Ross Donaldson

We investigate the feasibility of satellite-to-ground quantum key distribution (QKD) at multi-GHz clock rates, where secure key generation time is constrained, due to LEO satellite overpasses (≈ 300 s). Higher repetition rates present an immediate route to increased secure key rate (SKR), though performance is limited by detector jitter, coupling losses, and atmospheric turbulence, depending on the receiver architecture. We combine finite-key modelling, detailed detector timing characterization, adaptive optics (AO) modelling, and real-time free-space QKD experiments at 1 GHz to evaluate practical receiver configurations based on multi-mode fiber-coupled avalanche photodiodes (MMF-APDs) and single-mode fiber-coupled superconducting nano-wire single-photon detectors (SMF-SNSPDs), at projected rates > 1 GHz. The results indicate clear operating regimes: MMF-APDs maximize secure key rates below ∼ 1 GHz, predominantly due to their low coupling loss. However, low-jitter SMF-SNSPDs are required to obtain positive key rates at > 1GHz clock rates (estimated optimal operating point near ∼ 7.55 GHz), though AO correction is required for effective single-mode coupling under turbulence. The experimental and projected results provide realistic design targets for future high-rate satellite QKD systems, while highlighting the technological requirements needed to achieve end-to-end operation beyond the GHz regime.