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

Wiseman, H. M. and Vaccaro, J. A. (2002) Atom lasers, coherent states, and coherence II. Maximally robust ensembles of pure states. Physical Review A, 65 4: 043606. doi:10.1103/PhysRevA.65.043606

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Author Wiseman, H. M.
Vaccaro, J. A.
Title Atom lasers, coherent states, and coherence II. Maximally robust ensembles of pure states
Journal name Physical Review A   Check publisher's open access policy
ISSN 1050-2947
Publication date 2002
Sub-type Article (original research)
DOI 10.1103/PhysRevA.65.043606
Open Access Status File (Publisher version)
Volume 65
Issue 4
Start page 043606
Total pages 15
Editor B. Crasemann
Place of publication United States
Publisher American Physical Society
Collection year 2002
Language eng
Subject C1
240201 Theoretical Physics
780102 Physical sciences
Abstract As 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 rho(ss) 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 chi of the bosons in the laser mode, and the excess phase noise nu. 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 (nu=chi=0), the most robust states are coherent states. As the phase noise or phase dispersion is increased through nu or the self-interaction of the bosons chi, 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.
Keyword Optics
Physics, Atomic, Molecular & Chemical
Q-Index Code C1

Document type: Journal Article
Sub-type: Article (original research)
Collections: Excellence in Research Australia (ERA) - Collection
School of Mathematics and Physics
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Created: Tue, 14 Aug 2007, 17:15:09 EST