Experimental Study Of Fiber-based Continuous-Variable Quantum Key Distribution

Quantum key distribution(QKD)is one of the most important research field in quantum information.From an experimental point of view,QKD can be implemented with quantum discrete variables and continuous variables.What's remarkable here is that the continuous-variable(CV)QKD promises to achieve a higher secret key rate,and it is a highly compact combination of coherent communication,thus that it could be easily integrated into the current fiber network.Though the unconditional security of CV-QKD has been demonstrated,the secure distance(less than 80 km)and secret key rate(less than 10 kbps)are relatively low.Therefore,it is important for us to find out the limitations and then overcome these limitations when transforming the theoretical paradigm into a physical reality.The experimental achievements reported in the paper will include the latest theoretical works,integrate the advanced technologies,and it mainly includes:1.Ultra-high sensitive and wideband shot-noise-limited balanced homodyne detectors(BHD)are developed.Because of the launching power limitation of localoscillator(LO)in the long-distance CV-QKD,we developed an ultra-high sensitive BHD so that we can achieve the shot-noise limit detection as well as reduce the requirement of LO power at a distance beyond 100 km.The ratio between the shot noise and the electronic noise of our BHD is one order of magnitude larger than the reported state-of-art BHD,besides it has the ability to resolve the photon number.We also developed a BHD which can achieve the shot-noise limit within 1 GHz bandwidth,which is much higher than the reported state-of-art wideband shot-noise-limited BHD.2.CV-QKD with a secure distance beyond 100 km is experimentally demonstrated for the first time.It is the longest CV-QKD over an optical fibre channel yet demonstrated.Since the major limitation of the secure distance is the excess noise,our long-distance CV-QKD experiment combined with the excess noise analysis and controlling which utilized several advanced technical schemes,includes an ultra-high sensitive BHD,a high resolution phase compensation scheme,a high-efficiency classical reconciliation scheme and so on.On the security side,the secure distance can reach150 km with the consideration of the finite-size effects based on uncertainty principle,and the secure distance can also reach 100 km if we consider the recent composable security.The secure distance reported in this work almost doubles the record achieved in previous state-of-art experiment [Nat.Photonics 7,378(2013)].3.50 MHz CV-QKD is experimentally demonstrated for the first time,it is the highest repetition rate of the engineering system yet demonstrated.The major limitation of the repetition rate in a CV-QKD system is the bandwidth of BHD,and the major limitation of key rate per pulse is the excess noise.A wideband shot-noise-limited BHD and a high-speed and high-efficiency classical reconciliation are employed,so that we could improve the system repetition rate.Besides,the excess noise under the wavelength division multiplexing condition is analyzed and controlled.On the security side,the finite-size effects are considered,and the secret key rate of 1 Mbps is achieved at a distance of 25 km.Compared with the state-of-art experiment,owing to the great improvement of repetition rate,the secret key rate reported in this work is almost two orders of magnitude higher than the previous record.4.100 MHz one-way CV-QKD without sending a LO is experimentally demonstrated for the first time,and we also introduce and experimentally demonstrate a plug and play dual-phase-modulated coherent states CV-QKD scheme.At the present stage,almost all of the security loopholes are focused on the transmitted LO.For the oneway CV-QKD reported in this paper,the LO is generated at the receiver's side,so that we can defend all of the attacks associated with the LO.Meanwhile,we developed a new scheme to overcome the difficulty of conventional CV-QKD — two laser sources with high frequency stability are required.Based on the plug and play structure,we can use the dual-phase scheme to realize the CV-QKD with a locally generated LO using a single laser source,as well as keeping the security of the Gaussian scheme.5.Field CV-QKD network is demonstrated for the first time.This QKD network is based on the campus of Shanghai Jiao Tong University,four QKD nodes including Minhang,Xujiahui,Fahua,and Qibao are connected based on commercial telecommunication fiber links.In this paper,the excess noise caused by the field environment is analyzed and controlled,and we also provide some improved methods.