Free-space quantum communication for security and defence applications

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.