Abstract:

The unconditional security of continuous-variable quantum key distribution has been demonstrated under theoretical conditions. However, due to the non-ideal experimental devices in the experimental system, the ideal continuously distributed quantum signals are degraded, which in turn triggers the degradation of the system performance. To bridge the gap between the theory and practice of continuous-variable quantum key distribution, the article considering the impacts of the limited resolution of the modulator at the transmitting end and the detector at the receiving end on the system performance, proposed a model based on discretized polar modulation and coarse-grained measurement with finite range. It also evaluated the bias characteristics of parameter estimation under this model and its impact on the security key rate. Numerical results show that the coupling effect of the discrete effects at the transmitter and the receiver causes deviations in the estimated value of excess noise, leading to an initial increase followed by a decrease and ultimately a sharp decline in the system security key rate. When the channel transmission is between 0.2 and 0.6, the estimation deviation of excess noise can lead to a severe underestimation of the system security key rate, which in turn restricts the maximum transmission distance of the system. The finite range effect of the homodyne detector is the most significant factor inducing the estimation deviation of excess noise, which can be effectively compensated for by adjusting the modulation variance.

Key words: quantum key distribution, continuous-variable, discretized polar modulation, coarse-grained measurement with finite range