  The orbital angular momentum of photons, being defined in an infinitely
dimensional discrete Hilbert space, offers a promising resource for
high-dimensional quantum information protocols in quantum optics. The biggest
obstacle to its wider use is presently represented by the limited set of tools
available for its control and manipulation. Here, we introduce and test
experimentally a series of simple optical schemes for the coherent transfer of
quantum information from the polarization to the orbital angular momentum of
single photons and vice versa. All our schemes exploit a newly developed
optical device, the so-called "q-plate", which enables the manipulation of the
photon orbital angular momentum driven by the polarization degree of freedom.
By stacking several q-plates in a suitable sequence, one can also access to
higher-order angular momentum subspaces. In particular, we demonstrate the
control of the orbital angular momentum $m$ degree of freedom within the
subspaces of $|m|=2 \hbar$ and $|m|=4\hbar$ per photon. Our experiments prove
that these schemes are reliable, efficient and have a high fidelity.
