Research On Continuous Variable Quantum Key Distribution With A Local Local Oscillator In Real Optical Fiber System

In nowadays society where informatization is highly developed,ensuring the security of information transmission is crucial to all fields.Quantum key distribution protocol has received extensive attention because of its theoretical security,and continuous-variable quantum key distribution protocol(CVQKD)based on coherent state has the advantages of low-cost detection,easy integration,and compatibility with classical optical communication systems,making it become one of the most important research area in the field of quantum key distribution.At present,CVQKD protocol can be implemented in real system by transmitted local oscillator(LO)scheme or local local oscillator scheme.The latter has the advantages of eliminating the loopholes of transmitted LO,easy to reach the shot-noise limited detection,and simplifying the system structure,and therefore has achieved rapid development in recent years.However,at this stage,the large excess noise in real system causes limited performance,the actual channel impairments interfere with quantum key distribution,and practical security issues have not been fully resolved,which hinder the process of practical application.This dissertation focuses on CVQKD with a local LO in real system.It studies and analyzes the actual excess noise suppression,secret key rate improvement,carrier recovery,polarization compensation,the practical security of shot noise monitoring,and therefore promote the process of practical application of local-LO scheme.Related innovative work is as follows:1.Aiming at the problem of large excess noise in the preparation,transmission and detection of quantum signals in local-LO system,we propose a pilot-multiplexed scheme to suppress the excess noise.The scheme uses time-and polarization-multiplexing technology to isolate pilot and quantum signals to reduce crosstalk,and uses two sets of heterodyne detectors to detect two types of signals to reduce quantization noise.By constructing a general local-LO noise model and comparing from the aspects of linewidth tolerance,etc.,the low excess noise characteristic of this scheme is demonstrated.Experiments have proved that the excess noise in this scheme can be suppressed to 0.078 under the actual fiber link,indicating that it has a secret key distribution capability of 554 kbps within 15 km.This work achieves excess noise suppression,laying a foundation for the practical performance improvement.2.In order to further improve the secret key rate in the local-LO system,we propose a homodyne detection-based pilot-multiplexed scheme with higher quantum signal detection efficiency,adjust the wavefront arrival time through a dynamic delay line and use two-time phase compensation method to achieve high-precision phase synchronization at the transceiver,and optimize parameters such as the modulation variance of the quantum signal to improve secret key rate.Finally,discrete devices are used to build an experimental platform,which confirms that it can achieve a secret key rate of3.14 Mbps under the standard single-mode fiber of 25 km.Compared with the previous transmitted-LO scheme,the secret key rate in this work has increased by more than 3.3.Aiming at the problem that the frequency offset and phase noise in the local-LO system cause the quantum signal's phase misalignment at the transceiver,we study the carrier recovery for simultaneous quantum and classical communication protocol and measurementdevice-independent CVQKD protocol.For the former,we use the power algorithm to recover the classical signal and quantum signal.For the latter,we propose a frequency domain-based frequency offset estimation algorithm and a cross-correlation-based phase estimation algorithm.We verify the feasibility of schemes through simulation and test its carrier recovery capability under actual fiber system through experiment.This work paves the way for these two protocols to be implemented in the local-LO system.4.Aiming at the problem of the dynamic deviation of the polarization state of the quantum signal caused by the random birefringence effect in the actual fiber channel,we propose a polarization compensation scheme to recover the quantum signal.This scheme estimates the dynamic change of the polarization state through the Kalman filter algorithm,and then realizes the accurate polarization demultiplexing of the quantum signal and the pilot.We analyze the scheme's polarization tracking ability and anti-phase drift ability through simulation,and show that it can resist 1krad/s polarization rotation interference and has a key distribution capability of 8.4kbps under 20 km fiber through experiment.This work solves the problem of the dynamic deviation of the polarization of the quantum signal and helps the long-term automated operation of the real system.5.Aiming at the problem of imperfections introduced by shot noise monitoring in practice,we analyze the impact of imperfections on the practical performance of CVQKD and therefore make up for the vulnerabilities.We calculate the practical secret key bound when the length of the monitoring segment is limited and the device parameters in the monitoring module are limited,and analyze the optimal monitoring segment length value and device parameter range.This work guarantees the practical security of CVQKD under the shot noise monitoring,and guides the selection of the length of monitoring segment and device parameters.