  We show that the group velocity of a probe pulse in an ensemble of
$\Lambda$-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 to create a deterministic
single-photon source.
