Lasing without inversion experiments and theory
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Quantum coherence and correlations in atomic systems have many interesting and potentially useful consequences, such as cancellation of absorption, lasing without inversion, and enhancement of the index of refraction without absorption. One of the oldest demonstrations of the importance atomic coherence in radiative processes is the Hanle effect. Other historically important investigations of coherence effects in atom-field interactions involved population trapping. A main thrust of today's efforts in this field is focused on lasing without inversion (LWI). Several groups around the world are presently working on different possible LWI schemes. The approach of Harris et al. involves lasing through an upper-level doublet that is autoionizing or through an upper-dressed-state doublet originating from coupling with some auxiliary level. The approach chosen by Kocharovskaya et al. is based on an initially prepared coherence in a lower-level doublet. The work of Scully and co-workers dealt first with a scheme driven by a microwave field or a double- scheme utilizing Raman-like physics. Several groups have reported the experimental observation of LWI to date. Fry et al. used transitions in the sodium D1 manifold to demonstrate LWI, Nottelmann et al. were able to show the effect in samarium, and van der Veer et al. reported LWI in cadmium. The extent to which an earlier experiment by Gao et al. demonstrates LWI remains a source of discussion. It has been argued that in order to achieve LWI it is always necessary to have a hidden inversion in some dressed-state basis. To show that this is not true in general, we consider the special case of the double- scheme. The effects mentioned so far are all explainable in terms of a semiclassical theory in which the atoms are quantized but the field is treated classically. By using a fully quantized theory, several authors have predicted quantum-noise quenching, or even squeezing, in systems with atomic coherences. As a consequence, lasers without inversion may have subnatural line-widths, a highly useful feature for spectroscopy and interferometry.
