photonpair

A photon pair refers to two photons that are produced in correlated quantum states, often with entanglement in properties such as polarization, time-energy, or momentum. Photon pairs are essential resources in quantum information science and quantum optics experiments, enabling tasks that rely on strong correlations between light quanta.

Common generation methods include spontaneous parametric down-conversion (SPDC) in nonlinear crystals, where a pump photon splits into signal and idler photons while conserving energy and momentum. Type I SPDC yields similarly polarized pairs, whereas Type II produces orthogonally polarized pairs. Another method is four-wave mixing in optical fibers or microresonators, which also generates correlated signal and idler photons through the χ3 nonlinearity. The spectral and temporal characteristics of the pair depend on phase-matching conditions and the pump properties.

Photons in a pair exhibit measurable correlations that can be used to test fundamental quantum mechanics and to implement protocols in quantum information. The joint spectral amplitude describes frequency correlations, and Schmidt decomposition can quantify spectral purity. Hong-Ou-Mandel interference is a common technique to assess indistinguishability and entanglement. Detection relies on single-photon detectors and coincidence counting; heralding techniques use one photon to infer the presence of its partner.

Applications of photon pairs span entanglement-based quantum key distribution, quantum teleportation, and linear-optics quantum computing. They are also employed in quantum metrology, calibration of detectors, and fundamental tests of quantum mechanics. Design considerations focus on maximizing brightness and collection efficiency while preserving spectral purity and minimizing losses and detector noise, using approaches such as quasi-phase matching, cavity-enhanced sources, and waveguide or fiber architectures.