Quantum channels with beam wandering: an analysis of the Marcum Q-function

Beam wandering leads to strong disturbances in free-space quantum channels. This disadvantage can be overcome by post-selection of those events with the beam center being close to the center of the aperture. We provide an analytical analysis for the feasibility of this procedure, which requires a rigorous study of the Marcum Q-function. For this purpose, we develop an appropriate analytical approximation, which gives a reasonable accuracy and can be useful for many other applications.