Generating and probing a two-photon Fock state with a single atom in a cavity

A two-photon Fock state is prepared in a cavity sustaining a "source mode " and a "target mode", with a single circular Rydberg atom. In a third-order Raman process, the atom emits a photon in the target while scattering one photon from the source into the target. The final two-photon state is probed by measuring by Ramsey interferometry the cavity light shifts induced by the target field on the same atom. Extensions to other multi-photon processes and to a new type of micromaser are briefly discussed.