Professor Zeng Guihua's team at Shanghai Jiao Tong University experimentally verifies the feasibility of satellite ground continuous variable quantum key distribution

Research background

Long distance quantum key distribution is an important foundation for building a global quantum communication network. Compared with fiber optic links, satellites can enable quantum signals to propagate mainly in low loss free space, providing feasible paths for intercontinental and ultra long distance quantum communication. The satellite ground discrete variable quantum key distribution has achieved thousands of kilometers level key distribution and intercontinental encrypted communication, demonstrating the potential for building an integrated quantum network of space, sky, and earth. Continuous variable quantum key distribution has the characteristics of high channel capacity, low cost, and compatibility with classical coherent optical communication systems. It can also effectively suppress background light noise through coherent detection, and naturally has the ability to operate all day long. However, under high loss conditions, it is difficult to suppress excessive noise and the theoretical tolerable loss is limited, which has led to the fact that the satellite ground CVQKD has mainly remained in the theoretical analysis stage. Experimental verification requires both recovering the original key under high loss and establishing correlation with the modulated data, as well as proving that the noise is low enough to obtain a positive secure key rate.

Research Path and Achievements

In response to the contradiction between insufficient signal-to-noise ratio of pilot signals and increased pilot leakage noise in high loss satellite ground links, the research team constructed a local oscillator Gaussian modulation coherent state CVQKD system, which uses time polarization multiplexing to transmit quantum signals and pilot signals. The two-stage high extinction ratio amplitude modulator provides 80 dB time-domain isolation, and the polarization beam splitter provides 30 dB polarization isolation, achieving a total of 110 dB multiplexing isolation. While the pilot intensity is 67 dB higher than the quantum signal, it ensures sufficiently low photon leakage, achieving suppression of phase noise and photon leakage noise at a high loss of 41 dB.

The research team constructed a 38-43 dB equivalent loss link consisting of a free space channel, 50 km optical fiber, and variable optical attenuator. The experimental optical path is shown in Figure 1. The receiving end uses an independent local oscillator to perform coherent detection and achieves a quantization accuracy of 12 bitsOscilloscopeCollect quantum signals and pilot signals to provide experimental data for original key recovery and over noise evaluation under high loss conditions. The research team used efficient downsampling, Quinn high-precision frequency offset estimation, and pilot assisted phase recovery techniques to complete digital post-processing. At a loss of 41 dB, the recovered signal exhibits a unique and significant cross-correlation peak with the Gaussian modulated data at the Alice end (Figure 2), indicating the successful recovery of the original key and the establishment of virtual entanglement between Alice and Bob. In response to the significant statistical fluctuations in overnoise estimation under high loss of 41 dB, the team used an equivalent testing method and measured an average overnoise of 0.0167 SNU. After taking into account the ADC quantization noise and fading noise of the actual satellite ground channel, it is predicted that the over noise of the 1200 km link will be 0.0198 SNU (Figure 3). The key rate analysis shows that the system can still obtain a positive secure key rate under 41 dB loss (Figure 3), which preliminarily verifies the feasibility of satellite ground CVQKD.

development prospects

This study preliminarily verified the feasibility of continuous variable quantum key distribution in a 41 dB high loss satellite ground channel, breaking through the limitations of theoretical analysis and numerical simulation of satellite ground CVQKD. It provides experimental basis for quantum signal recovery and noise suppression in high loss and strong disturbance environments, and provides a new technical path for the design and deployment of actual satellite ground continuous variable quantum key distribution systems. It also provides important reference for building an integrated space ground quantum communication network.

quote:Professor Zeng Guihua's team at Shanghai Jiao Tong University experimentally verifies the feasibility of satellite ground continuous variable quantum key distribution【 Reference time: August 12, 2026 】