Experimental Research On Quantum Physical-layer Secure Communication

Quantum cryptography is a burgeoning interdiscipline of quantum mechanics and information theory.Its establishment has completely changed people’s cognition of cryptography.Various cryptographic tasks can be realized through the preparation,manipulation,transmission,measurement and other processes of quantum state,which shows obvious advantages over the classical methods.The research contents of quantum cryptography include quantum identity authentication,quantum key distribution,quantum relay,quantum teleportation,quantum secret sharing,quantum physical-layer secret communication,etc.Among them,quantum physical-layer secret communication,which is expected to achieve information-theoretic security of communication,has attracted extensive attention.It monitors the channel through real-time observation of bit error rate,thus limiting the amount of information obtained by eavesdroppers through eavesdropping channel.Similar to quantum key distribution,the security of quantum physical-layer secret communication is based on the fundamental principles of quantum mechanics and assumes that the eavesdropper has all the capabilities that do not violate the laws of physics,including unlimited computing resources and technologies.Since it was proposed in 2000,physical-layer secret communication protocols based on entanglement,such as BF protocol,two-step protocol,and protocols based on single-photons,such as two-way protocol,have been proposed.Different with the quantum key distribution,the quantum physical-layer secret communication does not require the two parties to share the keys in advance,so there is no need for key distribution and management,and no need to implement complex post-processing algorithms,encryption and decryption algorithms,etc.This property is of great significance for the construction of full quantum networks,such as real-time secure communication between quantum computers.In this paper,we study the experimental implementation and security proof of quantum physical-layer secret communication.At the beginning,the research background and motivation of quantum physical-layer secret communication are discussed.Then the research progress of quantum physical-layer secret communication in recent years is introduced.The content of the research could be divided into three parts: Firstly,through the study of the statistical characteristics of the optical field in frequency domain,we propose a multi-channel frequency coding scheme that works at the single-photon level.Secondly,an improved quantum physical-layer secret communication protocol is proposed,which overcomes the influence of channel loss and noise when using single photon as information carrier and demonstrates the quantum physical-layer secret communication for the first time in the world.Finally,on the basis of wire-tap channel model,a quantum physical-layer secret communication channel model is established.Based on this model,strict security proof of the improved protocol is given.The innovations of this thesis include:(1)A new quantum statistical method is proposed.The traditional time-domain statistical methods,such as the second-order correlation function and the mandel Q parameter,can only reflect the time-domain statistical characteristics of the optical field.In our method,the frequency domain quantum statistical properties of modulated light field are revealed by studying the spectral characteristics.Compared with the time-domain statistical method,our statistical method can directly show the modulation frequency information,which provide a new method for the quantum statistical analysis of optical field.(2)Direct transmission of information at the single-photon level is realized through a noisy channel.Based on the study of the spectrum characteristics of single photons modulation,a multi-channel frequency coding scheme work at the single-photon level is proposed.At the sending end,the information is encoded on the modulation frequency of the modulated optical field,and at the receiving end,the modulation frequency information is obtained by decoding the measurement results of the photon sequence through the Discrete Fourier Transform,so as to realize the information transmission.Compared with the existing channel coding scheme,this scheme has the advantages of larger transmission capacity and stronger anti-noise ability,and it is a communication scheme with the lowest requirement of optical signal energy.This scheme has important application values in interplanetary free space optical communication,quantum communication and other fields.(3)A new practical quantum physical-layer secret communication scheme is proposed and verified by experiment.The influence of channel loss and noise is overcomed by combining multi-channel frequency coding scheme with two-way quantum communication protocol.By monitoring the bit error rate in real time to limit the amount of information obtained by eavesdropper,the legitimate channel is guaranteed to be better than that of eavesdropper.In the experiment,16 frequency channels were used to transmit 4 bits of information.The integration time was 1 ms and the 4 kbps transmission rate was realized.The experiment proves that the physical-layer secret communication can realize the information-theoretic security.(4)The security proof of quantum physical-layer secret communication.Based on wire-tap channel model,a quantum physical-layer secret communication channel model is proposed,and the security proof is completed based on the model.The proof process could be divided into two parts: first,when the bit error rate is lower than the predetermined threshold value,the mutual information IAB between Alice and Bob is bigger than the mutual information IBE between eavesdropper Eve and Bob.Secondly,we prove that when once the condition IAB>IBE are satisfied,there always an wire-tap code that able to transmit information safely and accurately.The work of wire-tap code makes the research of quantum physical-layer secure communication transfer from theory to experiment.Our method for security proof is applicable to other physical-layer sceret communication protocols.