Addressing Pulse Correlations in QKD Security

One of the most important and common device imperfections in QKD, especially among high-speed systems, is pulse correlations. These arise when the setting choices made in one round affect subsequent rounds, typically due to memory effects in modulation devices (such as phase and amplitude modulators), potentially leaking key information to eavesdroppers without raising any red flags. While recent security analyses have attempted to tackle this issue, they rely on unrealistic assumptions that are not met in practice, notably the presumption of a finite and known maximum correlation length. Unfortunately, this means that the current QKD implementations are not secure against this common flaw.

To address this critical vulnerability, a team of QSNP researchers from Universidade de Vigo, introduced a new formalism that frees QKD security from the constraints of finite correlation lengths. By removing this assumption, this approach ensures the security of real-life QKD implementations against arbitrarily long pulse correlations. Moreover, this approach extends beyond pulse correlations to other imperfections such as discrete phase randomisation.

Enhancing QKD Security Against Pulse Correlations

To achieve this milestone, the researchers performed an in-depth analysis of existing security proofs, revealing the fragility of assumptions concerning correlation lengths. This is because, in reality, the length of these correlations could potentially be unbounded. In other words, the first emitted pulse could correlate with the last one, even if very faintly. To demonstrate the application of their formalism, they chose a scenario in which the emitted signals suffer from bit and basis correlations and assumed that the correlations decrease exponentially with their length. However, this framework also works for other types of correlations, such as intensity or phase-randomization correlations with an unbounded length. Furthermore, we can use it to incorporate other imperfections into existing security proofs that assume perfect devices.

As we look into the future, their focus shifts towards experimental validation and characterization of pulse correlations, a crucial step in refining our understanding of QKD’s implementation security. Additionally, efforts will continue to address and characterize other imperfections, paving the way for a future where the next generation of secure communications can withstand the sophisticated threats posed by quantum computers. Their research enhances the security of practical QKD systems, increasing its potential to allow fields reliant on secure communications—healthcare, finance, defence, telecommunications— operate with confidence in this digital world.

Curious about what’s next? Our News page is your ticket to staying in the loop.


Image credits: generated with openart.ai


“Quantum key distribution with unbounded pulse correlations”

Margarida Pereira, Guillermo Currás-Lorenzo, Akihiro Mizutani, Davide Rusca, Marcos Curty and Kiyoshi Tamaki.

arXiv:2402.08028 [quant-ph]

Published 12 Ferbuary 2024