Adjustable spatial filter for optimal free-space quantum communication round the clock.

Free-space quantum communication in daylight relies crucially on spatial filtering. The optimal filter configuration, however, depends on ever-changing link conditions. To this end, we developed an adjustable spatial filter that can be used to change the system field of view on the fly. We demonstrate its use in quantum key distribution over a 1.7-km free-space link. Furthermore, we compare it to filtering with multi-mode fiber coupling. Finally, we extrapolate our results to a broader range of realistic link conditions and show that active field-of-view optimization has the potential to substantially improve the overall secure key output of the system.