The orbital angular momentum carried by single photons represents a promising resource in the quantum information field. In this paper we report the characterization in the quantum regime of a recently introduced optical device, known as q-plate . Exploiting the spin-orbit coupling that takes place in the q-plate, it is possible to transfer coherently the information from the polarization to the orbital angular momentum degree of freedom, and viceversa. Hence the q-plate provides a reliable bi-directional interface between polarization and orbital angular momentum. As a first paradigmatic demonstration of the q-plate properties, we have carried out the first experimental Hong-Ou-mandel effect purely observed in the orbital angular momentum degree of freedom.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.