The Research Of Light Source And Detector In Continuous Variable Quantum Key Distribution

The information security has become a hot issue in modern society.The security of the modern encryption is based on the complexity of algorithm.Quantum key distribution(QKD)is a new subject,and the unconditional security is guaranteed by the basic principle of the quantum mechanics.It can discover any eavesdroppers' behavior,and has a broad application prospects in the fields,for example defense,finance,networking,etc.In the continuous variable quantum key distribution(CV-QKD),the quadratures of optical field are used as carriers of the information.It has advantages,for instance the simple light source,the high detector efficiency,the compatibility with the classic fiber-optic communication system,etc.So it has been highly valued by the research group at domestic and overseas,and the related theory and experiment is growing rapidly.This paper introduces the light source and detector in CV-QKD using Gaussian-modulated coherent states.Firstly,it reviews briefly the modern cryptography and the development history of QKD.Then,the related fundamental theory in this field is introduced.Furthermore,it explains the experiment scheme and unconditional security in detailed.At last,it carries out the research on the long-term stability of CV-QKD.The main work includes:1.In CV-QKD,the sender usually generate the high extinction ratio light pulses based on the two cascaded Lithium Niobate-based Mach Zehnder(LNMZ)intensity modulators,so that it reduces the excess noise produced by the photon leakage from the local oscillator pulse to signal pulse,and improves the stability of system.Firstly,due to the bias working point drifts over time,this paper proposes a bias locking method which is capable of locking two or more cascade LNMZ modulators.This technique is based on the scanning and the ten orders polynomial fitting,and it can get a higher accurate bias working point.Secondly,we design a pulse generator with its performance of small size,low electronic noise,high-drive capacity,short development period,easy integration and low cost.At last,combine with this method and the pulse generator,we can generate the light pulses with high extinction ratio(>80 dB)based on two cascade LNMZ modulators driven by the pulse generator,and demonstrate by a single photon detector.2.The time-domain pulsed homodyne detector is an important device in the CV-QKD.The measurement precision which is the key factor of the detector has an important impact on the excess noises and the safe key rate of the CV-QKD.It is known that the measurement precision of the detector depends crucially on the common mode rejection ratio(CMRR)and its long-term stability,In order to improve the measurement precision and avoid saturation of the detector,we propose and demonstrate an approach to balance the two output beams' intensity of the 50/50 fiber coupler of the time domain homodyne detector automatically.Firstly,we review briefly the principle and development of the time-domain pulsed homodyne detector.Secondly,a fiber-based variable attenuator based on computer-controlled linear stepper motor is designed.The principle of feedback control is described in detail.A method of variable step size according to the degree of balance is proposed to fulfill a fast auto-balance process.At last,using the auto-feedback-control system,the degree of balance about 1.56×10-6 is achieved.The auto-balance apparatus enables the time-domain homodyne detector to run stably in a long term with a high CMRR.The presented auto-balance time-domain pulsed homodyne detector can be well integrated into the CV-QKD system,and is expected to play an important role in improving the measurement precision and reducing the excess noises of the CV-QKD system.