Single-photon optical heterodyning
C. Davis · IEEE Journal of Quantum Electronics · 1979
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 to6 \times 10^{-15}W where the spacing between photons emitted from the source laser was\sim40 000times greater than their transit time through the apparatus.