Hong–Ou–Mandel effect

The Hong–Ou–Mandel effect is a two-photon interference effect in quantum optics that was demonstrated in 1987 by Chung Ki Hong (Korean: 홍정기), Zheyu Jeff Ou (Chinese: 区泽宇; pinyin: Oū Zéyǔ) and Leonard Mandel at the University of Rochester. The effect occurs when two identical single photons enter a 1:1 beam splitter, one in each input port. When the temporal overlap of the photons on the beam splitter is perfect, the two photons will always be detected in the same output mode, meaning that there is zero chance that they will be detected separately with one photon click in each of the two outputs giving a coincidence event. The photons have a 50:50 chance of being detected (together) in either output mode. If they become more distinguishable (e.g. because they arrive at different times or with different wavelength), the probability of them each being detected in a different detector will increase. In this way, the interferometer coincidence signal can accurately measure bandwidth, path lengths, and timing. Since this effect relies on the existence of photons and the second quantization, it can not be fully explained by classical optics.

The effect provides one of the underlying physical mechanisms for logic gates in linear optical quantum computing (the other mechanism being the action of measurement).

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