Abstract
We consider two-way continuous-variable quantum key distribution, studying its security against general eavesdropping strategies. Assuming the asymptotic limit of many signals exchanged, we prove that two-way Gaussian protocols are immune to coherent attacks. More precisely we show the general superadditivity of the two-way security thresholds, which are proven to be higher than the corresponding one-way counterparts in all cases. We perform the security analysis first reducing the general eavesdropping to a two-mode coherent Gaussian attack, and then showing that the superadditivity is achieved by exploiting the random on/off switching of the two-way quantum communication. This allows the parties to choose the appropriate communication instances to prepare the key, accordingly to the tomography of the quantum channel. The random opening and closing of the circuit represents, in fact, an additional degree of freedom allowing the parties to convert, a posteriori, the two-mode correlations of the eavesdropping into noise. The eavesdropper is assumed to have no access to the on/off switching and, indeed, cannot adapt her attack. We explicitly prove that this mechanism enhances the security performance, no matter if the eavesdropper performs collective or coherent attacks.
Highlights
Quantum Key Distribution (QKD)[1] is today one of the most advanced quantum technologies among those emerged from the fundamental research in quantum information
Gaussian CV-QKD has been achieved in in-field implementations[17], with practical performances comparable to those of discrete variables (DV)-QKD, despite the latter appears to be more robust for long-distances
In the following we focus on the use of reverse reconciliation (RR)[11,16]
Summary
Quantum Key Distribution (QKD)[1] is today one of the most advanced quantum technologies among those emerged from the fundamental research in quantum information. The typical scenario involves two parties, Alice and Bob, who want to share a secret message over an insecure channel[5] To achieve this goal they encode classical information in non-orthogonal quantum states, which are sent over a noisy quantum channel under control of an eavesdropper, Eve. The standard assumptions to analyze the security of QKD protocols are the following: Eve is computationally unbounded, but has no-access to the parties’ private spaces[2,5] and, most importantly, she is restricted by the no-cloning theorem[6]. The distribution of private keys is possible because any attempt to extract the encoded information unavoidably introduces noise on the quantum states Monitoring this noise the parties can quantify how much Eve has learnt on the secret key and, apply classical error correction and privacy amplification protocols reducing Eve’s information to a negligible amount. These natural properties make CV-QKD a promising candidate for future practical real-world implementations, especially in the mid-range distances like the metropolitan areas where high rates are desirable[23]
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