Álvaro Navarrete

Numerical security proofs offer a versatile approach for evaluating the secret-key generation rate of quantum key distribution (QKD) protocols. However, existing methods typically require perfect source characterization, which is unrealistic in practice due to the presence of inevitable encoding imperfections and side channels. In this paper, we introduce a novel security proof technique based on semidefinite programming that can evaluate the secret-key rate for both prepare-and-measure and measurement-device-independent QKD protocols when only partial information about the emitted states is available, significantly improving the applicability and practical relevance compared to existing numerical techniques. We demonstrate that our method can outperform current analytical approaches addressing partial state characterization in terms of achievable secret-key rates, particularly for protocols with non-qubit encoding spaces. This represents a significant step towards bridging the gap between theoretical security proofs and practical QKD implementations.

Chip-based quantum key distribution (QKD) systems offer improved efficiency but may also introduce previously unrecognized security vulnerabilities. In this work, we identify and experimentally characterize cross-polarization-intensity (CPI) correlations in a real-world chip-based QKD system. Moreover, we introduce a security analysis that incorporates CPI correlations and apply it to evaluate the performance of an integrated high-speed QKD system. Our results emphasize the need for rigorous security assessments in chip-based QKD implementations.

Recently, different modulator-free decoy-state quantum key distribution transmitters have been proposed. Among their advantages, they are essentially immune to information leakage, including that potentially induced by an adversary via e.g. a Trojan-horse attack. However, practical implementations of these transmitters emit, in addition to the desired signals, some extra pulses that are not used as quantum carriers, but still may contain sensitive information about the intensity and bit/basis encoding of the signals. This unwanted pulses can be actively blocked with an intensity modulator (or an optical switch), but the extinction ratio of these devices is always finite, and thus it is still crucial to account for the residual amount of information leakage at the security-proof level. In this work, we analyze the security of these transmitters and evaluate their performance in the presence of this kind of inherent information leakage. We find that the secret-key rate of the protocol is severely affected when the information leakage is not sufficiently attenuated, which highlights the importance of accounting for such type of imperfections.

A crucial assumption in most quantum key distribution (QKD) security proofs, is that no information about the selected settings is leaked to the channel. A secure space around the users' devices is usually required to ensure both parties can generate and handle classical data securely. However, this condition is not feasible in practice, since the devices usually leak some information passively, and an eavesdropper could even run a Trojan horse attack (THA) by injecting bright light into the QKD apparatuses, causing an active leak of information. In this paper, we present the first security proof for a decoy state protocol that considers an arbitrary leakage from every setting selected in the source due to passive or active information leakage. Furthermore, we apply our security proof to various cases of practical interest and we analyze the effectiveness of placing an extra phase modulator in the source to improve the secret key rate. Our analysis is also experimentally friendly, as it only requires one parameter to encapsulates all side-channel imperfections. We believe that our results constitute a vital step in closing the existing gap between theory and implementation in QKD.

The core of security proofs of quantum key distribution (QKD) is the estimation of a parameter that determines the amount of privacy amplification that the users need to apply in order to distil a secret key. To estimate this parameter using the observed data, one needs to apply concentration inequalities, such as random sampling theory or Azuma’s inequality. The latter can be straightforwardly employed in a wider class of QKD protocols, including those that do not rely on mutually unbiased encoding bases, such as the loss-tolerant (LT) protocol. However, when applied to real-life finite-length QKD experiments, Azuma’s inequality typically results in substantially lower secret-key rates. Here, we propose an alternative security analysis of the LT protocol against general attacks, for both its prepare-and-measure and measure-device-independent versions, that is based on random sampling theory. Consequently, our security proof provides considerably higher secret-key rates than the previous finite-key analysis based on Azuma’s inequality. This work opens up the possibility of using random sampling theory to provide alternative security proofs for other QKD protocols.

There is a large gap between theory and practice in quantum key distribution (QKD) because real devices do not satisfy the assumptions required by the security proofs. We close this gap by introducing a simple and practical measurement-device-independent-QKD type of protocol, based on the transmission of coherent light, for which we prove its security against any possible imperfection and/or side channel from the quantum communication part of the QKD devices. Our approach only requires to experimentally characterize an upper bound of one single parameter for each of the pulses sent, which describes the quality of the source. Moreover, unlike device-independent (DI) QKD, it can accommodate information leakage from the users’ laboratories, which is essential to guarantee the security of QKD implementations. In this sense, its security goes beyond that provided by DI QKD, yet it delivers a secret key rate that is various orders of magnitude greater than that of DI QKD.

In decoy-state-based QKD, GHz clocked or higher frequency transmitters exhibit correlations between the intensities of succeeding pulses. As a consequence, every pulse leaks partial information about previous intensity settings to an eavesdropper, thus invalidating the fundamental principle of the decoy-states method, i.e., the independent character of the yields from the intensity settings. In this work, we present a technique that allows to incorporate arbitrary intensity correlations to the decoy-state analysis, thereby solving a pressing problem in the race towards practical high-speed QKD systems. As a side contribution, we present a non-standard derivation of the asymptotic key rate formula from the non-asymptotic one, in so revealing a largely dismissed necessary condition for the significance of the former. We discuss this condition in full detail.