Quantum information transfer from spin to orbital angular momentum of photons

The optical ''spin-orbit'' coupling occurring in a suitably patterned nonuniform birefringent plate known as a ''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 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.