Research On Key Rate Optimization In Quantum Communication

In recent years,quantum communication has frequently appeared in various media and attracted wide attention because of its high confidentiality and high efficiency.As a key technology in quantum communication,quantum key distribution(QKD)shows some advantages in information encryption according to "Quantum non-cloning Theorem" and "Heisenberg uncertainty Theorem".In QKD,the sender encodes the information into the quantum state and shares the randomly generated key with the receiver through the channel.But in fact,there is a certain difference between the quantum key distribution system and the theory,which will lead to the efficiency of transmitting the key,that is,the key rate.Whether through free space or optical fiber channel,it will be affected by the inevitable noise,resulting in low key rate.In addition to the channel,it is also limited by the technical means used in QKD experimental construction,such as the low detection efficiency of the detector and the low proportion of single photons produced by the light source to carry information,which restricts the improvement of QKD efficiency.Key rate and maximum secure transmission distance are the key to measure the performance of a QKD protocol,and they are also two major problems we are facing at present.Since the first QKD protocol BB84 was proposed in1984,the QKD protocol has been constantly updated based on its performance optimization for more than three decades.Therefore,this paper mainly analyzes the factors that affect the key rate,and takes the improvement of the key rate as the ultimate goal,and studies the optimization of the key rate of QKD protocol from three aspects.The details are as follows:(1)Theoretical optimization: because most QKD protocols use weak coherent state light sources(WCS),the single-photon ratio is low.Thus,the techniques of orbital angular momentum(OAM)and pulse position modulation(PPM)are introduced into the quantum key distribution protocol published by Bennett in 1992(B92),and a B92 protocol based on OAM state and PPM modulation is proposed(PPM-OAM-B92).The position of the time slot in the PPM frame will carry n-bit coding information,and the information carried by a single photon will be enlarged to n times of the original.At the same time,the OAM state solves the problem of base dependence and increases the safe transmission distance.The experimental results show that PPM-OAM-B92 protocol can effectively improve the key rate and security.(2)Equipment optimization: a phase-matched quantum key distribution(PM-QKD)protocol based on heralded single photon source(HSPS)with three decoy states is proposed,which is called three-decoy state HSPS-PM-QKD protocol.By analyzing the performance of WCS and HSPS light sources,it is concluded that HSPS has a higher probability of generating single photons than WCS light sources,and can effectively reduce photon number splitting attack(PNS)and information leakage.The experimental results show that the improved protocol improves the key rate and secure transmission distance of the original protocol.At the same time,based on the limited decoy state protocol,the preparation is more simple and practical.(3)Technology introduction: because the development of quantum communication network brings a huge amount of calculation for setting parameters of QKD system,the machine learning method is introduced to quickly predict the "light intensity parameters"(the intensity and probability of signal state and decoy state)under the optimal key rate when the "experimental parameters"(such as channel loss,detector efficiency,phase offset,dark counting,etc.)are known.Based on the multilayer perceptron(MLP)model proposed by Wang W,ridge regression and batch normalization are introduced to get a new model(MLP-AN),which improves the fitting ability of the model and obtains better prediction results.Compared with the MLP model,the results show that the optimized model is more suitable for predicting the "light intensity parameters" of QKD.To sum up,this paper improves the QKD key rate from three aspects: PPM-OAM-B92 protocol,three decoy state HSPS-PM-QKD and MLP-AN model.They all get good results in key rate optimization.Finally,the optimization aspects of quantum key distribution system are summarized and prospected.