On the Integration and Control of Quantum Key Distribution over Free-Space Optics and 5G Networks

With ever increasing requirements for security and capacity, Quantum Key Distribution (QKD) over Free-space Optics (FSO) offers an alternative solution for 5G and beyond. The Innovate-UK AirQKD project addresses the possibility of metropolitan-scale 'last-mile' quantum secure connectivity through FSO-QKD. However, the practical implementation of FSO-QKD faces a multitude of engineering problems across software, hardware, and security. In this paper we examine the challenges of system architecture, integration, QKD transmission via FSO, and the post-processing of keys. We illustrate the design of an integrated system architecture with quantum and classical domains combining advanced FSO-QKD hardware and software, demonstrating its functionality for transmission, post-processing, network control and key management. Moreover, we show the results of an FSO-QKD link over 135 m at BT's Adastral Park in Suffolk, UK, and demonstrate a photon count rate up to 585 kcps over the channel loss of 15.9 dB, with 12 kHz background noise. The post-processing results show the potential key generation capability of the system and verify the feasibility of the successful transmission of orthogonal polarization states.