Angular momentum and the geometrical gauge of localized photon staes
  Localized photon states have non-zero angular momentum that varies with the non-unique choice of a transverse basis and is changed by gauge transformations of the geometric vector potential $\mathbf{a}$. The position operator must depend on the choice of gauge, but a complete gauge transformation of a physically distinct state has no observable effects. The potential $\mathbf{a}$ has a Dirac string singularity that is related to an optical vortex of the electric field. 
Margaret Hawton, William E. Baylis