Experimental effects of turbulence on coherent states in a free-space channel using adaptive optics for continuous variable quantum key distribution

Continuous variable quantum key distribution (CVQKD) is the sharing of a secret key between parties using the continuous quadratures of light, the phase and amplitude quadratures, which can infer the presence of an eavesdropper using fundamental quantum mechanics. A significant performance inhibitor of free-space CVQKD is turbulence which gives rise to optical wavefront distortions consisting of random phase and amplitude fluctuations (scintillation). In this work, we experimentally study the negative effects of turbulence on coherent states in a free-space channel for CVQKD. We demonstrate during coherent state transmissions from a continuous-wave laser in a turbulent channel, the interferometric visibility between a local oscillator and signal fluctuates and decreases with higher degrees of scintillation. This leads to the breaking of the phase-lock between the local oscillator and quantum signal and a decrease in the secret key rate (SKR). By incorporating an adaptive optics system, the degraded optical wavefront from turbulence can be corrected using a closed-feedback loop. This leads to the stabilisation and reduction in the decrease of the interferometric visibility in a turbulent channel. The improvement provided by adaptive optics leads to the increased performance of CVQKD, resulting in positive SKRs, which would have otherwise been infeasible in a turbulent channel.