Modelling and experimental testing of an optical synchronisation beacon designed for high-loss satellite quantum communication

Long-distance free space quantum key distribution based on CubeSats can be used to establish global quantum secure communication networks, with potential commercial applications benefitting from the low cost of its design and launch. Detecting single-photon level optical pulses sent from space requires highly accurate and robust timing systems to pick out signals from the noise. For such high-loss applications, we envisage a low-repetition (sub-MHz) beacon laser emitting short (ns) high-peak-power pulses from which interpolated quantum signal arrival windows can be derived. We firstly study theoretically the effects of jitter on the efficiency of gating quantum signals including all important jitter sources, and then experimentally investigated it by changing the clock jitter, and the result shows that the greater jitter will reduce the gating rate of the signal. The experimental interpolation error is tested against loss under laboratory conditions giving results close to our model. We also found that the jitter introduced by the Doppler effect can be ignored with a repetition rate larger than 1 kHz. This model can be directly used for the performance analysis and optimisation of all quantum and non-quantum systems using similar synchronisation schemes over terrestrial free space or fibre.