Continuous-variable quantum key distribution (CV-QKD) offers the advantages of high secret key rates in metropolitan areas. Optimization of modulation variance is an efficient method to improve the secret key rate of CVQKD system. However, in practical CV-QKD system, inevitable slight parameter fluctuation could occur after the modification of modulation variance, and controlling the modulation variance with arbitrary accuracy is also difficult. In this paper, we propose a two-step optimization for practical CV-QKD. The first step is to determine the optimal working state by combining the modulation variance optimization with error correction matrix optimization. The second step is to optimize the rate-adaptive reconciliation parameters to compensate the loss of secret key rate caused by inaccuracy modulation variance. Our results show that the secret key rate can be improved by 17.8% in comparison to one-step optimization method. Our method can be conveniently applied to CV-QKD protocol with homodyne and heterodyne detection, which will pave the way to the deployment of high stable and high performance for CV-QKD.
The throughput of error correction is the main bottleneck of continuous variable quantum key distribution (CV-QKD) postprocessing. Implementing the decoder of low-density parity-check (LDPC) codes based on FPGA with limited precision can improve the decoding throughput significantly. However, the limited precision on FPGA results in the existence of residual error-bits after decoding, which lowers the secret key rate and restricts the application of high-rate real-time CVQKD system. In this paper, an efficient decoding scheme is proposed to erase the residual error-bits and decrease the frame errors rate (FER), where the decoding process into two stages and some values of initial Log Likelihood Ratio (LLR) are adjusted according to the proposed principles before starting the second-stage decoding. For the rates 0.2 and 0.1 LDPC codes, numerical results demonstrate that the proposed decoding scheme decreases the FER obviously and the throughputs of 152.47Mbps and 88.32Mbps are achieved, which can be applied to support high-speed CV-QKD system under transmission distance of 25km and 50km respectively.
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