  Quantum Key Distribution (QKD) promises a provably secure method to transmit
information from one party to another. Free-space QKD allows for this
information to be sent over great distances and in places where fibre-based
communications cannot be implemented, such as ground-satellite. The primary
limiting factor for free-space links is the effect of atmospheric turbulence,
which can result in significant error rates and increased losses in QKD
channels. Here, we employ the use of a high-speed Adaptive Optics (AO) system
to make real-time corrections to the wavefront distortions on spatial modes
that are used for high-dimensional QKD in our turbulent channel. First, we
demonstrate the effectiveness of the AO system in improving the coupling
efficiency of a Gaussian mode that has propagated through turbulence. Through
process tomography, we show that our system is capable of significantly
reducing the crosstalk of spatial modes in the channel. Finally, we show that
employing AO reduces the quantum dit error rate for a high-dimensional orbital
angular momentum-based QKD protocol, allowing for secure communication in a
channel where it would otherwise be impossible. These results are promising for
establishing long-distance free-space QKD systems.
