Airborne quantum key distribution: a review

The security of communication has become one of the significant factors affecting defense, finance, and social stability. The emerging quantum technology represented by quantum key distribution (QKD) provides a brand-new solution to deal with the potential risks of classical cryptography and decoding challenges posed by quantum computation. Driven by the grand vision of building a global quantum Internet, the realization of all-weather, wide-coverage, high-bit rate free-space QKD is becoming one of the most important goals in the field of quantum information. With this regard, the terrestrial free-space QKD and satellite-to-ground QKD links have been extensively studied. Remarkable achievements have been witnessed, laying a solid foundation for the initial stage of the quantum Internet framework. However, due to the high cost, the limited working time window of the satellite-based platforms, and the deployment hurdles of ground-based fiber quantum networks, airborne QKD relying on aircraft is now attracting increasing attention and soon becoming a hot topic in recent years.


The research group led by Prof. Zhengfu Han and Wei Chen from the University of Science and Technology of China and the group led by Prof. Lei Shi from the Airforce Engineering University published a review article in Chinese Optics Letters, Volume 19, Issue 12, 2021(Y. Xue, et al., Airborne quantum key distribution: a review). This review summarized the recent progress of the free­space quantum communications and highlighted the QKD researches on airborne platforms. The basic concept of airborne QKD is introduced from system components, link configurations, key techniques, etc. The future development trend and realization routine of airborne QKD has also been revealed. The important technical challenges of airborne experiments at present and in the future are discussed in detail.


Airborne QKD refers to specific schematics to achieve quantum communication in free-space targeting the short-range, local-area networks and mobile scenarios. Compared with the existing terrestrial free-space experiments, airborne QKD features more drastic changes of quantum optical links between the transmitter and receiver. Meanwhile, it has a lower altitude, faster relative motions, and hardly any limitations to fixed orbits compared with the satellite-based platform. It is expected to complement the last piece of the puzzle towards the integrated quantum communication network. Airborne QKD will also provide a crucial approach to solve the last-mile access problem of the mobile quantum network under extreme conditions in a flexible, efficient, and cost-effective way. Figure 1 shows the blueprint of a hierarchical quantum network operating in different atmospheric layers. The QKD with GEO satellite aims at establishing quantum channels at the scale of thousands of kilometers. The QKD with LEO/MEO is suitable for constructing metropolitan backbone quantum links. The moving platforms like manned or unmanned aircraft and helicopters flying in the region of 5~15 km could be used for short-range quantum information transfer. In contrast, the high-altitude platforms (HAPs) such as hot-air balloons and airships can work above 15 km to play the role of intermediate nodes between space and ground. Miniature drones with propellers apply to near-surface regions below 500 meters.


Figure 1. Hierarchical quantum network operating in different atmospheric layers. LEO, low Earth orbit; MEO, medium Earth orbit; GEO, geostationary Earth orbit; HAP platform, high-altitude platform; UAV, unmanned aerial vehicle.


"Airborne QKD is an emerging field and now still in its infancy." Prof. Shi from the Airforce Engineering University said. He believes that investigating the airborne QKD is not only meaningful to expand the research scope of quantum key distribution, but also a powerful promotion for the in-depth military applications of quantum information technology.


In the future, with the rapid development of quantum information and the development of the unmanned aerial vehicle (UAV) field, the realization of airborne QKD based on UAV platforms will become the mainstream choice. The architecture of reconfigurable and long-distance hybrid QKD schemes represents a key strategy to the success of a full-fledged quantum network. As the competitive edges of airborne quantum communication, the scalability and flexibility are destined to be upgraded from 100 m to nearly 10~20 km and from point-to-point links to complicated multi-node networks.