A program for quantum key distribution in wireless communication has been developed.

Today, there are numerous commercial companies offering equipment for implementing Quantum Key Distribution (QKD) in fiber-optic communication lines (FOCL). Free-space QKD systems, such as atmospheric optical communication lines (AOCL), have several advantages but have yet to find widespread application due to the lack of serial devices that implement QKD technology in the atmosphere.

Free-space quantum communication devices currently exist only as experimental setups designed to solve specific scientific tasks. There are examples of systems utilizing horizontal atmospheric paths and artificial satellites, but they are not yet ready for practical use due to several unresolved technical issues. However, despite all the challenges, the prospects for developing QKD in free space appear quite promising. One of the most interesting solutions is a modular approach, where serial AOCL terminals are connected to commercial QKD blocks designed for FOCL. This solution was proposed by scientists from MTUCI.

“This allows for minimizing time and material costs. It is essential to consider that simply connecting different devices does not always enable stable distribution of quantum keys at the required speed. To compensate for dynamic losses on atmospheric paths, adaptive optics methods can be employed, and background illumination should also be taken into account, which necessitates conducting systematic research on the influence of numerous factors on the speed of quantum key formation on atmospheric paths,” noted Sergey Yuryevich Kazantsev, Doctor of Physical and Mathematical Sciences, Professor at the Department of NTC MTUCI.

At MTUCI, an hardware and software complex has been created to investigate the technology of quantum key distribution (QKD) in wireless communication systems based on serial modules.

“The overall scheme of the complex consists of two optical modules for signal reception and transmission in open space, the EMQOS 1.0 training setup, and fiber-optic cables. The terminals of the atmospheric optical communication line are installed on the roofs of two MTUCI buildings, and the training setup is connected to one of them via a fiber-optic cable. The distance between the modules is 180 meters, and their height above the ground is about 30 meters. The EMQOS 1.0 training setup employs a two-pass scheme for transmitting the quantum key via the BB84 protocol with phase encoding. This allows for investigating the impact of atmospheric conditions on the stability of quantum key transmission and developing methods to compensate for emerging disturbances,” explained Yuriy Mironov, Associate Professor, Candidate of Technical Sciences.

The complex allows for studying the impact of dynamic losses on the atmospheric path, as well as the effect of background illumination on the bit error rate and the speed of quantum key distribution.

“For this, other devices were connected to the complex instead of EMQOS 1.0: a radiation source with a wavelength of 1530-1550 nanometers, two photodetectors, and a spectrometer, which enabled us to verify the correlation between optical losses in the quantum channel and losses in the information and test channels of AOCL under various weather conditions. Moreover, it allowed for monitoring background illumination from natural and artificial radiation sources using a spectrometer and a polarimeter,” clarified Natalia Vladimirovna Pchelkin, a participant in the experiments with the developed complex and Candidate of Technical Sciences, Associate Professor at the Department of NTC.

Measurements showed that weather conditions do not significantly affect the change in photon polarization. However, slow polarization drift caused by external influences on the optical cable can considerably deteriorate the system's performance. Therefore, additional spectral filters were added to EMQOS 1.0,” reported Kirill Erokhin, a student at MTUCI and laboratory assistant in the NTC department's educational laboratories.

After installing a standard commercial optical filter in front of the single-photon detector of the EMQOS 1.0 complex, it became possible to transmit the quantum key over a distance of 180 m and examine the impact of weather conditions on the quantum key generation rate.

The created hardware and software complex allows for monitoring parameters of both classical and quantum communication channels implemented in free atmosphere under various weather conditions, and the modular structure of the complex enables the connection of other quantum equipment for comparative testing of the efficiency of quantum communication systems under real atmospheric path conditions.