Quantum key distribution (QKD) enables secure communication through quantum mechanics principles, but its practical implementation faces challenges from environmental noise, particularly during daylight. This research demonstrates a breakthrough: a Gaussian modulated coherent state continuous-variable QKD (GMCS CV-QKD) system operating effectively in daylight and even rainy conditions.
Traditional QKD systems are highly vulnerable to ambient light, which increases error rates or renders systems inoperable. Discrete Variable (DV) approaches require complex filtering techniques (spatial, spectral, and temporal) to mitigate background noise, significantly increasing system complexity and cost.
This CV-QKD implementation offers a more robust alternative by leveraging unique advantages of continuous variables in quantum communication. The system operates at 1550 nm wavelength to minimize solar radiation interference and uses a local oscillator polarization-multiplexed with the signal as both spatial and spectral filters, substantially reducing complexity compared to conventional approaches.
Through an 860-meter free-space link with active beam tracking, this system achieved an average Secure Key Rate (SKR) of approximately 317 kbps in real-time during daylight operations, and maintained around 40 kbps after finite-key analysis. The system remained operational even during rainy conditions, demonstrating exceptional environmental resilience.
This implementation integrates classical laser communication services for real-time post-processing, enhancing practical deployment capabilities. At a 10 MHz operating frequency, the system consistently maintains SKRs exceeding 100 kbps without requiring the complex filtering operations that burden traditional systems.
The demonstrated robustness of GMCS CV-QKD against environmental noise represents a significant advancement toward practical quantum communication applications. By combining quantum key distribution with classical communication techniques, this research provides a pathway to more reliable, efficient, and cost-effective quantum secure communications in real-world conditions.
This work not only addresses daylight operation challenges but also significantly advances the feasibility of widespread deployment of quantum communication technologies in practical settings.
Reference: Zheng, XT., Zhang, QF., Ling, J. et al. Free-space continuous-variable quantum key distribution under high background noise. npj Quantum Inf 11, 52 (2025). doi:10.1038/s41534-025-01009-w
