Hong-Ou-Mandel interference between triggered and heralded single photons from separate atomic systems

We present Hong-Ou-Mandel interference of single photons generated via two different physical processes by two independent atomic systems: scattering by a single atom, and parametric generation via four-wave mixing in a cloud of cold atoms. Without any spectral filtering, we observe a visibility of V = 62 +/- 4%. After correcting for accidental coincidences, we obtain V = 93 +/- 6%. The observed interference demonstrates the compatibility of the two sources, forming the basis for an efficient quantum interface between different physical systems.