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在连续变量量子密钥分发的实验系统中，由于调制器受限于分辨率有限的驱动电压，理想的高斯调制会退化成离散极化调制，进而引发系统性能的下降。本文提出并研究了线性光学克隆机改进的离散极化调制连续变量量子密钥分发方案。在接收端插入线性光学克隆机能够有效地补偿由幅度和相位离散化产生的综合效应所造成的系统性能损失，实现整体性能的提升。本文推导出了所提方案在非理想外差探测下可组合安全密钥率的表达式，并进行了数值仿真，仿真结果表明所提方案不仅能够通过灵活调谐线性光学克隆机的相关参数优化安全密钥率、提升过量噪声抗性，还能有效克服有限码长效应对安全性的影响，为推动连续变量量子密钥分发的实用化发展提供了切实有效的方法。In experimental setups of continuous-variable quantum key distribution (CVQKD) independently modulating the amplitude and phase of coherent states, the ideal Gaussian modulation will be degraded into discretized polar modulation (DPM) due to the finite resolution of the driving voltages of electro-optical modulators. To compensate for the performance degradation induced by the joint effect of amplitude and phase discretization, linear optics cloning machine (LOCM) can be introduced at the receiver side to reduce the impact of channel excess noise. Implemented by linear optical elements, homodyne detection and controlled displacement, LOCM introduces extra noise that can be transformed into an advantageous one to combat channel excess noise by dynamically adjusting the relevant parameters into a suitable range. In this paper, the prepare-and-measure version of LOCM DPM-CVQKD is presented, where the incoming signal state enters a tunable LOCM before being measured by the nonideal heterodyne detector. The equivalent entanglement-based model is also established to perform security analysis, where the LOCM is reformulated into combining the incoming signal state and a thermal state on a beam splitter. The composable secret key rate is derived to investigate the security of LOCM DPM-CVQKD. Simulation results demonstrate that the secret key rate is closely related to the tuning gain and the transmittance of LOCM. Once the two parameters are set to appropriate values, LOCM allows the promotion of the secret key rate of DPM-CVQKD, as well as its resistance to excess noise. Meanwhile, taking finite-size effect into consideration, LOCM can also effectively reduce the requirement for the block size of the exchanged signals, which is beneficial to the feasibility and practicability of CVQKD. Since the performance of LOCM DPM-CVQKD is heavily reliant on the calibrate selection of relevant parameters, further research may concentrate on the optimization of LOCM in experimental implementations, where machine learning related methods may be exploited.
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Keywords:
- Quantum key distribution /
- Continuous variable /
- Discretized polar modulation /
- Linear optics cloning machine
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