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The noise in optical source preparation needs to be characterized for the security of practical continuous-variable quantum key distribution (CVQKD). Two feasible schemes, based on either an active optical switch or a passive beamsplitter, are proposed to monitor the variance of the source noise. We derive the security bounds for both schemes against collective attacks in the asymptotical limit and find that the passive scheme performs better. The finite-size effect is also discussed briefly for both schemes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Source noise affects the security of continuous-variable quantum key distribution (CV QKD) and is difficult to analyze. We propose a model to characterize Gaussian source noise through introducing a neutral party (Fred) who induces the noise with a general unitary transformation. Without knowing Fred's exact state, we derive the security bounds for both reverse and direct reconciliations and show that the bound for reverse reconciliation is tight. [ABSTRACT FROM AUTHOR]