Single photon counting based synchronisation scheme for deep space quantum communication

The global quantum communications network through fibre-optic networks and low-orbit satellites has already made great advance in the past two decades, and therefore the ultra-long-range, high-loss QKD link in outer space attracts more and more interests with greater space exploration capabilities and the envisioning of extraterrestrial bases. Synchronisation system is indispensable for indexing each received photon based on the transmitter clock and provide precious gating window to filter the noise out. Deploying high-precision clocks gives a great synchronisation performance but with complexity and high cost, as well as the complex compensating system for frequency drift. Bright laser pulse transmitted coaxially with the quantum signal scheme is widely used in most of modern QKD system, with the advantages of simple and efficient but can’t tolerant further high loss due to the limited power and APD sensitivities. Here, we propose a scheme to use a bright pulsed laser as clock source and a SPAD for detection. The novelty is that we take advantage of the effect that multi-photon pulses can compress the jitter of SPAD greatly and constant received intensity can avoid time drift. A timing and sync system is developed and demonstrated the performance with BB84 protocol developed for cube-satellite, in which a closed-loop controlled variable optical attenuator is used to maintain the incident intensity higher than single photon and constant, with the feedback from a calibrated SPAD used for monitoring the intensity. The result shows that the detector jitter is reduced from 578 ps to 64 ps when the photon number reaches to 104 per pulse, in which case the loss tolerance can be improved by 44.2 dB without compromising synchronisation precisions and gating efficiency compared with most practical APD, and the photon accumulation method. The de Bruijn is used for modulating the laser pulse to reduce the processing time significantly and recovering the loss in the received photon sequence.