  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.
