  A sequence of single photons is emitted on demand from a single three-level
atom strongly coupled to a high-finesse optical cavity. The photons are
generated by an adiabatically driven stimulated Raman transition between two
atomic ground states, with the vacuum field of the cavity stimulating one
branch of the transition, and laser pulses deterministically driving the other
branch. This process is unitary and therefore intrinsically reversible, which
is essential for quantum communication and networking, and the photons should
be appropriate for all-optical quantum information processing.
