ReQON: Reconfigurable Quantum Optical Networking

Quantum Key Distribution (QKD) offers totally secure communications and can be realised across large distances using Free Space Optical Communications (FSOC) links. QKD systems using satellites are under heavy development, (e.g. the EU’s “EAGLE-1”), however such systems require permanent ground and space infrastructure. We present the ReQON (Reconfigurable Quantum Optical Networking) programme with the objective to demonstrate deployable, mobile Quantum communications links with reduced requirements for infrastructure and cost, therefore enabling QKD to be used in a wider variety of applications. In this first step of the programme, we will focus on a laboratory demonstration of “asymmetrical” communication links, with one “active” terminal which contains transmission lasers, reception systems, tracking systems and adaptive optics (AO); and one “passive” terminal, which contains a receiver and a retro-modulator to return modulated light back to the active terminal without the need for heavy and power-hungry lasers or tracking systems. The passive terminal may then be mounted on a low cost platform, such as a Unmanned Aerial Vehicle, and quickly deployed above the area of interest. Research will focus on combining two key areas in which the partners have had previous success – AO and retro-modulators. Durham University (DU) will focus on the development of the active terminal – a transportable Optical Ground Station (OGS) – with significant attention on creating the next generation of AO systems that can optimise the efficiency of the link even in the worst-case conditions. AO becomes critical in Quantum communications links as it is not acceptable to lose photons due to turbulence effects. New wavefront sensing solutions are required to operate through “deep” turbulence conditions found in horizontal links. Convolution Neural Networks operating on focal plane images detecting the electric field are a promising solution to this issue. They do not require dedicated wavefront sensing devices and can improve the performance of the OGS by detecting the electric field, hence better accounting for “branch points” and “branch cuts”. DU has significant experience in developing AO systems for astronomy, including the CANARY laser guide star demonstrator and leading development of the control system for the HARMONI Extremely Large Telescope instrument. Dr Andrew Reeves has recently joined the instrumentation group having previously led development of the German Aerospace Centre’s (DLR) AO system for their state-of-the-art OGS, the test facility for EAGLE-1. The University of British Columbia (UBC) will develop the passive terminal, including the next generation of retro-modulators that are compatible with QKD. Prof Jonathan Holzman and his lab has previously specialised in the development of retro-modulators for optical communications. New techniques to improve the modulation depth will be explored - the retro-modulator must be capable of deep modulation to ensure an acceptable signal to noise ratio when sending low power quantum signals. Entanglement of photons with differing polarisation state is the most suggested solution for QKD across FSOC links, this will likely be achieved by enhancing their ability to quickly and efficiently control the polarisation as well as amplitude of the light. The two ReQON terminals will be brought together for full system testing through turbulence at DU. DU and UBC are a unique team capable of delivering accessible, secure communications to a variety of applications including disaster relief, defence, finance and providing secure communications to remote areas and less developed nations.