  The interference of two single photons impinging on a beam splitter is
measured in a time-resolved manner. Using long photons of different frequencies
emitted from an atom-cavity system, a quantum beat with a visibility close to
100% is observed in the correlation between the photodetections at the output
ports of the beam splitter. The time dependence of the beat amplitude reflects
the coherence properties of the photons. Most remarkably, simultaneous
photodetections are never observed, so that a temporal filter allows one to
obtain perfect two-photon coalescence even for non-perfect photons.
