Nonvolatile optical memory via recoil-induced resonance in a pure   two-level system
  We report on the storage of light via the phenomenon of recoil-induced resonance in a pure two- level system of cold cesium atoms. We use a strong coupling beam and a weak probe beam to couple different external momentum states of the cesium atom via two-photon Raman interaction which leads to the storage of the optical information of the probe beam. We have also measured the probe transmission spectrum, as well as the light storage spectrum which reveals very narrow subnatural resonance features showing absorption and gain. We have demonstrated that this memory presents the unique property of being insensitive to the reading process, which does not destroy the stored information leading to a memory lifetime limited only by the atomic thermal motion. 
A. J. F. de Almeida, M.-A. Maynard, C. Banerjee, D. Felinto, F.   Goldfarb, and J. W. R. Tabosa
quant-ph