The paper presents a simulator of a controlled atmospheric channel that we developed to search for ways to increase the performance and flexibility of quantum key distribution systems in free space, allowing us to simulate various interfering factors (scattering aerosol, temperature and temporary inhomogeneity of the environment) on one trace. Preliminary testing of the performance parameters of the experimental atmospheric communication channel, such as transmission speed, frame rate, total number of frames in the presence and absence of thermal disturbances, was carried out; the loss of laser radiation power and the stability of its polarization parameters for generating a quantum key encoded in the polarization states of photons were assessed.
The paper presents a simulator of a controlled atmospheric channel that we developed to search for ways to increase the performance and flexibility of quantum key distribution systems in free space, allowing us to simulate various interfering factors (scattering aerosol, temperature and temporary inhomogeneity of the environment) on one trace. Preliminary testing of the performance parameters of the experimental atmospheric communication channel, such as transmission speed, frame rate, total number of frames in the presence and absence of thermal disturbances, was carried out; the loss of laser radiation power and the stability of its polarization parameters for generating a quantum key encoded in the polarization states of photons were assessed.
The results of physical simulation of various optical disturbances in the atmospheric segment of a quantum wireless communication system are presented. Experimental setups for evaluating changes in the characteristics of the quantum key distribution system in an atmospheric communication channel in the presence of optical inhomogeneities in free-space (thermal zone disturbance, scattering water aerosol, incoming airflow) are described. Dynamic changes in quantum bit error rate (QBER) and the quantum sifted key generation rate under the influence of interfering disturbances in the atmospheric channel were obtained. It is determined that the strongest fluctuations in the characteristics of the quantum channel occurred when the source of disturbances was placed directly near the optical collimator connected to the quantum key distribution unit, which housed the single photon source.
Subject of study. This study examined the applicability of quantum key distribution (QKD) technology under free-space atmospheric conditions. Aim of study. The objective was the development of a hardware and software system for investigating the applicability of QKD technology in wireless communication systems based on commercially available modules. The system includes optical blocks for atmospheric optical communication and a training setup for studying QKD in fiber-optic communication lines. Method. The characteristics of a quantum communication channel in free-space conditions were studied through experiments. The channel was formed by the QKD units of the EMQOS 1.0 scientific and educational complex, integrated with optical blocks for atmospheric optical communication. The systems were also upgraded to minimize losses in the quantum channel. Main results. A hardware and software system was developed, within which the transmission of a quantum key through free-space atmospheric conditions over a distance of 180 m was successfully demonstrated. The quantum bit error rate did not exceed 6%. This system enabled the study of the effects of weather conditions on optical losses in the quantum communication channel and the quantum key generation rate. Practical significance. The developed hardware and software system allows the monitoring of the parameters of both classical and quantum communication channels in free space under various weather conditions. Its modular structure enables the integration of other quantum equipment for comparative testing of the performance of quantum communication systems under real atmospheric transmission conditions.
This paper explores the feasibility of using an atmospheric optical channel in a quantum key distribution communication system for communicating with unmanned aerial vehicles. The paper examines the specifics of turbulence in the tropospheric ground layer where the atmospheric link terminals of communication systems are located. The paper describes the methodology and a laboratory experimental setup. The paper measures the key optical, polarization, and network parameters in the presence of disturbance sources in the atmospheric optical segment. An experimental study of the atmospheric channel communication system under various weather conditions was conducted. The results of the experimental study of the influence of climatic factors on the characteristics of the atmospheric optical channel of the communication system showed a significant impact of climatic factors. The data obtained also allow us to formulate recommendations on the feasibility of using a quantum key distribution communication system with an atmospheric optical channel under various climatic factors and atmospheric turbulence.
