  We consider correlation properties of twophoton polarization states in the
parametric down-conversion process. In our description of polarization states
we take into account the simultaneous presence of colored and white noise in
the density matrix. Within the considered model we study the dependence of the
von Neumann entropy on the noise amount in the system and derive the
separability condition for the density matrix of twophoton polarization state,
using Perec-Horodecki criterion and majorization criterion. Then the dependence
of the Bell operator (in CHSH form) on noise is studied. As a result, we give a
condition for determining the presence of quantum correlation states in
experimental measurements of the Bell operator. Finally, we compare our
calculations with experimental data [doi:10.1103/PhysRevA.73.062110] and give a
noise amount estimation in the photon polarization state considered there.
