Quantum Disorder in Macroscopic Systems of Interacting Atoms and Radiation Fields
The linear interaction between a system of two-level atoms and an electromagnetic field can be described as taking place through a number of elementary acts in which photons are absorbed or emitted, while atoms change their states. It is conceivable that these processes tend to modify the original statistical properties characteristic of the atomic system and of the electromagnetic field at t = 0, when we assume that the interaction is “turned on”. The problem is of conceptual importance, and might become of practical importance in connection with laser processes in unusual ranges of frequency. In fact, it has recently received increasing attention in the case of one-photon interactions  and also for large times, where the interest has been focused either mainly on the one-atom case [2–4] or on the many-atom case [5,6]. In all cases in which at t = 0 the atomic system has been taken to be in its unperturbed ground state and the electromagnetic field in a coherent state , the z-component of the total dipole atomic moment has been found to vanish at large times. As was noticed early , in the absence of relaxation times of any sort, this would seem to suggest caution in the use of the correspondence principle for large N, since a classical angular momentum behaves quite differently, exhibiting no damping in its z-component. Moreover, the initial coherence of the field has been shown to decrease with time during the interaction by calculating the appropriate correlation functions  or the variances of the field amplitude [5,6].
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