Quantum probes to experimentally assess correlations in a composite system

We suggest and demonstrate a technique to obtain relevant information about a composite system by performing measurements on only a small and easily accessible part of it, which we call a quantum probe. We show in particular how quantitative information about the angular correlations of pairs of entangled photons generated by spontaneous parametric down-conversion may be 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 the quantum probe, we provide a detailed analysis of the connections between the increase of the trace distance above its initial value and the amount of angular correlation.