Several Studies On Continuous-Variables Quantum Key Distribution

Quantum informatics is a rising inter-discipline of informatics and quantum mechanics. Quantum communication is an important branch of quantum informatics, which has advantages in improving the arithmetic speed, ensuring the security of information, increasing data capacity and improving information accuracy.Early QKD setups use so-called discrete variables, therefore requiring single-photon sources and detectors. However, it is difficult to prepare reliable single-photon sources or high-detect-efficiency photon detectors at present. In contrast, continuous-variable QKD (CVQKD) schemes typically use quadratures of light beams as information carriers and homodyne detectors rather than photon detectors. Such protocols eliminate the need for single photon technology, as they only require standard available telecom components, such as diode lasers, electro-optic modulators, and PIN photon-diodes.In this paper, two parts are mainly focused on, one for the analysis to the performances of the two main schemes of conciliation which can break the 3dB limit; the other for the analysis of the homodyne detection, the analysis of the vacuum noise and the imperfectness of beam-splitter and how to resolve these problems.First of all, the basic concepts of coherent states square are introduced, and then starting from the coherent state, we introduce the basic principles of quantum key distribution with coherent states, as well as the system architecture. After a brief introduction of the reconciliation protocol, the reverse reconciliation protocol and post-selection protocol are analyzed. We give a coherent state quantum key distribution system using homodyne detection systems and principles.Second, the three schemes of reconciliation are analyzed in detail. The security condition of QKD system is summarized according to the different schemes. According to the different characteristics of the channel, the advantages and disadvantages of the two schemes of reconciliation are analyzed. First of all., we take the attenuation and channel noise into regard. Then we consider the detection noise, including the electronic noise and the efficiency of detection, we derive the effect of the detection. Through the analysis, it is reported that post selection reconciliation has more advantages in a channel with large transmission, and has much more tolerance to noise, but is sensitive to detection noise. At last, we study the security of these two reconciliation protocol which can break the 3dB limit.Finally, the balanced homodyne detection is analyzed. We analyze the noise and the beam-splitter system which may bring problems, give the methods and demonstrate there feasibility. The inequality of beams will increase the noise of the detection system; in order to overcome this problem, we can try to attenuate the strong light. We prove the feasibility of the two ways to attenuate the light. Finally, we analyze the impact of electronic noise and how to reduce the electronic noise in some ways.