  The orbital angular momentum (OAM) of photons is a promising degree of
freedom for high-dimensional quantum key distribution (QKD). However,
effectively mitigating the adverse effects of atmospheric turbulence is a
persistent challenge in OAM QKD systems operating over free-space communication
channels. In contrast to previous works focusing on correcting static simulated
turbulence, we investigate the performance of OAM QKD in real atmospheric
turbulence with real-time adaptive optics (AO) correction. We show that, even
our AO system provides a limited correction, it is possible to mitigate the
errors induced by weak turbulence and establish a secure channel. The crosstalk
induced by turbulence and the performance of AO systems is investigated in two
configurations: a lab-scale link with controllable turbulence, and a 340 m long
cross-campus link with dynamic atmospheric turbulence. Our experimental results
suggest that an advanced AO system with fine beam tracking, reliable beam
stabilization, precise wavefront sensing, and accurate wavefront correction is
necessary to adequately correct turbulence-induced error. We also propose and
demonstrate different solutions to improve the performance of OAM QKD with
turbulence, which could enable the possibility of OAM encoding in strong
turbulence.
