SinglePhoton

A single photon is the quantum of the electromagnetic field that carries energy hν and momentum hν/c. In quantum optics it is described as a Fock state with photon number n=1, denoted |1>. Unlike classical light fields, a single photon cannot be described by a definite field amplitude; instead it exhibits quantum properties such as indivisibility, wave–particle duality, and the ability to interfere with itself in a beam splitter.

Single photons are produced by specialized light sources. True on-demand single-photon sources aim to emit exactly one photon per trigger, whereas heralded sources use nonlinear processes such as spontaneous parametric down-conversion, where detection of one photon heralds the presence of its twin. Quantum dots and certain solid-state color centers can serve as on-demand emitters. Attenuated laser pulses can approximate single photons but follow a Poisson distribution that includes multi-photon events.

Single-photon detection uses devices such as superconducting nanowire single-photon detectors (SNSPDs), silicon or InGaAs avalanche photodiodes, and photomultiplier tubes. The single-photon nature is tested via the second-order correlation function g^(2)(τ). For an ideal single-photon source, g^(2)(0) = 0; practical sources exhibit g^(2)(0) < 0.5, indicating suppressed multi-photon probability and antibunching.

Applications include quantum communication (quantum key distribution), linear-optics quantum computing, quantum metrology, and certain imaging techniques. Indistinguishability of photons from the same source is important for interference-based protocols and gates, making spectral and temporal control a key performance metric.

Key challenges involve improving efficiency, purity, and indistinguishability, reducing loss and timing jitter, and suppressing background noise to enable scalable quantum information processing.