= 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.
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