Quantum key distribution offers the promise of unconditionally secure communication, and researchers are now extending this technology beyond the laboratory and into real-world conditions. Tianxiang Zhan, Huasheng Li, Peng Huang, and colleagues demonstrate a significant advance in this field, achieving long-distance quantum key distribution through the open air. The team, spanning institutions including Shanghai Jiao Tong University and CAS Quantum Network Co., Ltd, successfully transmitted secure keys over 7 kilometres inland and 9. 6 kilometres over water, a feat previously hampered by atmospheric interference. This breakthrough, which utilises continuous-variable quantum key distribution without complex spectral filtering, overcomes challenges posed by daylight and turbulence, paving the way for practical satellite-based cryptography and integrated air-ground communication networks.

Long-Distance Free-Space Quantum Key Distribution Demonstrated

This research details a significant advancement in Quantum Key Distribution (QKD) technology, demonstrating a free-space QKD system capable of operating over 9. 6 kilometers with high key rates, even under challenging atmospheric conditions. The team built a continuous-variable QKD (CV-QKD) system using coherent states, aligning well with standard telecommunication infrastructure. They achieved secure key distribution over 9. 6 kilometers in a maritime environment, proving the feasibility of long-distance free-space QKD.

The system incorporates techniques to mitigate atmospheric turbulence, employing adaptive optics and optimized data processing to achieve relatively high key rates, making it practical for real-world applications. Rigorous security analysis confirms the system’s ability to provide secure key distribution. This research builds upon previous work in CV-QKD, improving performance and robustness. It demonstrates the potential of CV-QKD for building secure communication networks in real-world environments, highlighting the importance of addressing practical challenges and optimizing system parameters.

Atmospheric Turbulence Mitigated for Quantum Key Distribution

Researchers developed a novel approach to quantum key distribution over free space, successfully transmitting secure keys over distances of 7 kilometers inland and 9. 6 kilometers over open water, significantly exceeding previous limitations. This achievement hinges on overcoming atmospheric turbulence, which typically degrades quantum signals. The team addressed this issue by meticulously controlling and compensating for fluctuations in the transmission channel. A key innovation lies in the system’s ability to categorize raw key data based on instantaneous channel transmission efficiency, effectively grouping data acquired under similar atmospheric conditions.

This allows for more accurate parameter estimation and reduces the impact of noise introduced by atmospheric fading. By sorting data in this manner, the system minimizes excess noise and maintains security. This technique represents a substantial improvement over existing methods, which often struggle with the dynamic nature of free-space transmission. The experimental setup employs sophisticated techniques for generating and manipulating quantum states, utilizing cascaded acoustic optical modulators to create a high-stability coherent pulse train. Furthermore, the system incorporates high-precision tracking and pointing technology to maintain alignment between the transmitter and receiver, compensating for vibrations and atmospheric distortions. The researchers conducted field tests in both inland and maritime environments, demonstrating the robustness and adaptability of the system under varying atmospheric conditions. These successful demonstrations pave the way for the development of integrated air-ground quantum networks, offering a promising solution for secure communication in a wide range of applications.

Free-space CVQKD exceeds 9km distance

Researchers have achieved a significant breakthrough in secure communication by demonstrating continuous-variable key distribution (CVQKD) over unprecedented distances using free-space links. This technology enables the sharing of encryption keys with guaranteed security and is compatible with existing communication networks. The team successfully transmitted secure keys across a 7-kilometer inland atmospheric channel and a 9. 6-kilometer maritime channel, distances far exceeding previous free-space CVQKD demonstrations. This achievement opens the possibility of satellite-based cryptographic communication even during daylight hours.

The researchers developed methods to precisely manipulate and restore the polarization and phase of the quantum signals, effectively minimizing the impact of atmospheric fluctuations. By carefully monitoring and categorizing signal loss due to atmospheric conditions, they were able to significantly reduce excess noise and maintain the integrity of the quantum information. Measurements reveal that the average fading noise remains remarkably low across varying channel conditions. Statistical analysis shows that the majority of the signal falls within narrow channel loss intervals, indicating a high degree of stability and predictability. The successful demonstration of CVQKD over these distances represents a major step towards realizing integrated air-ground access networks with applications spanning various sectors, from secure financial transactions to confidential government communications. By combining the benefits of CVQKD with existing fiber optic networks, this technology promises a future where secure communication is both robust and seamlessly integrated into our daily lives.

Daylight CVQKD Over Kilometre Distances

This research demonstrates, for the first time, successful continuous-variable quantum key distribution (CVQKD) over long distances, 7 kilometers inland and 9. 6 kilometers over maritime channels, in both daylight and nighttime conditions. By developing techniques for precise manipulation of quantum states, accurate signal acquisition, and efficient free-space optical tracking, the team overcame the challenges posed by atmospheric effects and achieved secure key transmission distances exceeding the effective thickness of the atmosphere. These results suggest a viable pathway towards all-day satellite-based quantum cryptography using standard coherent optical communication components. The successful demonstration of long-distance CVQKD is significant because it offers a potentially compatible solution for integrating quantum communication with existing ground-based telecommunication networks, paving the way for integrated air-ground quantum access networks and cross-domain applications. Future work will focus on improving the free-space system and optimizing data accumulation to further extend the range and practicality of the system.

More information

Long-distance free-space quantum key distribution with continuous variables

ArXiv: https://arxiv.org/abs/2507.21546

Source：https://quantumzeitgeist.com/long-distance-free-space-key-distribution-achieved-via-continuous-variable-quantum-key-distribution/