  The orbital angular momentum of light (OAM) provides a promising approach for
the implementation of multidimensional states (qudits) for quantum information
purposes. In order to characterize the degradation undergone by the information
content of qubits encoded in a bidimensional subspace of the orbital angular
momentum degree of freedom of photons, we study how the state fidelity is
affected by a transverse obstruction placed along the propagation direction of
the light beam. Emphasis is placed on the effects of planar and radial
hard-edged aperture functions on the state fidelity of Laguerre-Gaussian
transverse modes and the entanglement properties of polarization-OAM
hybrid-entangled photon pairs.
