Storage and retrieval of coherent optical information in atomic populations

We propose a simple and powerful protocol to map an arbitrary atomic coherence between two quantum states into a population distribution of three metastable states, and later to retrieve the atomic coherence from the population distribution. The protocol applies simple sequences of radiation pulses with arbitrary temporal profile, either as coincident or as consecutive pulses. Mapping of rather short-lived atomic coherences into very long-lived atomic populations permits the prolongation of storage times (e.g. of optical information encoded in atomic coherences) by many orders of magnitude — without the need for complicated techniques to reduce homogeneous broadenings.