Four-state continuous-variable quantum key distribution with long secure distance

Abstract

The four-state continuous-variable quantum key distribution protocol has a long practical secure distance [A. Leverrier and P. Grangier, Phys. Rev. Lett. 102, 180504 (2009)], while it poses the difficulty of parameter estimation. We propose an improved four-state protocol where the covariance matrix can be estimated from experimental data without using the linear channel assumption, thus ensuring its unconditional security in the asymptotical limit. Our scheme maintains the advantage of high reconciliation efficiency of the four-state protocol, which ensures long practical secure distance its. Our scheme can be implemented with the current technology.