Hong–Ou–Mandel effect

The reduction or disappearance of joint detections at a beam splitter’s two output ports when indistinguishable photons enter opposite input ports. Their two-photon amplitudes interfere so both photons leave through the same port.

Beam splitter: An optical device that combines or separates light beams, often by transmitting part of the light and reflecting the rest. Its reflection and transmission amplitudes interfere for the two possible paths the photons can take.
Fock state: A quantum state with a definite number of particles in a specified mode. The standard input uses one photon in each of two beam-splitter input modes.
Linear optical quantum computing: A quantum-computing approach that uses photons, interferometers, detectors, and measurement-induced operations. Two-photon interference enables photon-photon logic operations despite weak direct interactions.
Chung Ki Hong: A physicist who coauthored the 1987 proposal and demonstration of two-photon interference at a beam splitter. Hong co-developed the experiment that established the effect.
Classical wave interference: Interference produced by the superposition of classical electromagnetic waves. Classical intensity interference does not reproduce the same two-photon coincidence suppression.
Two-photon interference: Interference between probability amplitudes for outcomes involving a pair of photons. The effect arises when amplitudes for the photons to exit through different ports cancel.
Creation and annihilation operators: Operators that add or remove particles from quantum modes. They compactly derive the output state and its missing one-photon-per-port term.
Quantum teleportation: A protocol that transfers an unknown quantum state using shared entanglement and classical communication. Bell-state measurements with photons rely on interference at beam splitters.
Zouheir Y. Ou: A physicist who coauthored the 1987 study of two-photon interference with Chung Ki Hong and Leonard Mandel. Ou helped formulate and experimentally test the coincidence suppression.
Distinguishable photons: Photons whose states differ in a degree of freedom that can, in principle, reveal their identities. Distinguishability leaves which-photon information and reduces the Hong–Ou–Mandel dip.

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Indistinguishable particlesBroader topic: The coincidence dip directly reveals how indistinguishable photons interfere.