Spontaneous parametric down-conversion is an important process in quantum optics. A nonlinear crystal splits incoming photons into pairs of photons of lower energy whose combined energy and momentum are equal to the energy and momentum of the original photon. "Parametric" refers to the fact that the state of the crystal is left unchanged in the process, which is why energy and momentum must be conserved (this is related to phase matching in nonlinear optics). The process is "spontaneous" in the same sense as spontaneous emission—it is stimulated by random vacuum fluctuations. Consequently, the photon pairs are created at random times. However, if one of the pair (the "signal") is detected at any time then we know its partner (the "idler") is present. This then allows for the creation of optical fields containing (to a good approximation) a single photon. As of 2005, this is the predominant mechanism for experimentalists to create single photons (also known as Fock states). The single photons as well as the photon pairs are often used in quantum information experiments and applications like quantum cryptography and the Bell test experiments. Image File history File links Size of this preview: 800 × 471 pixelsFull resolution (1277 × 752 pixel, file size: 25 KB, MIME type: image/png) Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. ... Image File history File links Size of this preview: 800 × 471 pixelsFull resolution (1277 × 752 pixel, file size: 25 KB, MIME type: image/png) Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. ... Quantum optics is a field of research in physics, dealing with the application of quantum mechanics to phenomena involving light and its interactions with matter. ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... In classical mechanics, momentum (pl. ... Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization P responds nonlinearly to the electric field E of the light. ... Nonlinear optics is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization P responds nonlinearly to the electric field E of the light. ... Spontaneous emission is the process by which a molecule in an excited state drops to the ground state, resulting in the creation of a photon. ... In the description of the interaction between elementary particles in quantum field theory, a virtual particle is a temporary elementary particle, used to describe an intermediate stage in the interaction. ... The optical field is a term used in physics and vector calculus to designate the electric field shown as E in the electromagnetic wave equation which can be derived from Maxwells Equations. ... A Fock state, in quantum mechanics, is any state of the Fock space with a well-defined number of particles in each state. ... In quantum mechanics, quantum information is physical information that is held in the state of a quantum system. ... Quantum cryptography is an approach based on quantum physics for secure communications. ... In quantum mechanics, Bells Theorem states that a Bell inequality must be obeyed under any local hidden variable theory but can in certain circumstances be violated under quantum mechanics (QM). ...