  A formalism for two-photon Stokes parameters is introduced to describe the
polarization entanglement of photon pairs. This leads to the definition of a
degree of two-photon polarization, which describes the extent to which the two
photons act as a pair and not as two independent photons. This pair-wise
polarization is complementary to the degree of polarization of the individual
photons. The approach provided here has a number of advantages over the density
matrix formalism: it allows the one- and two-photon features of the state to be
separated and offers a visualization of the mixedness of the state of
polarization.
