Improvement For Free-space QKD Based On The Random Characteristics Of Channel

In this information era, the need for secure communication is growing. The critical point of secure communication is to obtain the secure key to encrypt the information. Quantum key distribution has attracted great attention since it can offer great amount of secure key in real-time. Nowadays, QKD is the most promising implementation arising from quantum information theory. The secure key rate is a key indicator for the QKD in a practical environment. For free-space quantum key distribution system and especially the long range link, the transmission medium for the signal is the atmosphere, which is always in a movement. Therefore, the channel characteristic of atmosphere is an important consideration for a practical free-space QKD system.Based on the random characteristics of the atmosphere, this thesis centers on the influence of the atmospheric turbulence on the performance of free space QKD. On this basis, the relevant optimizing design of the system is proposed. In detail they are:1) From the respect of optical time delay fluctuation caused by the atmosphere turbulence, a geometrical optics model for shaping the turbulence is built up with the random lens theory. By shaping the eddies in the turbulence as the spherical lens with the graded refractive index, the behavior of light propagating in the turbulence is described. We then use the ray tracing method to analyze the fluctuation of transmission time and beam wandering effect.2) Base on the characteristics of atmosphere, the factors that decrease the synchronization accuracy have been evaluated. Based on the analysis, we propose a novel’sparse’synchronization scheme which may help improving the timing resolution for high speed QKD. It takes advantage of employing high power low repetition rate pulse laser to offer precise time reference for high speed QKD because of its narrow pulse width. And we also reduce the cost of system and the light interference on the quantum channel by making the timing light pulses sparse. Experimental results and calculation show that, our scheme is feasible.3) The impact of time jitter in the free space QKD system is analyzed. The relationship between the optimal width of the time gate and the variance of the time jitter is given. According to the time characteristics of the single photon detector, the corresponding relationship among the width of the time gate, noise level and the system loss is presented for the long range link between orbit satellite and ground station.4) According to the secure key rate resolving model, The effects of the scintillation is calculated for free space QKD system. The results show that scintillation has a minimal impact on the free space QKD system. By analyzing the key data process, the reason why the scintillation noise has no effect is present. From the characteristic of fluctuating-loss channel, a quantum-bit error rate estimation scheme based on the information of instant channel loss is proposed to help improving the performance of free-space QKD system, which is suffered from the influence of atmosphere turbulence. The theoretical and numerical results show that this scheme is a useful tool to increase the secure key rate especially for long range free-space QKD.