Shengkai Liao

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.

Satellite-based quantum key distribution (QKD) holds the potential to establish global quantum communication networks. While satellite QKD has been extensively demonstrated using polarization encoding, time-bin encoding offers distinct advantages, such as simplifying polarization-maintaining telescope designs and being insensitive to satellite-ground relative motion. In this work, we developed a high-speed 625-MHz QKD light source using a robust Sagnac-interferometer-based modulation scheme and subsequently demonstrated time-bin QKD over a 7 km urban terrestrial free-space channel. This experiment successfully operated over a channel traversing an equivalent atmospheric thickness and loss exceeding that of typical satellite-to-ground links, achieving a low quantum bit error rate of 0.87% and a secure key rate of 134 bps@51.4 dB. Furthermore, by using a half-wave plate to simulate the polarization basis rotation inherent in such links, we verified the robustness of time-bin encoding for satellite scenarios. The results validate time-bin encoding as a compelling alternative and lay the technical foundation for future satellite QKD applications.

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.