Research On High-dimensional Quantum Key Distribution Protocols Based On Photon's Orbital Angular Momentum

As mankind enters the information age,the demand for information transmission,storage and exchange is increasing,and this also makes information security and confidentiality issues increasingly serious.Scientists have tried to study the mechanisms and implementation methods of information security from different disciplines and perspectives.As a result,modern cryptographic communications based on public key cryptosystems based on computational complexity such as RSA,El Gamal,elliptic curve,DES,and AES have emerged.These cryptosystems based on computational complexity are all facing the deadly threat of quantum computers.In October 2019,Google's research published in Nature showed that its 53-bit quantum computer has achieved “quantum supremacy”.If the number of qubits is further increased,the traditional cryptosystem based on computational complexity will be unbearable because of the quantum computers.This has prompted people to develope a cryptographic system whose security does not depend on the limitations of computing power.Quantum cryptography came into being under this urgent need.It uses the basic principles of quantum physics to ensure the security of information transmission and theoretically has unconditional security.In 1984,the first quantum key distribution(QKD)protocol,the BB84(Bennett and Brassard in1984)protocol,was proposed,and many related researches followed,making the field of quantum cryptography progress by leaps and bounds.At present,the security of the QKD protocol has been verified by many experiments,and even some technology companies have developed QKD solutions suitable for military or commercial use.This has laid a solid foundation for the large-scale application of QKD in the future.However,the commercialization and large-scale application of the existing QKD protocol still face many challenges.First,according to the Pirandola-Laurenza-OttavianiBanchi(PLOB)basic key rate limit,the security key rate of QKD increases exponentially with the increase in transmission distance.Attenuation,which makes it difficult for the QKD protocol to achieve high-speed communication in long-distance communications;in addition,most of the existing QKD protocols use the polarization or phase of photons as the carrier of qubits.Those degrees of freedom are extremely susceptible to phase loss during transmission.The impact of the distribution reduces the practicability of the agreement.This thesis mainly focuses on the improvement of the existing QKD protocol by using the degree of freedom of the orbital angular momentum of the photon to solve the shortcomings of the existing QKD protocol based on the phase or polarization of the optical pulse,and to improve its security and security key rate.The work completed in this paper mainly includes the following aspects:(1)A more comprehensive introduction to the principles of the QKD system,a detailed introduction to the basic quantum key distribution protocol,post-processing methods and components of the QKD system are given.In this thesis,we also give a detailed introduction to the security analysis theory and security keys of the QKD protocol and some important knowledge such as rate analysis methods and decoy protocol.At the same time,this thesis also explains the basic knowledge of photon orbital angular momentum(OAM)degrees of freedom,and introduces its application in the QKD protocol and the latest developments.The thesis also introduced the Twin-Field QKD(TF-QKD)protocol and related work proposed in 2018 that can overcome the PLOB fundamental code rate limitation.(2)Based on the above background,this paper proposes a protocol that uses photon orbital angular momentum to improve TF-QKD,namely the OAM-TF-QKD protocol.The newly proposed protocol uses OAM to encode classic bits,and uses the rotation invariance of OAM to overcome the shortcomings of traditional optical pulse phase encoding methods that require calibration,thereby enhancing the practicability of TF-QKD.At the same time,because OAM has high-dimensional characteristics,it can expand the coding space.The protocol proposed in this paper uses this advantage of OAM to propose a high-dimensional OAM-TF-QKD,and gives a novel protocol theoretical scheme and a secure bit rate formula.The threshold range of the qubit error rate that can generate a secure key is increased,thereby improving the security of the protocol.Through the numerical simulation in the thesis,it can be seen that the highdimensional OAM-TF-QKD protocol can reduce the mutual information between the legal communicating parties and the eavesdropper compared with the two-dimensional TF-QKD,thereby improving the secure bit rate.(3)In addition,this paper also proposes a high-dimensional QKD protocol based on the total angular momentum of photons,and uses the idea of fountain codes to propose a practical privacy amplification scheme.With this scheme,the two parties in communication can be based on the eavesdropping detection results.The channel conditions dynamically adjust the parameters to achieve a balance between coding efficiency and security;at the same time,this paper also gives the numerical simulation results of the scheme to support the above results.