Study On Security And Performance For Practical Continuous Variable Quantum Key Distribution Systems

In the age of quantum information,classical cryptography relying on computational complexity may be insecure.Therefore,quantum cryptography is proposed by researchers based on the basic laws of quantum physics,which is a theoretical unconditionally secure technology.In particular,quantum key distribution(QKD)is a relatively mature branch in the field of quantum cryptography,and its theoretical security against general attack has also been fully proven by using some basic assumptions.However,there are some imperfections in practical QKD systems,which deviates these assumptions in security proofs.Here,some imperfections may open a security loophole for Eve to successfully obtain information about secret key without being detected,which is an effective quantum hacking attack strategy.The practical security of the system can be seriously destroyed by these quantum hacking attacks.The other imperfections may simply deteriorate the practical performance of the system.Accordingly,explorations of practical security and practical performance can effectively promote the commercial application of QKD.At present,QKD can be divided into the two following categories: discrete-variable quantum key distribution(DVQKD)and continuous-variable quantum key distribution(CVQKD).In this thesis,we focus on the research of the CVQKD with the Gaussian-modulated coherent states(GMCS).Specifically,we explore the potential problems about practical security and practical performance for the transmitter of GMCS CVQKD systems.In addition,we investigate the countermeasures against the quantum hacking attacks in complex communication environments.The research contents are summarized as follows:(1)We investigate the practical security of a GMCS CVQKD system with reduced optical attenuation.In a practical GMCS CVQKD system,the performance of the optical attenuator may deteriorate due to the intentional and unintentional damage of the device.We find that the secret key rate of the system may be overestimated based on the investigation of parameter estimation under the effects of reduced optical attenuation.Therefore,the reduced optical attenuation opens a security loophole for Eve to successfully perform an intercept-resend attack in a practical GMCS CVQKD system.To close this loophole,we add an optical fuse at Alice's output port and design a scheme to monitor the level of optical attenuation in real time,which can make the secret key rate of the system evaluated precisely.The analysis shows that this potential quantum hacking attack can be effectively resisted by using these countermeasures.(2)We investigate the security of the practical one-way GMCS CVQKD and GMCS continuous-variable measurement-device-independent quantum key distribution(CV-MDI-QKD)systems under laser seeding attack.In particular,Eve can inject a suitable light into the laser diodes of the light source modules in the two kinds of practical CVQKD systems,which results in the increased intensity of the generated optical signal.The parameter estimation under laser seeding attack shows that the secret key rates of these two schemes may be overestimated,which indicates that this attack can open a security loophole for Eve to successfully obtain information about secret key in these practical CVQKD systems.To close this loophole,we propose real-time monitoring scheme to precisely evaluate the secret key rates of these schemes.The analysis results indicate the implementation of the proposed monitoring scheme can effectively resist this potential attack.(3)The monitoring scheme against local oscillator(LO)attacks for a practical GMCS CVQKD system in complex communication environments is studied.To guarantee the practical security of the system,a general monitoring scheme has been previously demonstrated to resist the LO attacks,where the channel transmittance was regarded as a fixed value related to transmission distance.However,practical communication environments are complex,which may result in the time-varying transmittance.This deviation may affect the effectiveness of this monitoring scheme.Therefore,we first model the LO attacks on practical GMCS CVQKD systems in complex environments,where the channel transmittance is assumed to obey a fixed distribution.Then,the low bound of intensity disturbance of the LO signal for Eve successfully concealing herself is obtained based on this model,where we consider all noise that can be used by Eve in complex communication environments.Simultaneously,we obtain an optimal monitoring condition to resist the LO attacks.Our numerical analysis confirms that the proposed monitoring scheme can effectively resist the LO attacks on practical CVQKD systems in complex environments.Subsequently,the feasibility of this way is again demonstrated by using a quantitative example.In particular,this monitoring scheme can also be extended to resist other quantum hacking attack in complex environments.(4)We investigate the practical performance of GMCS CVQKD systems with imperfect randomness.In a practical GMCS CVQKD system,imperfect randomness on the state preparation may exist due to imperfections in Gaussian modulation or in random-number generators.Here,we first propose a weak randomness attack model in practical GMCS CVQKD systems to analyze the practical performance of the systems with imperfect randomness,where the prepared coherent states based on nonrandom state modulation are assumed to be known by Eve.Then,we perform parameter estimation and calculate the secret key rate of the systems based on the proposed attack model.The simulation results demonstrate that a small imperfect randomness can dramatically reduce the secret key rate of the systems.Therefore,true random numbers and perfect Gaussian modulation are key for practical GMCS CVQKD systems.(5)We investigate the improvement of practical performance for GMCS CVQKD systems with imperfect Gaussian modulation.In a practical GMCS CVQKD system,imperfection in the Gaussian modulation may introduce modulation noise that can deteriorate the performance of the system.First,we explore the several different origins,impacts,monitoring and evaluation schemes for the modulation noise in detail.Then,we discuss the practical performance of the system with an untrusted noise model and neutral party model,respectively.These analyses indicate that neutral party model should be reasonably regarded as a general noise model,and the establishment of a reasonable noise model can passively and effectively improve the performance of the system.Finally,we propose a dynamic auto-bias control scheme to actively resist the modulation noise which comes from the drift of bias point of the intensity modulator.Together these methods contribute to the improvement of the practical performance of GMCS CVQKD systems with imperfect Gaussian modulation.In particular,the whole investigations describe a complete and general framework to improve the practical performance of GMCS CVQKD systems with suchlike imperfections.