Liying Han

Satellite-based quantum key distribution (QKD) is crucial for establishing global quantum networks, while current implementations are restricted to low-Earth-orbit satellites and nighttime operations. Geosynchronous-Earth-orbit (GEO) satellites present a compelling alternative, offering continuous availability and wide-area coverage. The primary obstacles are the substantial link loss inherent to GEO distances and high daytime ambient noise, which severely degrade the signal-to-noise ratio and hinder secure key generation. To address the challenge of daytime ambient noise, we implemented a multi-faceted filtering strategy across spatial, spectral, and temporal domains. Notably, an additional background reduction of approximately 5.2 dB was achieved using the 854.45 nm Fraunhofer line for spectral noise suppression. Simultaneously, we employed deep-learning-based adaptive optics, improving single-mode fiber coupling efficiency by approximately 1.4∼5.2 dB. We then experimentally validated all-day QKD over a metropolitan 7-km free-space channel, demonstrating continuous QKD operation throughout the day with maximum tolerable channel loss exceeding 62 dB. This represents an improvement of over one order of magnitude and surpasses the projected daytime loss budget for GEO satellite links. These results signify a major advancement towards practical, globally accessible quantum communication via GEO satellites.

Qubit-based synchronization offers a novel approach for quantum key distribution (QKD), simplifying system architecture and reducing implementation costs by leveraging the exchanged qubits. However, the performance of such schemes hinges on accumulating sufficient qubit events, rendering them vulnerable to local clock drift—particularly in high-channel-loss scenarios. This study investigates the impact of frequency instability in non-ideal oscillators, emphasizing clock drift-induced deterioration on QKD performance in these challenging, lossy conditions. We develop a computational model to quantify the secure key rate of QKD systems as a function of clock frequency stability. Through simulations and experimental validation with two distinct clock configurations, we demonstrate that oscillator stability becomes a key bottleneck in high-loss scenarios. By integrating target frequency scanning and clock offset recovery method, we verify our model via Monte Carlo simulations. Experimental validation confirms these findings, demonstrating a secure key rate of 0.37 bps at 67 dB channel loss—empirically validating the trade-off relationship among clock frequency stability, channel loss and acquisition time.

Quantum key distribution (QKD) based on the fundamental laws of quantum physics can allow the distribution of secure keys between distant users. However, the imperfections in realistic devices may lead to potential security risks, which must be accurately characterized and considered in practical security analysis. High-speed optical modulators, being as one of the core components of practical QKD systems, can be used to prepare the required quantum states. Here, we find that optical modulators based on LiNbO3, including phase modulators and intensity modulators, are vulnerable to photorefractive effect caused by external light injection. By changing the power of external light, eavesdroppers can control the intensities of the prepared states, posing a potential threat to the security of QKD. We have experimentally demonstrated the influence of light injection on LiNbO3-based optical modulators and analyzed the security risks caused by the potential green light injection attack, along with the corresponding countermeasures.