Teleportation theory
The scheme of Bennett et al. Alice wishes to teleport an unknown quantum state |Yñ (the "teleportee") to Bob. They agree to share an entangled pair of "ancillary" photons. Alice then performs a Bell-state measurement on the teleportee and her ancillary photon, and obtains one of the four possible Bell results. This measurement also projects Bob's ancillary photon into a well defined quantum state. Alice then transmits her result to Bob, who performs one of four unitary operations to obtain the original state.