We consider two-way continuous-variable quantum key distribution, studyingits security against general eavesdropping strategies. Assuming the asymptoticlimit of many signals exchanged, we prove that two-way Gaussian protocols areimmune to coherent attacks. More precisely we show the general superadditivityof the two-way security thresholds, which are proven to be higher than thecorresponding one-way counterparts in all cases. We perform the securityanalysis first reducing the general eavesdropping to a two-mode coherentGaussian attack, and then showing that the superadditivity is achieved byexploiting the random on/off switching of the two-way quantum communication.This allows the parties to choose the appropriate communication instances toprepare the key, accordingly to the tomography of the quantum channel. Therandom opening and closing of the circuit represents, in fact, an additionaldegree of freedom allowing the parties to convert, a posteriori, the two-modecorrelations of the eavesdropping into noise. The eavesdropper is assumed tohave no access to the on/off switching and, indeed, cannot adapt her attack. Weexplicitly prove that this mechanism enhances the security performance, nomatter if the eavesdropper performs collective or coherent attacks.
展开▼
