  We consider the bipartite state of a two-photon polarization system and
obtain the exact analytical expression for the von Neumann entropy in the
particular case of a 5-parameter polarization density matrix. We investigate
and graphically illustrate the dependence of the entropy on these five
parameters, in particular, the existence of exotic, transition from exotic to
non-exotic, and non-exotic states, where the quantum conditional entropy is
negative, both positive and negative, and positive, respectively. We study the
"cooling" or "heating" effect that follows from the reduced density of photon 2
when a measurement is performed on photon 1.
