In the space channel continuous variable quantum key distribution (CVQKD), the quantum coherent state modulation system is more versatile with the classical optical communication equipment, and has better compatibility with the fiber channel. It is one of the future space channel CVQKD network construction schemes. Aiming at the effects of beam expansion, absorption, scattering, and drift of optical quantum signals in turbulent channels, as well as the phase difference jitter between local oscillator light and signal light, we construct a simulation model of CVQKD key rate under turbulent channels. The model focuses on analyzing the influence of turbulent channel parameters, phase delay and detection method on the system key rate. The results show that the total excess noise of the system is positively correlated with the transmission distance, the delay time between the local oscillator light and the signal light, and the intensity of atmospheric turbulence. Heterodyne detection has a higher key rate in short-distance transmission, and homodyne detection can achieve longer transmission distance. The simulation results can provide reference for the design and optimization of actual free-space CVQKD systems.
In the common time division multiplexing CVQKD system, the phase difference jitter between the local oscillator light and the signal light will increase the channel excess noise and reduce the system key rate. In this paper, the effects of turbulent channel parameters, phase delay and detection methods on system noise and key rate are studied by establishing a simulation model of CVQKD under turbulent channels. The principle of the simulation model in this paper includes key rate theory, turbulent channel transmission efficiency theory, interruption probability theory, local light and signal light phase difference jitter theory. Then, the corresponding simulation work is carried out, and the channel excess noise caused by the phase difference jitter between the local oscillator light and the signal light is discussed.
According to the theoretical model and under specific simulation parameters, the following simulations are carried out: the probability distribution of the system transmittance (Fig. 3), the influence of different time delays on the total system noise (Fig. 4), the influence of different turbulence intensities on the total system noise (Fig. 5), and the influence of different turbulence intensities on the system key rate (Fig. 6). It can be seen from Fig. 3 that under the condition of a fixed transmission distance, there is a peak in the transmittance distribution, and the probability density first decreases and then increases with the increase of distance. It can be seen from Figs. 4 and 5 that with the increase of time delay and atmospheric turbulence intensity, the system key rate decreases. It can be seen from Fig. 6 that heterodyne detection has a higher key rate in short-distance transmission, and homodyne detection can achieve longer transmission distance.
Turbulent channel CVQKD has important applications in wide-area quantum communication networks. It is necessary to study the influence of turbulent channel structure parameters on free-space CVQKD system. In order to meet this demand, the CVQKD model of turbulent channel is established in this paper, and the effects of turbulent channel parameters, local oscillation light and signal light delay time and different detection methods on CVQKD system noise and key rate are studied. According to the simulation results, the total excess noise of the system increases slowly and then rapidly with the increase of distance. Increasing the delay time will increase the total noise of the system. The actual CVQKD system needs to select the delay time according to the specific situation. The increase of atmospheric turbulence intensity will increase the total noise of the system, and the phase noise spectral density is positively correlated with the product of signal transmission distance and atmospheric turbulence intensity. The enhancement of atmospheric turbulence will reduce the channel transmittance, increase the system interruption probability and excess noise, and make the system key rate drop rapidly. Heterodyne detection has a high key rate in short-distance transmission, and homodyne detection can achieve longer transmission distance. In the actual system, the receiving antenna size can be appropriately expanded, the delay time between the local oscillation light and the signal light can be optimized, or the adaptive optical system can be used for phase compensation, and the appropriate optical carrier wavelength and detection mode can be selected, so as to improve the key rate or the safe transmission distance of the system. This paper can provide reference for the design and optimization of high performance and practical CVQKD system under turbulent channels.
