Hong–Ou–Mandel Effect - Quantum-mechanical Description - Experimental Signature

Experimental Signature

When two photodetectors monitor the output modes of the beam splitter, the coincidence rate of the detectors will drop to zero when the identical input photons overlap perfectly in time. This is called the Hong–Ou–Mandel dip, or HOM dip, shown in figure 2. The coincidence count reaches a minimum, indicated by the dotted line in figure 2. The minimum drops to zero when the two photons are perfectly identical in all properties. When the two photons are perfectly distinguishable, the dip completely disappears. The precise shape of the dip is directly related to power spectrum of the single-photon wave packet, and is therefore determined by the physical process of the source. Common shapes of the HOM dip are Gaussian and Lorentzian.

A classical analog to the HOM effect occurs when two coherent states (e.g. laser beams) interfere at the beamsplitter. If the states have a rapidly varying phase difference (i.e. faster than the integration time of the detectors) then a dip will be observed in the coincidence rate equal to one half the average coincidence count at long delays (Nevertheless it can be further reduced with a proper discriminating trigger level applied to the signal.). Consequently, to prove that destructive interference is two-photon quantum interference, the HOM dip must be lower than one half.