  Single-photons of well-defined polarisation that are deterministically
generated in a single spatio-temporal field mode are the key to the creation of
multi-partite entangled states in photonic networks. Here, we present a novel
scheme to produce such photons from a single atom in an optical cavity, by
means of vacuum-stimulated Raman transitions between the Zeeman substates of a
single hyperfine state. Upon each transition, a photon is emitted into the
cavity, with a polarisation that depends on the direction of the Raman process.
