THE FORMULATION OF REQUIREMENTS FOR THE LENGTH OF THE FIBER-OPTIC LINE WHEN INTRODUCING A TWO-STAGE SYNCHRONIZATION ALGORITHM IN THE SYSTEM OF QUANTUM KEY DISTRIBUTION
The features of time synchronization of stations in systems of quantum key distribution (QKD), taking into account the quantum nature of the photodetector output signal, are analyzed. It is emphasized that the implementation of the synchronization process in the multi-photon mode potentially simplifies the unauthorized access to unauthorized access to unauthorized access to the confidential information. Methods for enhancing the security of the synchronization subsystem are discussed, among which the approach is focused on using a photon pulse as the sync signal with an average number of photons less than 1. The aim of the research is to establish the functional dependence of the length of a fiber-optic communication line (FOCL) on the parameters of single-mode optical fibers and single-photon avalanche photodiodes (SAPD) used to formulate the prospects for using a two-stage algorithm for one-photon synchronization of the receiving and transmitting and coding stations of the autocompensation QKD system without dividing time frames on temporary segments. In the analyzed algorithm at the receiving end, the repetition period and duration of optical sync signals are known. The equipment at the search stage registers the reception of a photon or a dark current pulse (DCP) in the first time frame. If a photon is not registered in the frame, the search continues in subsequent frames. If a photon is accepted when analyzing a time frame, the equipment proceeds to testing, where the second survey is performed only during the gating of a single-photon photodetector. The study shows the prospect of using a two-stage synchronization algorithm for a QKD system using single-mode optical fibers with a minimum attenuation and single-photon avalanche photodiodes with a smaller DCP generation frequency. The implementation of a two-stage algorithm is possible in local networks with a fiber optic length of 20 ... 30 km on almost any single-mode optical fibers in the case of SAPD with an DCP generation frequency of less than 10 Hz. The construction of communication lines with a length of more than 50 km is possible only with a focus on the use of SAPD with a frequency of DCP generation of less than 1 Hz.