  A quantum field theory approach is put forward to generalize the concept of
classical spatial light beams carrying orbital angular momentum to the
single-photon level. This quantization framework is carried out both in the
paraxial and nonparaxial regimes. Upon extension to the optical phase space,
closed-form expressions are found for a photon Wigner representation describing
transformations on the orbital Poincare sphere of unitarily related families of
paraxial spatial modes.
