Phys. Rev. X 3, 041001 (2013)
Anatoly Svidzinsky and co-workers in the USA have proposed a new scheme for obtaining quantum gain without generating a population of excited-state atoms. Their scheme involves quantum amplification by superradiant emission of radiation (QASER). The researchers say that the approach holds promise for amplification of coherent extreme-ultraviolet radiation and is many orders of magnitude more efficient than nonlinear multiphoton processes. The team considered a medium composed of two-energy-level atoms and analysed the evolution of a superradiant pulse at the atomic transition frequency, which is periodically modulated by a coherent driving field. They found that amplification by the QASER is possible if one eliminates the time-independent Stark shift of the atomic transition frequency caused by the driving field. This is achievable by using a nearly circularly polarized driving field, or, more generally, by applying an additional magnetic field. The amplification mechanism of QASER is governed by the difference combination parametric resonance that occurs when the drive-field frequency matches the frequency difference between two close, high-frequency normal modes of the coupled light–atom system. Such a superradiant resonance holds promise for the development of a new class of radiation sources that generate high-frequency coherent light by utilizing a low-frequency coherent source.
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