时域脉冲平衡零拍探测器的高精度自动平衡∗

摘要

时域脉冲平衡零拍探测器能够直接对光场量子态的正交分量进行测量，是连续变量量子密钥分发系统的核心测量器件，其测量精度与密钥分发系统的额外噪声及安全密钥速率密切相关。本文理论分析了探测器双臂不平衡对测量精度的影响，并实验设计、研制了双臂可精确自动平衡的时域脉冲平衡零拍探测器。通过高精度控制探测器一臂的光纤圆环曲率半径，实现了光纤内部光场强度的精密衰减，进而获得了探测器的精确自动平衡。实验测试结果表明时域平衡零拍探测器双臂具有10−5以上的平衡精度，能够长时间稳定运行，有效避免了探测器的输出电压进入非线性区或饱和区。该探测装置可应用于连续变量量子密钥分发系统，有效降低由于量子态信号探测过程引入的额外噪声，提高系统的长期稳定性。%Quantum key distributions, which could make legitimate communication parties Alice and Bob achieve the same random key with unconditional security, will have broad applications in defense, commerce, and communication. The protocol of the continuous variable quantum key distribution (CVQKD) has many advantages, such as easy preparation of the light source, high detector effciency, and good compatibility with the classic fiber-optic communication systems. In recent years, great progress in the research of CVQKD has been made both theoretically and experimentally. In the protocol, the quadratures of the optical field with Gaussian or Non-Gaussian modulation are employed as the carriers of the key. The quadratures of the pulsed optical quantum states in CVQKD can be detected with a time-domain pulsed homodyne detector. The performance of the detector has great influences on the excess noises and the safe key rate of the quantum communication system. The measurement accuracy, which depends crucially on the common mode rejection ratio and the long-term stability, is the key performance of the detector. In order to improve the accuracy of measurement and avoid saturating the detector, we propose and demonstrate a technique to balance the two output beams of a 50/50 fiber coupler of the homodyne detector automatically. The auto-balance technique, which improves the long-term stability and high common mode rejection ratio, is described in the following. Firstly, the relation between the balance degree and the measurement accuracy is theoretically analyzed in detail. The result shows that a balance degree larger than 10−4 should be reached to ensure a high precision measurement when the intensity of the local oscillator pulse is 108 photons per pulse. Secondly, a fiber-based variable attenuator based on computer-controlled linear stepper motor is designed. The linear stepper motor that is used to drive the fiber coils has a small dimension of 20 cm × 20 cm × 28 cm and a minimum step size of 78 nm, and is controlled through the I/O port of a multifunction data acquisition card connected to a computer. The attenuations of the fiber coils of different radii are detected. The precision of attenuation is estimated to be on the order of 10−6 per 100 nm. The principle of the feedback control is described. A method of changing step-size which depends on the balance degree is proposed to fulfill a fast auto-balance process. Using the auto-feedback-control system, a balance degree of about 1.56 × 10−6 can be achieved. The procedure of auto-balance takes about 1 s, and the evolution curves that represent the transformation process from various unbalanced states to the balanced state are presented. The auto-balance apparatus can ensure that the time-domain pulsed homodyne detector run stably in a long-term with a high common mode rejection ratio. The nonlinear and saturation effects due to the drift of the balance point are eliminated. The presented auto-balance time-domain pulsed homodyne detector can be well integrated into the continuous variable quantum key distribution system, and is expected to play an important role in improving the measurement accuracy and reducing the excess noises of the system. We believe that it could also be found to have potential applications in other areas.