Single-atom single-photon coupling facilitated by atomic-ensemble dark-state mechanisms

We propose to couple single atomic qubits to photons incident on a cavity containing an atomic ensemble of a different species that mediates the coupling via Rydberg interactions. Subject to a classical field and the cavity field, the ensemble forms a collective dark state which is resonant with the input photon, while excitation of a qubit atom leads to a secondary ``dark'' state that splits the cavity resonance. The two different dark-state mechanisms yield zero and π reflection phase shifts and can be used to implement quantum gates between atomic and optical qubits.