Single-photon interference experiment using a mechanical frequency shifter

A variation of the interference experiment in the single photon has been performed. By means of a mechanical effect, it is changed the frequency of the photon only on one arm of a Mach-Zender interferometer. The beat signals even when the intensity is so small that only one photon at a time passes through the interferometer are observed. Our experiment gives a further proof of the classical behaviour of the electromagnetic radiation at arbitratily small intensity, in agreement with the probabilistic interpretation of quantum mechanics