Diffusion-induced decoherence of stored optical vortices

We study the coherence properties of optical vortices stored in atomic ensembles. In the presence of thermal diffusion, the topological nature of stored optical vortices is found not to guarantee slow decoherence. Instead the stored vortex state's decoherence is surprisingly larger than the stored Gaussian mode. Furthermore, calculation of the coherence factor shows that the center of the stored vortex becomes completely incoherent once diffusion begins and, when a reading laser is applied, the optical intensity at the center of the vortex becomes nonzero. Its implication for quantum information is discussed. A comparison of classical diffusion and quantum diffusion is also presented.