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Uspekhi Fizicheskih Nauk

"Jarring" of a quantum system and the corresponding stimulated transitions

Александр Михайлович Дыхне, Gennady L. Yudin

Published Jan 1, 1978• 66 citations
Publisher
DOI
10.3367/ufnr.0125.197807a.0377

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A systematic theory of sudden perturbations is derived for quantum systems whose states are described both by wave functions (a pure ensemble) and by a quantum density operator (a mixed ensemble). A perturbation series is written in powers of the parameter ωτ, which is small when the perturbation is sudden; ω is the typical eigenvalue of the unperturbed system; and τ is the characteristic collision time. When the perturbation (t), taken at different times, commutes with itself, the theory yields a compact analytic expression for the probabilities for stimulated transitions for any value of Vτ /. The results of many cross-section calculations for atomic collision processes are discussed from a common standpoint: the processes are treated as jarring processes which stimulate transitions in the quantum system. If a momentum δp is rapidly transferred to the system in a collision, regardless of the physical nature of the jarring, the probabilities for the stimulated transitions are governed by the parameter N ~δpδR/ where δR is a measure of the uncertainty in the coordinates which is due to the relatively slow motions in the unperturbed system.

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A systematic theory of sudden perturbations is derived for quantum systems whose states are described both by wave functions (a pure ensemble) and by a quantum density operator (a mixed ensemble). A perturbation series is written in powers of the parameter ωτ, which is small when the perturbation is sudden; ω is the typical eigenvalue of the unperturbed system; and τ is the characteristic collision time. When the perturbation (t), taken at different times, commutes with itself, the theory yields a compact analytic expression for the probabilities for stimulated transitions for any value of Vτ /. The results of many cross-section calculations for atomic collision processes are discussed from a common standpoint: the processes are treated as jarring processes which stimulate transitions in the quantum system. If a momentum δp is rapidly transferred to the system in a collision, regardless of the physical nature of the jarring, the probabilities for the stimulated transitions are governed by the parameter N ~δpδR/ where δR is a measure of the uncertainty in the coordinates which is due to the relatively slow motions in the unperturbed system.
Recently, a new decoding rule called jar decoding was proposed, under which the decoder first forms a set of suitable size, called a jar, consisting of sequences from the channel input alphabet considered to be closely related to yn, and then takes any codeword from the jar as the estimate of the transmitted codeword. In this paper, we show that under jar decoding, the analysis of low density parity check (LDPC) codes is much easier compared to maximum a posteriori (MAP) or maximum likelihood (ML) and Belief Propagation (BP) decoding, and new general LDPC coding theorems can be established. Specifically, it is proved that LDPC codes can approach the mutual information, with diminishing bit error probability, of any binary input memoryless channel with uniform input distribution when the average variable node degree is large. Moreover, simulation shows an interesting connection between jar decoding and BP decoding, i.e., BP decoding can be regarded as one of many ways to pick up a codeword from the jar for LDPC codes when it succeeds in outputting a codeword.