摘 要 量子密码术是基于量子力学测不准原理和单量子态不可克隆定理发展起来的一种新型保密通信技术，在理论和实验上已被证明是绝对安全、不可破译的。由于其绝对安全性，量子密码术在军事、外交、通信、电子商务等领域有广泛的应用前景。 论文首先对量子密码术的基本原理、各种量子密钥分配（QKD）协议以及各种量子编码技术作了比较全面的介绍，进而对各种QKD系统的结构、特性及其优缺点作了系统的分析和论述，指出了即插即用QKD系统存在着特洛依木马光子攻击的危险，存在不安全因素。而相位编码和偏振编码QKD系统受环境干扰因素多，存在着不稳定、损耗大等缺点。论文进而提出了一种新型的相位调制偏振编码的QKD系统，对这种QKD系统的工作原理、实验方法、实验结果作了详细的论述。并提出了利用相位调制偏振态的方法实现六态编码的新方案。 因为QKD系统是对单量子态的操作，因此QKD系统本质上是量子力学系统。量子信息的编码、解码过程须用量子力学方法来描述，为此，论文首先发展了一套全量子力学的描述方法，对QKD系统所有的光学器件及其所构成的量子编码器和解码器，全部用量子力学算符来表示。对用来编码的所有量子态，全部用量子力学的态矢量来表示。这种描述方法的优点是能对QKD系统的量子信息编码、解码过程给出严格的理论描述。在此基础上，论文首次提出了利用相位调制实现偏振编码的新方法。即用两个正交的偏振态通过偏振干涉合成一个新的偏振态，这个合成的偏振态由两个正交偏振态的位相差决定，改变位相差，就可以合成各种不同的偏振态，而位相差的改变可以通过位相调制器来实现，从而可以通过位相调制方法实现对光子偏振态的编码和解码。根据这种原理，我们首先研制成功量子编码器件—相位-偏振控制器，然后利用相位-偏振控制器研制成四态量子编码器、解码器以及偏振补偿器。四态量子编码器可以随机地制备45o线偏振、135o线偏振、右旋圆偏振、左旋圆偏振等四种非正交偏振态的光子，四态量子解码器可以随机地制备45o线偏振、135o线偏振、右旋圆偏振、左旋圆偏振等四种非正交偏振态的检偏器。偏振补偿器可以对任意偏振态进行精确补偿，使其恢复原来的偏振状态。这种偏振编码方法的优点是调制电压低（10V左右）、调制速率高（可以达到GHz），并且可以进行偏振补偿，从而有效地降低误码率。 论文提出了利用相位-偏振控制器实现六态量子编码的新方法，并提出了一种扩展六态量子编码新协议。根据该协议，Alice只需产生：0、V0/2、V0、3V0/2四种控制电压（ V0为相位调制器的半波电压），就可以产生：0o 线偏振、45o 线偏振、90o 线偏振、135o 线偏振、右旋圆偏振 、左旋圆偏振等六种非正交偏振态；同时，Bob也只需产生：0、V0/2、V0、3V0/2四种控制电压，就可以制备0o 线偏振、45o 线偏振、90o 线偏振、135o 线偏振、右旋圆偏振 、左旋圆偏振等六种非正交偏振态的检偏器，对Alice发送的光子的偏振态进行检测和解码。然后，Alice和 Bob通过比对测量基以后就可以获得共同的密钥。这种扩展六态量子编码协议可以极大地简化量子密钥的安全性分析，并降低了窃听者获取信息的几率。

Abstract Quantum key distribution (QKD) is a new kind of cryptography, which is based on the Heisenberg’s uncertainty principle and the no-cloning theorem of unknown quantum state. It can be proven theoretically to be unconditional secure. Because of its absolute security, quantum cryptography has potential application in many fields such as military, diplomacy, communication and electronic commerce. After a comprehensive introduction to the progress in quantum cryptography, the principles of quantum key distribution and the protocol of quantum key were discussed in this paper. Furthermore, the characteristic of many kinds of QKD system has been analyzed in detail. Following this discussion, concentration was focused to discuss a new kind of QKD system proposed firstly in this paper, which is based on polarization coding by phase modulation. The principles of this new QKD system were described by complete quantum mechanics method. The experimental method and the experiment result were discussed in detail. Moreover, a new six-state QKD implementation based on polarization coding by phase modulation and an expanded six-state protocol were proposed in this paper. Because QKD system operated on individual quanta state, it is a natural quantum system. The encoding and decoding of quantum information in QKD system should be described by quantum mechanics method. In order to understand the quantum information processing in QKD system, complete quantum mechanics method was employed to describe QKD system. All of the optical components used in QKD system were described by quantum mechanics operators, and all of quantum states used to encode quantum information were described by quantum state vectors. Based on this theory, a new polarization coding method firstly proposed in this paper was analyzed theoretically in detail. According to the superposition principle of quantum mechanics, any polarization state of a photon can be synthesized by two orthogonal polarized states. And the synthesized polarization state is only determined by the phase difference between the two orthogonal polarized states. Thus by modulating the phase difference, any polarization state can be produced. Following this idea, a quantum encoder, called phase-polarization modulator, has been designed to produce all polarization states. By means of the phase-polarization modulator, a four-state quantum encoder, a four-state quantum decoder and a polarization compensator have been made. The four-state quantum encoder can randomly produce four nonorthogonal polarization states such as 45o linear polarization, 135o linear polarization, right-hand circular polarization and left-hand circular polarization. The four-state quantum decoder can randomly produce four nonorthogonal polarization analyzers such as 45o linear polarization analyzer, 135o linear polarization analyzer, right-hand circular polarization analyzer and left-hand circular polarization analyzer. The polarization compensator can provide precise compensation for any polarization state and can make a polarization photon return to its original polarization state. The advantages of the polarization coding by phase modulation are lower controlling voltage (10V), higher bit rate and lower error rate. A new implementation for six-state quantum key distribution and an expanded six-state protocol are also proposed in this paper. Based on polarization coding by phase modulation technique, Alice can produce six nonorthogonal polarization states such as 0o linear polarization, 45o linear polarization, 90o linear polarization, 135o linear polarization, right-hand circular polarization and left-hand circular polarization. While Bob can produce six nonorthogonal polarization analyzers such as 0o linear polarization analyzer, 45o linear polarization analyzer, 90o linear polarization analyzer 135o linear polarization analyzer, right-hand circular polarization analyzer and left-hand circular polarization analyzer. According to the expanded six-state protocol, Alice can exchange her quantum key with Bob by providing only four kind of input voltage such as 0、V0/2、V0、3V0/2 (V0 is the half-wave voltage of the phase modulator).