  We suggest and demonstrate experimentally a strategy to obtain relevant
information about a composite system by only performing measurements on a small
and easily accessible part of it, which we call quantum probe. We show in
particular how quantitative information about the angular correlations of
couples of entangled photons generated by spontaneous parametric down
conversion is accessed through the study of the trace distance between two
polarization states evolved from different initial conditions. After estimating
the optimal polarization states to be used as quantum probe, we provide a
detailed analysis of the connection between the increase of the trace distance
above its initial value and the amount of angular correlations.
