| Quantum key distribution (QKD) is the use of photon status as an information carrier through the quantum channel for key distribution, whose core is the security.Unlike classical key distribution, the security of QKD is ensured by physical laws.Quantum no-cloning theorem shows that the eavesdropping operation of unknown quantum states leaves a "trace",which make the eavesdropper's behaviors discoverable. When the eavesdropping is found, the corresponding key will be discarded. Using the key that is distributed without eavesdropping and the "one-time pad" encryption will achieve the unconditional security of confidential communications in theoretical.QKD protocols mainly include two major technical approaches: discrete variables (DV-QKD) and continuous variables (CV-QKD). DV technology based on single photon protocol has made remarkable achievements, but its quantum state preparation and detection technology are extremely difficult. The detector of the CV-QKD protocols is the balance homodyne detector constructed with a photodiode,which is low cost and high reliability. What's more,the most important CV-QKD protocol is a one-way Gaussian-modulated coherent-state protocol, where the coherent state can be prepared by using semiconductor lasers. Thus, the cost and reliability advantages of such protocol are more prominent. Although CV-QKD protocol is superior to the maturity and stability of the device, the security proof of the protocol has a certain imperfection in the past for a long time. So it is less attractive and significant than the DV-QKD protocol. The theoretical security of CV-QKD protocols have shown a great leap in recent years and has been well proved. So far,there are three kinds of CV-QKD protocols that have reached the same security proof level as the DV-QKD protocol: one-way protocol based on Gaussian-modulated coherent-state & heterodyne detection, one-way protocol based on Gaussian-modulated and squeezed-state & homodyne detection,measurement-device-independent protocol based on Gaussian-modulated coherent-state. In particular, the one-way protocol based on Gaussian-modulated coherent state - heterodyne detection is rapidly moving towards practicality.Although the theoretical security proof of the continuous-variable protocol has developed rapidly in recent years, there are still some problems that have not yet been solved, the security proof of some new CV-QKD protocols especially is still blank.Compared with the Gaussian-modulated one-way protocol, these new CV-QKD protocols have advantages in terms of experimental implementation, practical security and protocol performance. For example, one-way protocol based on four-state modulation can simplify the difficulty of data processing and improve the efficiency of error correction, thus improving the performance of the protocol. The measurement-device-independent protocol can be detected by the eavesdropper,naturally against all detector's side channels, thereby improving the practical security.The two-way protocol can use two quantum channels to increase the channel capacity,thereby improving the performance of the protocol. If they are further refined on the proof of security proof, there is a broader development. The research goal of this paper is to study the security of the one-way protocol based on four-state modulation,the measurement-device-independent protocol and the two-way protocols, and to improve the security of the protocol. And on this basis, the protocol improvement program was presented, Improving the secret key rate, transmission distance and tolerable noise of protocol, including:1. The security proof of the four-state modulated one-way protocol: a security analysis method based on source monitoring is proposed and the secret key rate calculation method based on collective attack is presented. It is proved that the protocol is security under collective attack. This method solves the problem of linear channel hypothesis in the original security proof. Under the premise of no linear channel hypothesis, the security of the protocol under collective attack is proved, and the security proof of the protocol is improved.2. The propose of measurement device independent protocol and the security proof: the measurement-device-independent protocol of coherent state and squeezed state is proposed, and the Prepare-and-Measure (PM) model and Entanglement-based(EB) model is given. The secret key rate calculation method based on collective attack is presented. We prove that the protocol under collective attack is security. The numerical simulation results show that the squeezed state measurement-device-independent protocol has an advantage over the coherent state measurement-device-independent protocol in the transmission distance and the tolerable noise. In addition, an improved squeezed state measurement-device-independent protocol is proposed, which can further improve the performance of the protocol, and under the existing experimental conditions, it is expected to achieve the key distribution at 18km distance. Finally, we prove that the coherent state measurement-device-independent protocol is security under the framework of composable security, and the secret key rate calculation method and numerical simulation result are given.3. The security proof of the two-way protocol: the security of the two-way protocol based on the coherent state and homodyne detection is analyzed under the two-mode attack, and the secret key rate calculation method under all type of the two-mode attack is given. The security of the protocol under the collective attack is proved. Moreover, through the numerical simulation, we find the optimal two-mode attack approach for the two-way protocol of coherent state and homodyne detection is found. At the same time, in this optimal two-mode attack,comparing the performance of the two-way protocol with the corresponding one-way protocol, it is found that the advantages of the two-way protocol in noise resistance can still be maintained.4. The improvement of continuous-variable protocol using optical amplifier: the continuous-variable protocols of three kinds of optical amplifiers are analyzed, phase sensitive optical amplifiers, phase non-sensitive optical amplifiers, noise-free linear optical amplifiers. And three improved schemes are proposed: improved schemes for measurement device independent protocols using noise-free linear optical amplifiers,improved schemes for two-way protocol based on coherent state and homodyne detection using phase-sensitive optical amplifiers, improved schemes for two-way protocol based on coherent state and heterodyne detection using phase-insensitive optical amplifiers. Moreover, through the numerical simulation, the working range and conditions of the three schemes are given. Adequate numerical calculation and analysis are implemented to achieve these improvements in the experiment. |
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