Photon-number selective group delay in cavity induced transparency.

We show that the group velocity of a probe pulse in an ensemble of Λ-type atoms driven by a quantized cavity mode depends on the quantum state-of-the input probe pulse. In the strong-coupling regime of the atom-cavity system the probe group delay is photon-number selective. This can be used to spatially separate the single photon from higher photon-number components of a few-photon probe pulse and thus create a deterministic single-photon source.