Polarization transformations of biphotons

A method of polarization ququart`s bases states preparation using polirization transformations is discussed. All states out of three mutually unibased bases were prepared experimentally with high fidelity values. The proposed method can be used in quantum key distribution protocols using polarization ququarts as information carriers. The advantages of such realization of ququarts are the possibility to generate all the required states with single nonlinear crystal, low measurement losses and the fact that the experimental setup is free from any unstable interferometric schemes. All bases states out of the chosen basis can be distinguished deterministicaly using a simple scheme that makes polarization ququarts a useful object for quantum key distribution.