| Information security is an essential part of national security.Encryption algorithms in cryptography are one of the core technologies to ensure information security,while the security of commonly used algorithms mainly depends on the computational complexity.With the increase of computing power,the improvement of mathematical methods,and the rapid development of quantum computing,the security of existing cryptography is facing severe challenges.Quantum cryptography can provide theoretically unconditional secure communication based on the laws of quantum mechanics,combined with the "One-time pad" encryption method.Quantum key distribution(QKD)is the core content of quantum cryptography.Since its proposal in 1984,many outstanding results have been revealed both in theoretical and experimental fields.QKD has embarked on the road of engineering and robust implementations,and has been the closest application to industrialization in the field of quantum information.However,plenty issues still need to be addressed before its wide adoption,such as performance,cost,mass production,and standardization.Rapidly evolving integrated photonics can provide an ideal platform for the technical implementation of QKD.With the advantages of small size,high precision,low cost and scalable production,integrated devices can bring significant improvements to the core performance of QKD,such as stability,secure key rate and practical security.Several research results have been published on integrated QKD systems,involving multiple integrated photonics platforms and various QKD protocols.Previous researches have verified the feasibility,but only a few studies have been focused on improving the capability of anti-disturbance,conducting practical security analysis and optoelectronic integration under realistic application conditions,which are still urgently needed.During the doctoral period,the author conducted research on integrated QKD codecs,focusing on the critical challenge of self-compensation against polarization perturbations,for the needs of miniaturization,high stability and scalable deployment of practical QKD systems.The main contents of the research and the results obtained are as follows.1.To address the requirements of codecs in QKD systems,we summarize the characteristics of several material platforms in integrated photonics and analyze their advantages and applicability scenarios.Selecting the silicon-on-insulator and silicaon-silicon platforms,we develop integrated units required for QKD codecs through the complete process,including design,simulation,fabrication,packaging,and testing.This work lays the foundation for implemening on-chip QKD systems and practical security research.2.For the demand of phase encoding in QKD systems,an asymmetric FaradayMichelson interferometer(AFMI)with a large arm-length difference is developed using the silica-on-silicon platform.Compared with the previous works,the AFMI can selfcompensate the polarization perturbation caused by the environment in a wide temperature range.Moreover,based on the light source model and single-photon interference,a method for measuring the symmetry between two interferometers is proposed and utilized to achieve high-precision symmetry adjustment when packaging.The experimental results show that the single-photon visibility can maintain above 98.5%for a long time under random polarization perturbation in the channel,which meets the requirements of the practical phase-encoded QKD systems.3.Based on the AFMI structure,an integrated QKD codec chip for the time-bin scheme is developed,and a 1.25 GHz QKD system is built upon it.Considering the conditions such as the afterpulse effect of the single-photon avalanche detectors and the finite-key effect,the theoretical analysis model of the QKD system is improved,and better system operating parameters are obtained.The secure key rate is about 1.34 Mbit/s ±13 kbit/s under a 50 km fiber channel with random polarization perturbation,which means the standard deviation is less than 1%.This result verifies that the system can resist channel polarization perturbations and provides an essential technical reserve for realizing a highly stable and miniaturized QKD system.The innovations of this paper are as follows.1.To solve the polarization perturbation problem in QKD,a hybrid package structure of silica-on-silicon chip and Faraday mirrors is proposed to implement the AFMI chip,which can automatically compensate the change of photon polarization over a wide temperature range.Compared with previous solutions operating at a specific temperature point,it can avoid the high power consumption and control stability caused by the large difference between the operating temperature and the ambient temperature,which makes it more suitable for satellite-based applications.2.Aiming at the challenge of online high-precision detection of symmetry between interferometers,a measurement method based on the single-photon interference results is proposed,which can complete the detection of symmetry with an accuracy better than 100 fs only with the QKD instruments.This method provides an effective analysis means for the fine quantitative evaluation of interferometers and system states and offers a supporting technology for the large-scale network deployment of QKD.3.Based on the AFMI structure,an integrated time-bin encoded QKD codec chip is designed and implemented.The QKD system built by this chip has the ability of self-compensating polarization perturbation over a wide temperature.range,and can obtain low quantum bit error rate(QBER)and high secure key rate(SKR)only with a phase tracking method,which provides a way to realize a self-stabilized integrated QKD system.4.Considering the sources of QBER in the time-bin coding QKD system operating at a high repetition rate,such as afterpulse and the finite extinction ratio,the parameter selection method is optimized and the calculation formula with finite-key analysis is improved.The analytical model can portray the actual system more accurately and acheive a higher SKR in the experiment. |
PDF Full Text Request