Subject of study. An optical system, composed of transmitting and receiving telescopes, was designed for quantum key distribution, based on the subcarrier wave modulation method, over an atmospheric channel. Aim of study. The aim of the study was to develop an optical system that allowed the use of single-mode fibers as the input and output of the system, ensured the transmission of an ultralow-power optical signal without introducing additional distortion, and implemented a protocol for distributing a quantum key over an atmospheric optical communication channel at a distance of up to 25 m. Method. An experimental setup with an atmospheric quantum communication channel was developed, which showed stable quantum key distribution without interruptions throughout the measurement process. Radiation propagation simulations were also performed to estimate the insertion loss of the optical signal. Main results. Simulations revealed that the optical losses in the quantum key distribution system amounted to approximately 17 dB. The atmospheric communication channel was studied at distances of 5, 10, and 25 m. Considering all optical elements of the system, including the ferrules of the output and input fibers of the quantum key distribution modules, the attenuation of the optical signal was approximately 8, 13, and 18 dB, respectively. Based on the atmospheric optical losses, the quantum key generation rates were of the order of 500, 350, and 190 bits/s with increasing distance and insertion losses. The probability of a quantum error was in the range of 3% to 5%, which was lower than the threshold of 6%, ensuring the legitimacy of the generated key and excluding the possibility of an attack on the communication channel. Practical significance. The compatibility of the developed optical system with single-mode optical fibers allows its integration into the existing infrastructure of data transmission lines without the need for additional optical radiation converters. Owing to the analyzed characteristics of the optical system for distributing a quantum key over an atmospheric communication channel, it can be applied in communication systems based on the Internet of Things, unmanned vehicles, and other moving objects.
When light is propagated through a channel that is not a waveguide, such as a fiber, it is known as a free-space optical channel. When used in communication systems, free-space optical channels provide the advantages of high communication rates, lower power, and minimal infrastructure installation. When the free-space optical channel has a random medium in it, such as the atmosphere, the distortions will be applied to the phase. The phase distortions can lead to intensity fluctuations and additional beam spreading. To compensate for the distortions adaptive optics can be used, allowing for the stability of the system to be improved. Additionally, post processing techniques on transmitted data can also be used to improve the stability of the channel. By significantly decreasing the number of photons transmitted, a classical communication channel can be considered as a quantum channel. Encoding information into the quantum states of the transmitted photons, quantum communication can be performed. When transmitting on the order of 1 photon per pulse, classical post processing techniques cannot be used to improve the stability of the channel. To stabilize the free-space optical channel, adaptive optics can be used at the transmitter or receiver. As no measurement of the quantum state is performed by the adaptive optics system, no information in the photon is lost. In this work, quantum key distribution will be implemented over the quantum channel. The focus of this work is on the design and study of how the channel degradations can be overcome, as well as the establishment of a free-space optical channel with adaptive optics in a quantum networking testbed. Quantum key distribution systems were simulated over a 30 km maritime channel as well as over a underwater channel. For the 30 km channel, the use of multiple parallel spatial modes results in the highest increases of the secure key rate at the cost of transmitting a lower average number of photons. When adaptive optics is applied, the secure key rates have a small increase, however as the strength of the turbulence increases from weak to moderate, the relative improvement is higher. For the underwater channel, operation at lower depths is optimal as fewer background photons reach the receiver and the channel attenuation is lower. In the experimental setup, it was found that the single photon detectors that are being used are the limiting the channel loss to be at most 6 dB. The free space optical link established between the Electrical and Computer Engineering building and College of Optical Sciences uses a corner cube retro-reflector to yield a distance of 1.45 km. The channel has been characterized at various times of day to determine the strength of the atmospheric turbulence, which ranges from a scintillation index of 0.07 to 2.21. A classical communication experiment was performed, resulting in error-free transmission while the input power to the EDFA before the photodetector was consistently above -25 dBm.
The effect of atmospheric turbulent environment on the performance of free-space quantum communication is analysed. Based on the existing mathematical model of atmospheric turbulent fluid motion, the three most influential physical factors, i.e. wind speed, temperature and humidity, are abstracted out and simulated to study the variation of quantum channel utilisation, quantum channel entanglement, channel capacity and fidelity in amplitude damped channels under the atmospheric turbulent environment parameters. After a comprehensive analysis of the simulation results, it can be preliminarily judged that an increase in all three physical factors will increase the intensity of atmospheric turbulence and reduce the performance of quantum communication, and the magnitude of the influence is: temperature > wind speed > humidity. Therefore, corresponding adjustment strategies should be adopted to improve the communication performance of quantum communication and ensure the reliability of communication.