Quantum communications: Tetrat coding
G. A. Barbosa · Physical Review A · 1998
This work shows that an orthogonal tetrat basis element made up of two photons can be generated with two optical parametric down-converters and identified with a multiport detection system for coincidences. The transmission gain over the usual coding in bits is $g=2.$ The information is assigned to each photon pair using a single conjugate beam of the parametric down-conversion. This system also demonstrates a way to get a pair of disentangled polarized photons by handling just one photon of an entangled pair.