Margarida Pereira

Quantum key distribution (QKD) promises information-theoretic security based on quantum mechanics, but practical implementations face security vulnerabilities due to device imperfections. While recent advances have separately addressed source and detector imperfections, real-world QKD systems suffer from both simultaneously. Here, we demonstrate that existing phase-error-estimation-based security proof techniques can be integrated into a unified security proof that simultaneously accounts for both types of imperfections. This represents an important step toward closing the gap between theoretical security proofs and practical QKD implementations.

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.

Bandwidth-limited devices in the transmitter of fast QKD implementations cause pulse correlations that leak information about previous setting choices. To take them into account in the existing security proofs, a measure of their strengths is needed. This is experimentally challenging, especially for long-range correlations, which are not experimentally accessible. In this work, we propose a new characterization method that exploits a linear model of the modulation devices. We show that this model predicts an upper bound for arbitrary order correlations that makes their characterization possible. We also present experimental results using the proposed method. In doing so, we can retrieve security even in the presence of arbitrary long correlations, with similar performance to classical security proofs.

Quantum key distribution (QKD) has raised as an attractive alternative to classical cryptography due to its security being provided by quantum mechanics rather than relying on algorithms that could potentially be broken in the future, rendering current communications insecure. However, many of the security proofs rely on assumptions that may not agree with reality, for instance, device imperfections can open loopholes that could potentially be exploited by a malicious party in order to extract part, if not all, of the secret key.

Quantum key distribution (QKD) promises theoretically unbreakable encryption by exploiting the principles of quantum mechanics. However, the security of real-world implementations is compromised by inevitable device imperfections, unless these are accounted for in the security proof. In this work, we introduce an innovative and powerful security proof framework that guarantees robustness against all practical source imperfections while maintaining high performances, thereby significantly bridging the gap between the theoretical promise and practical realization of QKD. In combination with measurement-device-independent QKD, which closes all security loopholes related to the measurement units, our framework can guarantee an unprecedented level of implementation security.

Typical security proofs of quantum key distribution (QKD) require that the emitted signals are independent and identically distributed. In practice, however, this assumption is not met because intrinsic device flaws inevitably introduce correlations between the emitted signals. Although analyses addressing this issue have been recently proposed, they only consider a restrictive scenario in which the correlations have a finite and known maximum length that is much smaller than the total number of emitted signals. While it is expected that the magnitude of the correlations decreases as the pulse separation increases, the assumption that this magnitude is exactly zero after a certain point does not seem to have any physical justification. Concerningly, this means that existing analyses cannot guarantee the security of current QKD implementations. Here, we solve this pressing problem by developing a general framework that can handle pulse correlations of unbounded length. Our framework allows us to directly use existing proofs addressing this imperfection without the need to construct them from scratch, thus reestablishing the security of QKD in a simple and versatile manner.

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.