Heterodyne interferometry at the single-photon level

Heterodyne interferometry uses beats produced by superposing two light waves with different frequencies at a photodetector. The output from the photodetector then contains a component at the beat frequency which can be processed to obtain information on changes in the optical path difference. We present experimental results which show that beats can be observed down to power levels at which the mean time interval between the arrival of photons at the photodetector is much greater than the transit time from the laser source to the detector and also when the mean time interval between photons is greater than the period of the beat. These results show that the beats are not due to the interference of one photon with another, but arise from a succession of single-photon events. It follows that measurements can be made even at extremely low photon fluxes by using homodyne detection and extending the integration time