Xoel Sixto

Decoy-state quantum key distribution (QKD) is undoubtedly the most efficient solution to handle multi-photon signals emitted by laser sources, and provides the same secret key rate scaling as ideal single-photon sources. It requires, however, that the phase of each emitted pulse is uniformly random. This might be difficult to guarantee in practice, due to inevitable device imperfections and/or the use of an external phase modulator for phase randomization, which limits the possible selected phases to a finite set. Here, we investigate the security of decoy-state QKD with arbitrary, continuous or discrete, non-uniform phase randomization, and show that this technique is quite robust to deviations from the ideal uniformly random scenario. For this, we combine a novel parameter estimation technique based on semi-definite programming, with the use of basis mismatched events, to tightly estimate the parameters that determine the achievable secret key rate. In doing so, we demonstrate that our analysis can significantly outperform previous results that address more restricted scenarios.

The decoy-state method is a prominent approach to enhance the performance of quantum key distribution (QKD) systems that operate with weak coherent laser sources. Current experimental decoy-state QKD setups increase their secret key rate by raising the repetition rate of the transmitter, which can lead to correlations between subsequently emitted optical pulses. This phenomenon leaks information about the encoding settings, including the intensities of the generated signals, thus invalidating a basic premise of decoy-state QKD. Here, we experimentally characterize intensity correlations between the nearest-neigbouring optical pulses in two commercial prototypes of decoy-state BB84 QKD systems and show that they significantly reduce the asymptotic key rate. In addition, we study intensity correlations between pulses spaced further apart (higher-order correlations) and find that, in contrast to what has been conjectured, their impact on the intensity of the generated signals can be much higher than that of the nearest-neighbour (first-order) correlations.

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.