EVALUATION OF THE FUNCTIONAL ELEMENTS INFLUENCE ON THE PARAMETERS OF THE QKD SYSTEM BASED ON B92 PROTOCOL

Abstract

The influence of the parameters of functional elements on the energy, time and probabilistic
characteristics of the quantum key distribution system (QKD) based on the B92 protocol is studied.
The dependences of the probability of writing correct and erroneous bits into a raw quantum
key sequence are plotted for various lengths of a fiber-optic communication line (FOCL) and the
use of various lasers (EML-laser; DFB-laser; VCSEL-laser and FP-laser) and photodetector
modules (id201; id210; id220; id230). Thus, changes in the probability of writing a correct bit into
a raw quantum key sequence are much more significant than changes in the probability of writing
an erroneous bit (50.9 times versus 3.3 times with FWHM=80 pm and a change in the length of
the FOCL from 10 to 100 km). This is due to the fact that with an increase in the length of the
FOCL, the probability of the absence of registration at the receiving station of photons or dark
current pulses (DCP) sharply increases. Numerical material indicates a direct proportional dependence
of the probability of writing an erroneous bit on the frequency of generation of noise
pulses of single-photon avalanche photodiodes (SAPD). So, with an increase in the frequency of
occurrence of DCP by 60 times (from 100 to 6000 Hz), the probability of recording an erroneous
bit also increases by 60 times (for example, with a FOCL length of 100 km – 6.39 versus 383.3).
It has been established that the root-mean-square deviation of the photon delay time is directly proportional to the length of the FOCL and the width of the laser spectrum. With a spectrum width
of FWHM=10 pm and an increase in the FOCL length from 10 to 100 km (by a factor of 10), the
standard deviation of the photon delay time also increases by a factor of 10 (from 4.16 to 41.6 ps).
To achieve the best performance of the QKD system as a whole, it is advisable to use a laser with
a minimum width of the radiation spectrum, for example, an EML-laser. However, EML-lasers are
considered the most complex and expensive of all the considered types of lasers, so the use of
EML-lasers significantly increases the cost of the entire QKD system.