Quantum ternary logic based on biphotons

Summary form only given. Consider a pair of collinear frequency-degenerate photons-a biphoton, which can be obtained by spontaneous parametric down-conversion (SPDC). Being a three-state quantum system, it is a ternary analog of a qubit. Its state vector is |/spl psi/>=c/sub 1/|2,0>+c/sub 2/|0,2>+c/sub 3/|1,1>, with |n,m>,n+m=2 denoting n X-polarized photons and m Y-polarized photons. The first two terms describe type-I biphotons (correlated photons have similar polarizations), and the third one type-II biphotons (correlated photons have orthogonal polarizations). An arbitrary biphoton state is given by four parameters.