  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 $\pi$ reflection phase shifts and can be used to implement quantum gates
between atomic and optical qubits.
