Features of detection of a single-photon pulse at synchronisation in quantum key distribution systems

The object of this study is a two-pass fiber-optic quantum key distribution system (QKDS) with phase coding states of photons in synchronisation mode. The QKDS is based on a scheme with automatic compensation of polarisation distortions. We use single-photon avalanche photodiodes (SPAD) as optical radiation detection devices. We also evaluate the impact of parameters used in QKDS optical radiation detectors on probabilistic and time characteristics of the detected time interval with a weak optical pulse in sync. In this paper, we describe the methodology of the design process to detect a time interval with an optical pulse during synchronisation taking into account the features used in QKDS SPAD. The aim of our work is to evaluate the impact of QKDS changing hardware synchronisation settings on probabilistic and time characteristics of the detection time interval. We describe an algorithm that we have developed to synchronise a QKDS that considers the time to restore the operating state of a photon after registration during synchronisation. For evaluation, we report computer modelling results of the design process to detect a time interval according to the changing parameters applied in SPAD quantum key distribution systems.