Quantum Key Distribution System Of Polarization States Encoded By Phase Modulation

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 produceall 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 45° linear polarization, 135° 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 45° linear polarization analyzer, 135° 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 0° linear polarization, 45° linear polarization, 90° linear polarization, 135° linear polarization, right-hand circular polarization and left-hand circular polarization. While Bob can produce six nonorthogonal polarization analyzers such as 0° linear polarization analyzer, 45° linear polarization analyzer, 90° linear polarization analyzer 135° 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...