  The optical "spin-orbit" coupling occurring in a suitably patterned
nonuniform birefringent plate known as `q-plate' allows entangling the
polarization of a single photon with its orbital angular momentum (OAM). This
process, in turn, can be exploited for building a bidirectional "spin-OAM
interface", capable of transposing the quantum information from the spin to the
OAM degree of freedom of photons and \textit{vice versa}. Here, we
experimentally demonstrate this process by single-photon quantum tomographic
analysis. Moreover, we show that two-photon quantum correlations such as those
resulting from coalescence interference can be successfully transferred into
the OAM degree of freedom.
