Applications
SPDC 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). Recently, an alternative electrically driven semiconductor source was proposed based on the newly observed effect of two-photon emission from semiconductors. The single photons as well as the photon pairs are often used in quantum information experiments and applications like quantum cryptography and Bell test experiments.
Read more about this topic: Spontaneous Parametric Down-conversion