  The recently demonstrated possibility of entangling opposite values of the
orbital angular momentum (OAM) of a photon with its spin enables the
realization of nontrivial one-photon spinorbit four-dimensional states for
quantum information purposes. Hitherto, however, an optical device able to
perform arbitrary unitary transformations on such spinorbit photon states has
not been proposed yet. In this work we show how to realize such a ``universal
unitary gate'' device, based only on existing optical technology, and describe
its operation. Besides the quantum information field, the proposed device may
find applications wherever an efficient and convenient manipulation of the
combined OAM and spin of light is required.
