  We propose an efficient free-space scheme to create single photons in a
well-defined spatiotemporal mode. To that end, we first prepare a single source
atom in an excited Rydberg state. The source atom interacts with a large
ensemble of ground-state atoms via a laser-mediated dipole-dipole exchange
interaction. Using an adiabatic passage with a chirped laser pulse, we produce
a spatially extended spin wave of a single Rydberg excitation in the ensemble,
accompanied by the transition of the source atom to another Rydberg state. The
collective atomic excitation can then be converted to a propagating optical
photon via a coherent coupling field. In contrast to previous approaches, our
single-photon source does not rely on the strong coupling of a single emitter
to a resonant cavity, nor does it require the heralding of collective
excitation or complete Rydberg blockade of multiple excitations in the atomic
ensemble.
