The Quantum Signal Transmitter (satellite payload)

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The Payload is divided into five subsystems: the Central Payload Controller (CPC); the Optical Transmission Alignment Module (OPTAM); the Downlink Laser Beacon (DLB); the Earth Observation Camera (EOCAM); the Quantum Subsystem proper. The SPOQC mission is unique in its ambition to launch a dual quantum source operating a Continuous-Variable (CV) QKD protocol (engineering model developed by Dr Rupesh Kumar and his support team at the University of York); and a Discrete-Variable (DV) QKD protocol (engineering model developed by Dr Siddarth Joshi and his support team at the University of Bristol). The DV source will employ weak coherent pulses (approximate single photons) in a decoy-state implementation of the well-known BB84 (Bennett-Brassard) QKD protocol. The source will use two separate wavelengths, in order to increase the quantum key rate. The CV source will transmit modulated quantum light pulses, alongside a reference beam for measurement. It is designed to explore the potential advantages of CV, including daylight operation. The source outputs combine in a passive optical device in the satellite, before entering the OPTAM for transmission to the ground. Only one source (DV or CV) will operate at any time, with the corresponding receiver being active.