  We analyze features of mixed biphoton polarization states which arise from
pure states of polarization-frequency biphoton ququarts after averaging over
frequencies of photons. For mixed states we find their concurrence C, Schmidt
parameter K and degree of polarization P, as well as the von Neumann mutual
information I. In some simple cases we find also the relative entropy S_{rel}
and the degree of classical correlations C_{cl}. We show that in mixed states
the Schmidt parameter does not characterize anymore the degree of entanglement,
as it does in pure states. Nevertheless, the Schmidt parameter remains useful
even in the case of mixed states because it remains directly related to the
degree of polarization. We compare results occurring in the cases of full pure
polarization-frequency states of ququarts, mixed states (averaged over
frequencies) and states with separated high- and low-frequency parts.
Differences between these results can be seen in experiments with and without a
dichroic beam-splitter, as well as with and without frequency filters in front
of detector.

Quantum Physics