1,721,106 research outputs found
Two-way Gaussian quantum cryptography against coherent attacks in direct reconciliation
Get PDFWe consider a two-way quantum cryptographic protocol with coherent states assuming direct reconciliation. A detailed security analysis is performed considering a two-mode coherent attack, which represents the residual eavesdropping once the parties have reduced the general attack by applying symmetric random permutations. In this context we provide a general analytical expression for the key rate, discussing the impact of the residual two-mode correlations on the security of the scheme. In particular, we identify the optimal eavesdropping against two-way quantum communication, which is given by a two-mode coherent attack with symmetric and separable correlations
Improving the lower bound to the secret-key capacity of the thermal amplifier channel
Get PDFWe consider the noisy thermal amplifier channel, where signal modes are amplified together with environmental thermal modes. We focus on the secret-key capacity of this channel, which is the maximum amount of secret bits that two remote parties can generate by means of the most general adaptive protocol, assisted by unlimited and two-way classical communication. For this channel only upper and lower bounds are known, and in this work we improve the lower bound. We consider a protocol based on squeezed states and homodyne detections, in both direct and reverse reconciliation. In particular, we assume that trusted thermal noise is mixed on beam splitters controlled by the parties in a way to assist their homodyne detections. The new improved lower bounds to the secret-key capacity are obtained by optimizing the key rates over the variance of the trusted noise injected, and the transmissivity of the parties’ beam splitters. Our results confirm that there is a separation between the coherent information of the thermal amplifier channel and its secret key capacity
Continuous variable entanglement and quantum state teleportation between optical and macroscopic vibrational modes through radiation pressure
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Gaussian two-mode attacks in one-way quantum cryptography
Get PDFWe investigate the asymptotic security of one-way continuous variable quantum key distribution against Gaussian two-mode coherent attacks. The one-way protocol is implemented by arranging the channel uses in two-mode blocks. By applying symmetric random permutations over these blocks, the security analysis is in fact reduced to study two-mode coherent attacks and, in particular, Gaussian ones, due to the extremality of Gaussian states. We explicitly show that the use of two-mode Gaussian correlations by an eavesdropper leads to asymptotic secret key rates which are strictly larger than the rate obtained under standard single-mode Gaussian attacks
Parameter estimation with almost no public communication for continuous-variable quantum key distribution
Get PDFOne crucial step in any quantum key distribution (QKD) scheme is parameter estimation. In all known QKD protocols, if prior information is not available, the users have to sacrifice part of their raw data to estimate the parameters of the communication channel as, for example, the error rate. This introduces a tradeoff between the secret key rate and the accuracy or parameter estimation in the finite-size regime. Here we show that continuous-variable (CV) QKD is not subject to this constraint and the whole raw keys can be used for both parameter estimation and secret key generation, without compromising the security. First we show that this property holds for measurement-device independent (MDI) protocols, as a consequence of the fact that in an MDI protocol the correlations between Alice and Bob are post-selected by the measurement performed by an untrusted relay. This result is then extended beyond the MDI framework by exploiting the fact that MDI protocols can simulate device-dependent one-way QKD with arbitrarily high precision
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