Effective two-state behaviour of a collisionally perturbed Zeeman degenerate atomic transition and its application to optical bistability

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Abstract
In real atoms, levels connected by a dipole transition contain a number of degenerate Zeeman states. Thus in practice, the phenomena of optical pumping and collisional transfer amongst states severely restrict the application of a simple two-state model. There are a number of cases, however, where the macroscopic dipole induced in an atomic gas has the same general form as predicted by the two-state model, and these are presented here. The effects of optical pumping and collisional transfer, and the implications for the description of optical bistability, are discussed. In particular, ground-state optical pumping can, under certain conditions of low perturber density, considerably reduce the power necessary to achieve bistability.