Feasibility of multi-gigahertz satellite-to-ground secure quantum communication

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 low-Earth orbit (LEO) satellite overpasses ( ≈ 300 s). Higher repetition rates offer a direct route to increased secure key rate (SKR), though system performance is fundamentally limited by detector timing jitter, optical coupling losses, and atmospheric turbulence, depending on receiver architecture. We combine finite-key modeling, detailed detector timing characterization, adaptive optics (AO) modeling, 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 nanowire single-photon detectors (SMF-SNSPDs), at projected rates >1 GHz. The results indicate clear operating regimes across a range of atmospheric conditions: MMF-APDs maximize secure key rates below ∼ 1 GHz, mainly due to their low coupling loss. However, low-jitter SMF-SNSPDs are required to obtain positive key rates at higher clock rates—with modeling and measured timing responses of our hardware indicating an estimated optimal operating point near ∼ 7.2 GHz—though AO correction is required for effective single-mode coupling under turbulence. The experimental and projected results provide quantitative performance bounds and 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.