical Society. This is the author-manuscript version of this paper. Reproduced in accordance with the copyright policy of the publisher. Please refer to the journal's website for access to the definitive, published version.