Practical Security Analysis For Twin-Field Quantum Key Distribution

Quantum Key Distribution(QKD)enables legitimate communication parties to share secret keys over long distances.However,traditional QKD protocols cannot overcome the rate-distance limit of QKD without quantum repeaters,which restricts the vast application of QKD technology to some extent.Compared with traditional QKD protocols,Twin-Field QKD(TF-QKD)protocol,which is based on the characteristics of single-photon interference,greatly improves the point-topoint QKD transmission distance and successfully overcomes the fundamental rate-distance limit of QKD without quantum repeaters.The secret key rate of TF-QKD scales with the square-root of the channel transmittance,achieving a quadratic improvement over the traditional QKD protocols whose secret key rate scales linearly with the channel transmittance.At present,TF-QKD has made tremendous progress both theoretically and experimentally,showing a broad application prospect.Practical security analysis is crucial to the development and application of QKD.In this thesis,we focus on the impact of practical factors such as finite-key length,asymmetric channels,and intensity fluctuations on the practical security of TF-QKD protocol.The main contexts are as follows:1.Finite-key analysis for TF-QKD with symmetric channels.Security analysis under the condition of finite-key length is one of the important issues to be studied in the practical security analysis of QKD.Based on the composable security analysis method,the secret key length formula of the decoy-state TF-QKD in finite-key regime is provided.Based on statistical fluctuation analysis tools such as Hoeffding's inequality,Multiplicative-Chernoff bound and ImprovedChernoff bound,the statistical fluctuations of the total amount of successful detection events are estimated.Furthermore,the upper bounds of the phase error rates of the two decoy states and three decoy states TF-QKD protocol are provided.With the upper bound of the phase error rate,the secret key rate formula of the TF-QKD in finite-key regime is given.According to the symmetrical channel loss model,numerical simulations of the secret key rates of the decoy-state TF-QKD under the condition of finite-key length are performed.The performances of the decoy-state TF-QKD with different statistical fluctuation analysis tools,different dark count rates,and different decoystate schemes are estimated.Simulation results demonstrate that under the condition of finite-key length,secure communication distance of the decoy-state TF-QKD declines significantly,and the secret key rate greatly reduces.Additionally,in practical TF-QKD systems,three decoy states TFQKD scheme can obtain higher secret key rates and longer communication distances.2.Finite-key analysis for TF-QKD with asymmetric channels.In practical TF-QKD systems,the distances of quantum channels between legitimate users and the central node are often unequal,which are referred to as asymmetric channels.Therefore,the security analysis of decoy-state TF-QKD with asymmetric channels is of great practical significance.Based on the characteristics of asymmetric channels and variant intensity setting strategies adopted by communication parties,we estimate the statistical fluctuations of the total amount of successful detection events of decoystate TF-QKD with asymmetric channels.Furthermore,the upper bound of phase error rate and secret key rate formula of the decoy-state TF-QKD are given under the condition of finite-key length and asymmetric channels.According to the asymmetrical channel loss model,the secret key rate of decoy-state TF-QKD is numerically simulated.Simulation results demonstrate that under the circumstance of asymmetric channels,higher secret key rates and longer communication distances can be acquired by optimizing the intensities of signal states for both communication parties.Moreover,the performance of decoy-state TF-QKD is significantly weaken under the condition of asymmetric channels and finite-key length.The results of the security analysis can provide necessary theoretical guidance for the application of practical decoy-state TF-QKD.3.Finite-key analysis for TF-QKD with intensity fluctuations.Intensity fluctuations caused by unstable light sources might bring in correlations between detection events.In order to study the practical security of TF-QKD with symmetric and asymmetric channels when both intensity and statistical fluctuations coexist,statistical fluctuation analysis of the total amount of successful detection events is provided based on Azuma's inequality.The upper bound of phase error rate and the secret key rate of the decoy-state TF-QKD with both symmetric and asymmetric channels are estimated.The effect of intensity fluctuations on the security of TF-QKD with finite-key length is analyzed through numerical simulations.Simulation results demonstrate that whether the quantum channels are symmetrical or asymmetrical,the secure communication distance of TF-QKD is seriously shorten when intensity and statistical fluctuations coexist.Considering the effect of intensity fluctuations,the performance of decoy-state TF-QKD is more significantly weaken due to the effect of finite-key length.