All-day free-space quantum key distribution with thermal source towards quantum secure communications for unmanned vehicles

Unmanned vehicles (UV) demand highly secure communication system with high-cost-effectiveness. Bypassing the use of quantum coherent source and active modulations, passive-state-preparation (PSP) continuous-variable quantum key distribution (CVQKD) with thermal source provides a favorable solution for all-day cryptography communication between UVs. However, the field experiment of free-space PSP CVQKD has still not been realized due to the lack of efficient excess noise suppression techniques via high-loss free-space channels. Here, we realize the PSP CVQKD field test over an urban free-space channel with record-breaking attenuation from −12.24 dB to −15.59 dB. Specifically, a novel scheme is proposed to reduce excess noise from PSP, and efficient quantum coherence detection alongside advanced digital signal processing algorithms is developed to achieve low-noise synchronized raw data acquisition. The secure keys are successfully generated, with statistical summation values of 0.85 kbps during the day and 1.52 kbps at night, proving the viability for UV secure communication.