 Two-photon interference

Interference between indistinguishable alternatives involving two photons. The Hong-Ou-Mandel effect is an example: for identical photons arriving at opposite inputs of a balanced beam splitter, the two alternatives for one photon in each output cancel. A Bell state analyzer exploits the dependence of the output statistics on the symmetry of the joint photon polarization state. Matching arrival time, spectrum and spatial mode prevents those alternatives from being distinguished by unobserved degrees of freedom.