  We present the emergence of coherent two-photon backscattering, a
manifestation of weak localization, in multiple scattering of maximally
entangled pure and fully mixed two-photon states and examine the effect of
entanglement and classical correlations. Quantum correlations in backscattering
are investigated for finite three-dimensional disordered structures in the weak
localization regime as well as systems of a small number of scatterers with
specified spatial arrangements. No assumptions are made on the statistical
behavior of the scattering matrix elements. Furthermore, we study the interplay
between quantum correlations induced by multiple scattering and the
correlations that may be present in the illumination fields, and how they are
manifested in the output modes. We study the effect of the dimensionality of
the entanglement and the angular distribution of the jointly measurable photon
pairs on the emergence of enhancement and angular quantum correlations and show
how quantum correlations can be used as a probe of the entanglement
dimensionality. We show that by increasing the disordered material density, the
width of the coherent two-photon backscattering cones increases, in accordance
with the reduction of the mean free path length within the structure.
