Rayleigh backscattering noise in a Sagnac twin-field QKD set-up

Abstract

Twin-field quantum key distribution (TF-QKD) improves the distance of secure communication unprecedentedly without using quantum repeater. It can be used to build QKD network in long distances. Sagnac loop TF-QKD network has the advantage of providing phase stability and removing the need of interferometer stabilization. However, it suffers from Rayleigh backscattering problem at long fiber distances. With the bidirectional signal propagation, the backscattering noise will arrive at the detectors with the QKD signals when the signals are sent continuously. In my simulation, the secure key rate will approach to 0 when the loop distance is about 200km. Another graduate student Reem Mandil in our group has found that using signal patterning technique can mitigate the Rayleigh backscattering noise in the Sagnac network over long distances. In this thesis, I will first introduce the Rayleigh backscattering problem, the CAL-protocol used in the simulation and the decoy state method. Based on Reem's work, I will discuss about how to get a higher key rate by using time post-selection and setting duty cycle with 0dB insertion loss. I will also consider how to mitigate the Rayleigh backscattering noise under more general conditions, such as considering insertion loss, varying the intensity from Charlie, reducing finite data size. Besides, I will compare the distance limit of Sagnac TF-QKD with and without signal patterning technique. With my method after the optimization, the distance limit will be extended to more than 500km.