Nonclassical excitation for atoms in a squeezed vacuum

Abstract

The recent development of frequency tunable nonclassical light sources have opened up possibilities for performing spectroscopy with nonclassical light. Various different avenues in this new field can be naturally organized in two groups: atomic measurements with the sensitivity beyond the standard quantum limit and fundamental alterations of atomic radiative processes due to nonclassical nature of the e.-m. field. While the number of theoretical predictions has been rapidly growing for the last decade the experimental progress has been achieved only recently and so far only in the first group: sub-shot-noise spectroscopy. We report here the first experiment that belongs to the second group: driving multilevel atoms with nonclassical light. More precisely we report the observation of the fundamental nonclassical behavior of a three level atom driven with squeezed vacuum via a two-photon excitation process. It is well known that the rate of the two-photon excitation Г₂ for thermal or coherent light is proportional to the square of the intensity of the excitation field (only weak excitation is considered here, i.e., no saturation effects are involved). By contrast, the manifestly quantum correlations of squeezed vacuum can enhance this rate such that it becomes a linear function of intensity. We present here the experimental observation of the rate which approaches this linear nonclassical behavior.

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