  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 fibre 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.

Quantum Physics;Optics