An upper bound on the rate of information transfer in optical vortex beams

Light endowed with orbital angular momentum, commonly termed optical vortex light, has an azimuthal phase indexed by the orbital quantum number l. In contrast to the two basis states of the optical spin angular momentum, the interest in the information content of optical vortex beams is centred on the assumption that forms a countably infinite set of basis states. The recent experimental observation that group velocity is inversely proportional to l provides a theoretical basis for a practical measure of information transfer. This letter sets an upper bound on that measure in free-space propagation. (letter)