  We consider the two-color photooassociation of a quantum degenerate atomic
gas into ground-state diatomic molecules via a molecular dark state. This
process can be described in terms of a lambda level scheme that is formally
analogous to the situation in electromagnetically-induced transparency (EIT) in
atomic systems, and therefore can result in slow light propagation. We show
that the group velocity of the light field depends explicitly on whether the
atoms are bosons or fermions, as well as on the existence or absence of a
pairing gap in the case of fermions, so that the measurement of the group
velocity realizes a non-destructive diagnosis of the atomic state and the
pairing gap.
