Polarization state of biphoton light generated via collinear frequency-degenerate spontaneous parametric down-conversion is considered. A biphoton is described by a three-component polarization vector, its arbitrary transformations relating to the SU(3) group. A subset of such transformations, available with retardation plates, is realized experimentally. In particular, two independent orthogonally polarized beams of type-I biphotons are transformed into a beam of type-II biphotons. Polarized biphotons are suggested as ternary analogs of two-state quantum systems (qubits).
@article{arxiv.quant-ph/9907099,
title = {Polarization state of a biphoton: quantum ternary logic},
author = {A. V. Burlakov and M. V. Chekhova and O. A. Karabutova and D. N. Klyshko and S. P. Kulik},
journal= {arXiv preprint arXiv:quant-ph/9907099},
year = {2009}
}