Theoretical Study Of Long Distance Quantum Key Distribution Protocol

Throughout the ages,in order to guarantee the security of information,the way of encryption has been constantly changing from manual encryption to machine.However,the most of modern cryptography based on computational security,which can only guarantee the security of the keys within a certain period of time.With the increasing computing power of the computer and the development of deciphering algorithms,decoding time will be decreased drastically.Especially after the concept of quantum computer was proposed,the encryption method based on computational complexity became more and more fragile.To solve this problem,the researchers proposed quantum cryptography system that distributes keys in a quantum manner.Its security depends on quantum mechanics,and it could theoretically prove to be unconditional security.At present,the quantum key distribution protocol has gradually entered the practical stage from the laboratory stage,which brings confidence and dawn for the future information security.This paper focuses on the long-distance quantum key distribution protocol.Firstly,the paper introduces the background and the necessity of the quantum key distribution technology,and then emphatically expounds the development of long-distance quantum key distribution protocol and the problems faced by the practical quantum key distribution system.On this basis,combine with the current research on the long-distance key distribution protocol,the paper makes corresponding improvement and supplement:1.This paper studies the influence of quantum memory on the performance of BB84 protocol.At present,the ideal single photon source is usually replaced by the weak coherent state source in most quantum distribution key protocol experiments.The weak coherent state source has a certain probability to send the vacuum state pulse,and the photon state may also become a vacuum state due to the loss of optical fiber channel and other optical components which will aggravate the negative effect of dark count rate on the final secure key rate.To solve this problem,this paper proposes a scheme of introducing the heralded quantum memory into the receiver and constructs the system model.According to the system model,the single-photon yield and single-photon error rate are estimated by the decoy-state method.Then the numerical simulation results show that the heralding characteristic of the quantum memory can reduce the valid dark count rate of receiver's detectors,reduce the bit error rate,and thus improve transmission distance.2.Under the practical experimental conditions,this paper studies the effect of intensity fluctuations on twin-field quantum key distribution protocol.In the decoy-state quantum key distribution protocol,the intensity modulator can modulate different intensity pulses.However,the defects in the intensity modulator and the intensity misalignment will affect the final secure key rate of the quantum key distribution protocol.Then,this paper models and analyzes the influence of intensity fluctuations on the performance of twin-field quantum key distribution protocol,and the numerical simulation is carried out.The simulation results show that with the increase of transmission distance,the intensity fluctuation can reduce the secure key rate and transmission distance of quantum key distribution to some extent.But in general,the transmission distance is less affected by the intensity fluctuation.