Practically Useful Decoy-state Measurement-device-independent Quantum Key Distribution

Since twenty century,the quantum mechanics is fast developed and quantum cryptography has been a very remarkable field.And in which,the quantum key distribution(QKD),that can guarantee unconditional security in communication is an very important applications.However,in the real word setup,due to the practical equipments are not as idealized as theory,so is the unconditional security.So these years,to guarantee the safty of real setup QKD system experimently has been an extremely important task in quantum communication.To overcome all kinds of unperfect,the decoy state method and the measurement-device-independent quantum key distribution(MDI-QKD)have been developed.We focus on this field,and study the 3-intensity protocol.We present a tighter explicit formula to estimate the lower bound of the yield of two-single-photon pulses sent by Alice and Bob.Moreover,we show that the bounding of this yield and phase flip error of single-photon pulse pairs can be further improved by using other constraints which can be solved by a simple and explicit program.Our methods here can significantly improve the key rate and the secure distance of MDI-QKD with only three intensities.And we also study the application of Heralded Single-Photon Sources in the MDI-QKD.And then,we present formulas that tighten the upper bound for the phase-flip errors in the decoy-state method by using four intensities.Our result compressed the bound to about a quarter of the known result for MDI-QKD.Based on this,we find that the key rate is improved by more than 100% given weak coherent state sources,and even more than 200% with heralded single-photon sources.In the practical application of MDI-QKD protocol,the statistical fluctuation must be considered.Then the relevant relatively low key rate will be the major barrier to its practical use.We present a 4-intensity protocol for the decoy-state MDI-QKD that hugely raises the key rate,especially in the case the total data size is not large.Also,calculation shows that our method makes it possible for secure private communication with fresh keys generated from MDI-QKD with a delay time of only a few seconds.The experiment using this protocol has been implemented in 2016 by the University of Science and Technology of China,and the communication distance has reached 404 km while the key rate has increased 500 times,in which 50 times is obtained by our protocol.And we also study the behavior of MDI-QKD with statistical fluctuation using quantum blockade source.Numerical simulation for a type of 4-intensity protocol shows that,after parameter optimization,this source can improve the final key rate by 100 times compared with traditional weak coherent state sources.