Sub-Poissonian Photon Statistics in Resonance Fluorescence
When completely coherent light falls on a photoelectric detector, the number of photoelectric counts n registered in some finite time interval T obeys Poisson statistics. This is characterized by the fact that the variance ‹(∆n)2› of n equals the mean number ‹n›. When the radiation field is describable in terms of classical waves with a fluctuating light intensity, then ‹(∆n)2› in general exceeds ‹n›, as a consequence of the intensity fluctuations. However, within the framework of quantum electrodynamics there also exist quantum states of the electromagnetic field for which n is sub-Poissonian,or ‹(∆n)2› ‹ ‹n›. These states have no classical description, and the observation of sub-Poissonian statistics in a photoelectric counting experiment therefore represents evidence for the quantum nature of the field. We describe the results of such an experiment below.
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