Atom lasers, coherent states, and coherence: II. Maximally robust ensembles of pure states.

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Title Atom lasers, coherent states, and coherence: II. Maximally robust ensembles of pure states.
Author Wiseman, Howard Mark; Vaccaro, J.A.
Journal Name Physical Review A: Atomic, Molecular and Optical Physics
Year Published 2002
Place of publication USA
Publisher American Physical Society
Abstract s discussed in the preceding paper [Wiseman and Vaccaro, preceding paper, Phys. Rev. A 65, 043605 (2002)], the stationary state of an optical or atom laser far above threshold is a mixture of coherent field states with random phase, or, equivalently, a Poissonian mixture of number states. We are interested in which, if either, of these descriptions of ρss as a stationary ensemble of pure states, is more natural. In the preceding paper we concentrated upon the question of whether descriptions such as these are physically realizable (PR). In this paper we investigate another relevant aspect of these ensembles, their robustness. A robust ensemble is one for which the pure states that comprise it survive relatively unchanged for a long time under the system evolution. We determine numerically the most robust ensembles as a function of the parameters in the laser model: the self-energy χ of the bosons in the laser mode, and the excess phase noise ν. We find that these most robust ensembles are PR ensembles, or similar to PR ensembles, for all values of these parameters. In the ideal laser limit (ν=χ=0), the most robust states are coherent states. As the phase noise or phase dispersion is increased through ν or the self-interaction of the bosons χ, respectively, the most robust states become more and more amplitude squeezed. We find scaling laws for these states, and give analytical derivations for them. As the phase diffusion or dispersion becomes so large that the laser output is no longer quantum coherent, the most robust states become so squeezed that they cease to have a well-defined coherent amplitude. That is, the quantum coherence of the laser output is manifest in the most robust PR ensemble being an ensemble of states with a well-defined coherent amplitude. This lends support to our approach of regarding robust PR ensembles as the most natural description of the state of the laser mode. It also has interesting implications for atom lasers in particular, for which phase dispersion due to self-interactions is expected to be large.
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Copyright Statement Copyright 2002 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 link for access to the definitive, published version.
Volume 65
Page from 043606.1
Page to 043606.15
ISSN 1050-2947
Date Accessioned 2003-04-11
Language en_AU
Research Centre Centre for Quantum Dynamics
Faculty Faculty of Science
Subject PRE2009-Theoretical Physics
Publication Type Journal Articles (Refereed Article)
Publication Type Code c1

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