Advanced study on continuous-variable quantum key distribution

近年来，连续变量量子信息处理受到越来越多的关注。本文报道了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发的实验演示。CV量子信息的一个重要优点是，量子信息处理中的基本步骤的有效实现可以通过当前可用的技术来实现。例如，在室温下操作的零差检测器提供了用于以近单位效率测量电磁场的正交幅度的工具。对于使用相干态的CV QKD的情况，理想光源的制备容易通过仅衰减激光来实现。此外，预计CV QKD将允许实现比单光子QKD方案更高的密钥速率。在这项研究中，我们实现了连续变量（CV）量子密钥分配（QKD）的“即插即用”和自由空间实现。在CV QKD系统中，零差检测器用于通过将信号与强度比信号强得多的本振（LO）叠加来检测弱信号光。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。在实验中，我们使用了声光调制器，并成功地控制了信号和本振的强度。在自由空间实现CV QKD时，双干涉仪的稳定性是一个关键问题。利用电光晶体的双折射特性，我们在比脉冲相干时间更长的时间内分离了信号和本振。在该装置中，信号和本振沿沿着相同的光路通过，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.