Title: Airborne Quantum Key Distribution and Free-Space Optical Communications. Simulations and experimental verifications for practical quantum communications, including investigations into event-driven detection and processing algorithms.

Quantum key distribution (QKD) is a secure encryption key sharing protocol that addresses potential vulnerabilities in encryption key sharing and encrypted data storage. Although it is well established for optical fibre channels, its use in free space channels is less well developed, but it is attracting rapid growth in investment primarily because satellite platforms are the most efficient route to achieving a global reach network. A free-space network also enables connectivity to remote/nomadic platforms and offers a flexible and rapidly reconfigurable and deployable network, compared to a fixed fibre network. This studentship will investigate the practicality of a QKD communications channel integrated to a optical communications channel for a free-space encrypted data link. Free-space optical channels can transmit over a broad optical spectrum. However, free-space implementations of QKD have focused on a few narrow wavelength bands because they are compatible with current commercial-off-the-shelf components. Free-space transmission presents additional challenges compared to fibre-based systems: atmospheric absorption, scatter and scintillation degrade bit error rate and cause signal drop-out. These are affected by wavelength, but the optimum choice of wavelength remains an open question and depends on a complete understanding of the optical communications system. An additional practical consideration is maintaining a line-of-sight during data transmission to a moving platform such as a drone, aircraft or satellite and, therefore, tracking presents a problem especially if the trajectory is not known beforehand. Possible solutions to these problems include the use of adaptive optics to correct the wavefront error caused by turbulence to maintain signal-to-noise and high speed computational imaging algorithms to determine the centroid of the beam and track it across the field of view. Event-driven detection and neural networks might provide new innovative solutions for high-speed data processing for rapid tracking of the beam and sightline stabilisation for moving platforms. The technical scope of the studentship combines fundamental advances in QKD with engineering problems in optical transceiver design to understand the practical implementation of secure free-space optical communication FSOC. It is supported by related research at Heriot-Watt University developing single photon counting lidar and image processing. The student will work within an established academic research lab team led by Dr Ross Donaldson to develop performance prediction models and simulations of a practical free-space QKD communications channel. Simulations will then be validated using a test rig to collect real data. The test rig will also be used to assess new components (such as single photon detectors) as they become commercially available and the processing algorithms that the student will develop. Industrial supervision by Leonardo will ensure that the student gains a valuable insight into the engineering challenges associated with a practical system and equip the student with a sound understanding of typical use case requirements for optical transceiver design for free space transmission. As a Physics PhD student at Heriot-Watt University, the student will also be enrolled in the Scottish University Physics Alliance and the internal Postgraduate Development