Security Analysis For Continuous-variable Quantum Cryptography

Quantum cryptography promises to achieve secure communications between legitimate parties in the age of information and networks,and can also provide unconditionally secure protection for our personal information,privacy,and important data in our daily life.Compared with the conventional cryptography,the security of quantum cryptography is guaranteed by the laws of quantum mechanics,which can achieve the informationtheoretical security.However,in practice,due to device imperfections,the security of a quantum cryptosystem could be compromised.Thus,investigating the loopholes in quantum cryptosystems becomes crucial in reality.In this thesis,we exactly fucus on the practical security of continuous-variable(CV)quantum key distribution(QKD)and try the best to enhance the performance and reliability of practical systems.First of all,we independently propose a wavelength attack on a practical CVQKD system with a heterodyne protocol,in which the transmittance of beam splitters at Bob's station is wavelength-dependent.Our strategy shows that when the two light beams,from Eve to Bob,transmit through the homodyne detector,shot noise will be introduced,which becomes the major contribution to the deviation of Bob's measurement from Eve's,so it must be considered accurately.Under this circumstance,we firstly analyze the solutions of the equations specifically that must be satisfied in this attack.Then we calculate the shot noise of the homodyne detector accurately and conclude that the wavelength attack can be implemented successfully in some parameter regime.Furthermore,we consider the security of practical CVQKD implementation with the local oscillator(LO)fluctuating in time,which opens a loophole for Eve to intercept the secret key.We show that Eve can simulate this fluctuation to hide her Gaussian collective attack by reducing the intensity of the LO.Numerical simulations demonstrate that,if Bob does not monitor the LO intensity or scale his measurements with the instantaneous intensity values of LO,the secret key rate will be compromised severely.Additionally,it is also found that the fluctuations of LO intensity not only make the normalization of Bob's measurement outcomes difficult,but also can change the signalto-noise ratio(SNR)of an imperfect balanced homodyne detector(BHD),which may lead the security of a practical system of CVQKD to be compromised severely.Then,we propose that the LO intensity can be manipulated by the legitimate parties,i.e.,being tuned and stabilized to a required constant value,to eliminate the impact of LO fluctuations and defeat Eve's potential attack on the LO.Moreover,we show that the secret key rate can be increased over a noisy channel,especially the channels of metropolitan QKD networks,by tuning the intensity of LO or the SNR of a practical BHD to an optimal value.We find that,counterintuitively,the requirement on BHD(i.e.,high detection efficiency and low electronic noise)can also be reduced in this case.To realize this manipulation,we give a schematic setup which thus can be used to enhance the security of a practical CVQKD system.After investigating the practical issues in one-way CVQKD systems,in addition to overcoming specific imperfections and loopholes,we also propose a measurementdevice-independent(MDI)QKD scheme,which can close detection loopholes once for all.Compared with similar schemes proposed by other research groups,we use continuous variables instead of discrete ones,i.e.,with the source of Gaussian-modulated coherent states.This protocol not only leaves the detection procedure to the untrusted third partner and thus is immune to all detector side-channel attacks,but also has high key rates compared to its discrete counterpart.Hence,it will be very suitable for the construction of high-security quantum information networks.Then,we prove the security of this protocol of CV MDI-QKD against one-mode and two-mode attacks,respectively.Based on the optimality of collective Gaussian attack and the monogamy(i.e.,nonshareability)of entanglement,we show that two-mode coherent attack,i.e.,two quantum channels are anti-correlated by a potential eavesdropper Eve,is suboptimal,while instead one-mode collective attack,for example,two independent entangling cloner attacks on each channel,is optimal in the asymptotic regime.In the latter case,a lower bound of secret key rate is obtained.Moreover,we also show that,for such relay-based protocols,those multi-mode attacks which tends to neutralize the relay should be reduced to one-mode attack due to the untrust of relays,which also extremely simplifies the security analysis.Therefore,injecting entanglement or correlated noise into channels does not bring much more advantages to Eve for interception.Finally,to make CV MDI-QKD applicable to secure communications,we propose an experimental scheme to truly realize this protocol based on the measurement principle of locally generating LO.This scheme not only solves the problems about calibration and synchronization of the reference frames between Alice,Bob,and Charlie(or Bell relay)by data post-processing,but also greatly simplifies the optical layout both in the side of senders and receiver,which is thus easily compatible with chip-scale implementation.Besides,we also explore the manufactures of balanced homodyne detector and high-power pulsed laser,and make clear their interior configurations,which is very helpful and significant to construct the high secure CVQKD systems.