An optical switch and optical storage loop are used as the basis of a
single-photon source and a quantum memory for photonic qubits. To operate
as a single-photon source, the techniques include a source of a pair of
photons, such as a parametric down-conversion crystal, which is known to
emit photons in pairs. The detection of one member of the pair activates
the switch, which re-routes the other member into the storage loop. The
stored photon is then known to be circulating in the loop, and can be
switched out of the loop at a later time chosen by the user, providing a
single photon for potential use in a variety of quantum information
processing applications. To operate as a quantum memory for photonic
qubits, a single-photon in an arbitrary initial polarization state is
coherently stored in the loop, and coherently switched out of the loop
when needed.