Quantum Memory with a Single Photon in a Cavity

The quantum information carried by a two-level atom was transferred to a high- $Q$ cavity and, after a delay, to another atom. We realized in this way a quantum memory made of a field in a superposition of 0 and 1 photon Fock states. We measured the “holding time” of this memory corresponding to the decay of the field intensity or amplitude at the single photon level. This experiment implements a step essential for quantum information processing operations.