Hong–Ou–Mandel effecteffect

Discovered in 1987 by Chung Ki Hong, Zhe-Yu Ou, and Leonard Mandel, this effect revealed strange quantum behavior: two identical photons meeting at a beam splitter always stick together. A beam splitter is like a half-silvered mirror, reflecting half the light and transmitting half. From a classical perspective, sometimes the photons would end up in different detectors, but in the quantum world, interference of probability amplitudes completely excludes that outcome. This is a vivid demonstration that photons are indistinguishable particles obeying Bose–Einstein statistics, which favors occupying the same state. The original experiment used parametric down-conversion to create photon pairs and revealed a sharp dip in coincidence counts.

In our ordinary world, if two particles strike a half-silvered mirror from opposite sides, each can be reflected or transmitted independently, sometimes ending up on different sides. But for identical photons (particles of light with identical properties, such as color and arrival time), quantum rules forbid that: they always exit together. This happens because photons are bosons (particles inclined to bunch up in the same state), and their quantum waves interfere (overlap like ripples on water, sometimes reinforcing, sometimes canceling) in such a way that the splitting option completely vanishes.

The Hong–Ou–Mandel effect is a two-photon quantum interference in which two indistinguishable photons incident on different inputs of a 50:50 beam splitter always leave together through one output. This 'bunching' is a consequence of the bosonic nature of photons and the superposition of probability amplitudes, leading to destructive interference for the outcome with one photon in each output. The effect is the simplest non-classical interference and a cornerstone of linear optical quantum computing.