Advanced study on continuous-variable quantum key distribution

近年来，人们对连续变量量子信息处理的兴趣与日俱增。报道了CV量子隐形传态、量子密集编码、量子远程克隆和量子密钥分配(QKD)的实验演示。共轭量子信息的一个重要优点是利用现有的技术可以有效地实现量子信息处理中的基本步骤。例如，在室温下工作的零差探测器提供了一种工具，用于以接近单位的效率测量电磁场的正交幅度。对于使用相干态的连续量子密钥分配的情况，只需衰减激光就可以很容易地实现理想光源的制备。此外，人们期望CV量子密钥分发方案能够获得比单光子量子密钥分发方案更高的密钥速率。在本研究中，我们实现了连续变量量子密钥分配(QKD)的即插即用和自由空间实现。在连续量子密钥分配系统中，零差探测器通过将信号与比信号强得多的本机振荡器(LO)叠加来检测微弱的信号光。在传统的“即插即用”(或自动补偿)系统中，两个脉冲穿过相同的光路，并且它们的强度变得相等。在我们的实验中，我们使用了声光调制器，并成功地控制了信号和LO的强度。对于连续量子密钥分配的自由空间实现，双干涉仪的稳定性是一个关键问题。我们利用EOM晶体的双折射特性，在比脉冲的相干时间更长的时间内实现了信号和LO的分离。在这种装置中，信号和LO沿同一光线路径移动，从而大大提高了干涉仪的稳定性。

Recently there have been growing interests in continuous-variable (CV) quantum information processing. Experimental demonstration of CV quantum teleportation, quantum dense coding, quantum telecloning, and quantum key distribution, (QKD) have been reported. One important advantage of CV quantum information is that efficient implementation of the essential steps in quantum information processing is achievable with currently available technologies. For example, a homodyne detector operating at room temperature provides a tool for measuring with near-unit efficiency the quadrature amplitude of the electromagnetic field. For the case of CV QKD using coherent states, the preparation of an ideal light source is easily actualized by just attenuating a laser light. In addition, it is expected that CV QKD will allow one to achieve higher secret key rates than single-photon QKD schemes. In this research, we have achieved "plug & play" and free-space implementations of continuous-variable (CV) quantum key distribution (QKD). In a CV QKD system, a homodyne detector is used to detect a weak signal light by superposing the signal with a local oscillator (LO) whose intensity is much stronger than the signal. In conventional "plug & play" (or auto compensating) systems, two pulses traverse an identical optical path and the intensities of them become equal. In our experiment we use an acousto-optic modulator and have successfully controlled the intensities of the signal and the LO. For free-space implementation of CV QKD the stability of the double interferometer is a crucial problem. We have separated the signal and LO in time longer than the coherence time of the pulse by exploiting the birefringence of EOM crystals. In this setup the signal and LO traverse along the same ray path, so the stability of the interferometer is greatly improved.