Quantum Crptography And An Effective Scheme Of Realizing CONT Gate In Quantum Computing

Quantum physical systems, such as photon, electron, iron, atom, can serve as an efficient carrier of quantum information. As chargeless particles, photons have extensive applications in quantum communication, quantum key distribution and quantum computation because of its low loss, impartibility, getting phase delay and combining on beamsplitters easily.Owing to the important role of single photon in quantum information process, the technique of generating single photon is developing. Nowadays, There are many ways to generate single-photon, e.g. by attenuated pulse laser, quantum dot, pulsed excitation of a single dipole, single nitrogen-vacancy color center in a diamond nanocrystal, cavity-QED and spontaneous parametric down-conversion (SPDC). Among these methods, generating single photon by SPDC is one of the effective techniques. SPDC creates photons in pairs by nonlinear process, in which one photon spontaneously splits into two daughter photons—traditionally called signal and idler photon. If one photon is found in one arm of the twin-photon, we know there must exists a single-photon state in another arm. Thus we can confirm the single-photon without detecting it. And because of the phase matching, the wavelength, polarization and position of the photon can be predefined. Based on it, we can use one of the down converted photons as a trigger to control the other photons in sequence for the application of quantum network.In this thesis, an efficient scheme of generating single photon with SPDC is presented, and the important application in quantum key distribution is also introduced. Based on the scheme, the controlled Not (CNOT) gate with the single photon is developed. The main parts of the thesis are:1. The different quantum key distribution schemes are introduced. And the principle and process of BB84, B92 and the EPR protocols are analyzed. Then the data correction and privacy amplification are given.2. A theoretical scheme of generating high efficiency single photon using multi-dots delay detection in the process of SPDC is presented, by which the enhancement of the efficiency of controlled not gate is also studied.The creative works are as follows:1. We show a scheme to generate high efficiency single photon via multi-dots delay detection with SPDC. And using this scheme, the implementation of quantum key distribution and quantum logic gate are presented.2. This scheme of generating efficient single photon is designed for the prevention of the problem of multi-photon input encountered in the laser pulses attenuation, and it can be used to improve the efficiency of CNOT gate.