Observation of spontaneous coherence grating transfer in cold cesium atoms

We report on the direct observation of spontaneous coherence grating transfer between different pairs of Zeeman sublevels belonging to different cesium hyperfine states. Coherent Bragg diffraction is employed as a signature of the transfer mechanism; this spectrum shows a subnatural linewidth. Theoretically we model the observed effect using a tensorial density matrix formalism to describe the light interaction with a pair of degenerate two-level systems coupled by spontaneous emission. The possibility of transferring a short-lived atomic coherence into a long-lived one might be of considerable importance for the growing field of quantum-information processing.