Single-photon optical heterodyning

Optical heterodyning has been observed at the single-photon level by recording the sinusoidal variation in the probability for detection of a photoelectron from a photomultiplier tube illuminated with a FM laser and a local oscillator laser. The single-photon character of the heterodyne process was demonstrated by observation of beat signals at total power levels down to
6 \times 10^{-15}
W where the spacing between photons emitted from the source laser was
\sim40 000
times greater than their transit time through the apparatus.