Wired and fibre-based communication are the backbone of the current global communication network, thanks to their large bandwidth, reliability and stability, capability to serve many users and low cost over their lifetime. However, they show several important limitations when applied in the defence and military domain, mostly due to the possibility of physical attacks and disruption, the requirement of a pre-installed infrastructure and limited flexibility. Free-space and wireless communication work perfectly as a complementary technology and become enabler of many fundamental applications in the military domain. Free-space quantum communication is a growing field that promises to empower and enable new cryptographic applications and distributed sensing and computing use cases. In this paper, we review the most prominent applications of free-space quantum communication in the military domain, with a focus on Quantum Key Distribution. Then we critically analyse important results in the field in the last 15 years, focusing first on ground-to-ground implementations and then on mobile platforms, concluding with satellite-based quantum communication.
Wired and fibre-based communication are the backbone of the current global communication network, thanks to their large bandwidth, reliability and stability, capability to serve many users and low cost over their lifetime. However, they show several important limitations when applied in the defence and military domain, mostly due to the possibility of physical attacks and disruption, the requirement of a pre-installed infrastructure and limited flexibility. Free-space and wireless communication work perfectly as a complementary technology and become enabler of many fundamental applications in the military domain. Free-space quantum communication is a growing field that promises to empower and enable new cryptographic applications and distributed sensing and computing use cases. In this paper, we review the most prominent applications of free-space quantum communication in the military domain, with a focus on Quantum Key Distribution. Then we critically analyse important results in the field in the last 15 years, focusing first on ground-to-ground implementations and then on mobile platforms, concluding with satellite-based quantum communication.
Quantum key distribution is now a mature quantum communication protocol which allows the verifiably secure sharing of encryption keys between two communicating parties. It seeks to address potential vulnerabilities of data transmission and storage, offering a realistic possibility to share encryption keys which are robust to eavesdropping attacks and future-proof against hacking. Fibre-optic implementations of quantum key distribution currently have a limited practical transmission distance, of less than 400 km, making commercial applications limited. Quantum-specific amplifier/repeater technology is not yet mature enough to increase the transmission distance to achieve global capabilities. Optical fiber is also impractical and expensive for applications where a remote area or moving platform are involved. In recent years, long-distance free-space quantum communications using low-Earth orbit satellites has seen an increase in interest from the academic community as well as from industrial organisations and national research institutes. The source of this new interest was a series of proof-of-principle demonstrations of satellite-based quantum key distribution in 2017. The use of free-space channels implementing airborne or satellite platforms also opens a range of new applications for quantum communications, as they allow coverage of remote areas, moving platforms, and also avoid the requirement of spooling fibre through volatile regions. This presentation will give a general introduction to satellite-based quantum communications, an overview of the field, and discuss future endeavours. The talk will also include an overview of our research into novel photonic technology, such as the use of detector array technology, for the optical ground station receiver.
The optical quantum network plays an important role in quantum information technology, enabling important applications including quantum communication, distributed quantum computing, quantum remote sensing, etc. Without the fiber connection limitation, free-space quantum network is free from fiber scattering loss, and enables more flexible quantum network connections. In recent years, rapid progress has been achieved in free-space quantum networks using fixed ground stations, satellites, and mobile platforms. Fixed ground stations are the primary testbeds for free-space experiments, and link distances of up to 144 km have been demonstrated between selected locations. Satellites, free from atmospheric turbulence and absorption, further boost the distance to over a thousand kilometers, enabling intercontinental connections. Besides fixed ground stations and satellites, mobile platforms offer highly flexible connections to end users, as fully presented using drones, after preliminary works using balloons, aircraft, helicopters, etc. Here we review the free-space experiments based on these three platforms, and analyse and compare their characteristics, advantages, and limitations in different scenarios, respectively. In the prospectives, we discuss their unique and complementary roles, and the key challenges and enabling technologies toward the ultimate goal of a practical quantum network, in conjunction with the fiber-based quantum network.
Due to the recent rapid growth of computing technologies, today's encrypted information based on classical computational complexities will soon no longer remain secure. Quantum cryptography, or quantum key distribution (QKD), relying on the physical laws of quantum mechanics, stands out as a more powerful cryptographic solution that enables the secret key to be shared securely over publicly unsecured communication channels in the presence of eavesdroppers. QKD can be possibly implemented over both optical fiber and free-space optical (FSO) links. Compared to optical fiber, FSO has the immediate advantage of flexibility and cost-effectiveness in terms of infrastructure deployment. More interestingly, recent advancements in satellite communications using FSO links promise the future of a global QKD network for highly secure applications. This paper reviews the development of QKD over FSO for both terrestrial and satellite channels, which highlights the most up-to-date QKD protocols. Owing to how the information is encoded, different QKD protocols are classified into technology-focused categories, where their properties are reviewed and compared. The feasibility of current QKD protocols is then investigated with respect to its specific application scenarios for terrestrial and satellite FSO links, equipped with results from both theoretical and experimental studies. Based on the latest free-space QKD achievements, novel perspectives for future developments and challenges to enhance the secrecy performance and reduce implementation cost are then summarized and comprehensively discussed. The potential combination of QKD and physical layer cryptography for the newly emerging wireless mobile communications networks involved with unmanned aerial vehicles (UAV) is also introduced.
Abstract Concerted efforts are underway to establish an infrastructure for a global quantum Internet to realise a spectrum of quantum technologies. This will enable more precise sensors, secure communications, and faster data processing. Quantum communications are a front‐runner with quantum networks already implemented in several metropolitan areas. A number of recent proposals have modelled the use of space segments to overcome range limitations of purely terrestrial networks. Rapid progress in the design of quantum devices have enabled their deployment in space for in‐orbit demonstrations. We review developments in this emerging area of space‐based quantum technologies and provide a roadmap of key milestones towards a complete, global quantum networked landscape. Small satellites hold increasing promise to provide a cost effective coverage required to realise the quantum Internet. The state of art in small satellite missions is reviewed and the most current in‐field demonstrations of quantum cryptography are collated. The important challenges in space quantum technologies that must be overcome and recent efforts to mitigate their effects are summarised. A perspective on future developments that would improve the performance of space quantum communications is included. The authors conclude with a discussion on fundamental physics experiments that could take advantage of a global, space‐based quantum network.
Long-distance quantum communication is one of the most important tasks in quantum information science. It contains two main motivations: one is the fundamental interests, and the other is the practical requirements. The physical distance of quantum communication over free-space channel has been extended from several meters in the lab to 1000 km through satellite. In this mini-review, we emphasize the progress made toward the global quantum network, mainly following the world’s first satellite-to-Earth quantum communication in 2017. This includes the development of commercial small-scale quantum satellite constellations and high-orbit satellites capable of all-time coverage, as well as the feasibility of quantum communication network. On this basis, we then discussed the possibility of cross-application of free-space quantum communication technology in fundamental physics, precision measurement, and even astronomical observations. Free-space quantum communication presents both challenges and opportunities, playing a crucial role in the future global quantum network.
Worldwide free-space quantum communications (QC) experiments over the past decade are reviewed and discussed with attention to technological QC trends. Experiments reported in the open literature have included those conducted along horizontal propagation paths of varying distances, as well as communication paths from ground-to-aircraft, ground-to-space, and demonstrations in the laboratory. Available data characterize propagation distances, transmission speeds, quantum key distribution (QKD) protocols, and quantum bit error rates (QBER). While fiber optic implementations of quantum communications technologies are currently being tested for communications infrastructure it is important to also consider that free-space quantum communications will play an important role in securing such applications as earth-to-satellite, end of line connects, and defense implementations.