Research On The Time-domain Balanced Homodyne Detector In Continuous Variable Quantum Key Distribution System

Information security is crucial in today’s society,as is obvious.Complex encryption methods,including the DES symmetric encryption algorithm and the RSA asymmetric encryption algorithm,are frequently used in traditional communication processes to ensure the security of communication.The traditional encryption strategy,which relies on algorithm complexity to assure information security,will face challenges with the emergence of quantum computers.Quantum communication based on the basic principles of quantum mechanics and with unconditional security has attracted wide attention.It can detect the eavesdropping behavior of the third party in time during the communication between two parties and realize the absolute security of communication.Continuous variable quantum key distribution(CV-QKD)is one of the significant research directions in quantum communication.At the sending end of the information,continuous light or pulse light is usually used as the carrier of the key information for transmission.At the receiving end of the message,the security key information consistent with that at the sending can be obtained by means of homodyne or heterodyne detection.Therefore,quantum communication can achieve perfect compatibility with modern optical communication networks.Based on the above advantages,the theoretical research and experimental progress of CV-QKD have been greatly developed in the past two to three decades,especially in the communication rate,transmission distance,and secure proof of the protocol.However,due to the imperfection of devices,there are still controversial issues in practical security.At the same time,the high cost of the current quantum key distribution system is also one of the main factors limiting its large-scale application.To solve the above problems,the author studies the calibration of shot noise in the current communication system,realizes the accurate calibration of shot noise,and develops a multifunctional time-domain balanced homodyne detector,which can effectively simplify the CV-QKD communication system,and enhance the security of the system.The related research work is introduced in detail below.In the CV-QKD system,various variances collected at the receiving end should be normalized to the shot noise variance,so the accuracy of shot noise calibration directly affects the system’s parameters,performance,and even communication security.This thesis presents a method to calibrate the shot-noise-limited of time-domain balanced homodyne detector.By precisely calibrating the integral detector’s gain line and the homodyne detector’s gain line and making them coincide,the homodyne detector can be guaranteed to be in the shot-noise-limited.In the experiment,the intensity noise carried by the local oscillator can be eliminated by introducing a high-pass filter,and the measurement error in the calibration process of shot noise can be reduced to 1/1000 level.In order to avoid security vulnerabilities caused by measuring errors during the quantum secret key distribution system,the author proposes a method to ensure the security of quantum communication by decreasing the gain of the detector.The above calibration method for shot noise can be applied to the transmitted local oscillator(TLO)scheme or the local local oscillator(LLO)scheme to achieve accurate calibration of shot noise and avoid safety problems caused by the absence of accurate calibration of shot noise.The time-domain balanced homodyne detector plays a very important role as the core receiving device in the quantum key distribution system.Based on the function of the original time-domain balanced homodyne detector,a multifunctional time-domain balanced homodyne detector is developed by making full use of the photocurrent signals generated at both ends of each photodiode in the cascade photodiode.The detector can simultaneously measure the quadrature of the signal field,monitor the local optical power in real-time,and generate the clock signal.At the same time,it can be well integrated into the CV-QKD system,reduce the complexity of the receiving end of the system,avoid the security loopholes caused by the change of the fiber beam splitter ratio,and enhance the actual security of the system.