  Improving axial resolution is of paramount importance for three-dimensional
optical imaging systems. Here, we investigate the ultimate precision in axial
localization using vortex beams. For Laguerre-Gauss beams, this limit can be
achieved with just an intensity scan. The same is not true for superpositions
of Laguerre-Gauss beams, in particular for those with intensity profiles that
rotate on defocusing. Microscopy methods based on rotating vortex beams may
thus benefit from replacing traditional intensity sensors with advanced
mode-sorting techniques.
