Modeling and Characterization of Arbitrary Order Pulse Correlations for Quantum Key Distribution

TL;DR

This paper introduces a linear model to characterize and bound arbitrary order pulse correlations in quantum key distribution systems, addressing a key security challenge caused by device memory effects.

Contribution

It provides a simple method to derive upper bounds on pulse correlation strength of any order from system step response, ensuring QKD security against unbounded correlations.

Findings

Successfully characterized short-range correlations experimentally.

Applied the model to account for long-range, even infinite order, correlations.

Demonstrated the method's effectiveness in ensuring QKD security.

Abstract

In quantum key distribution (QKD) implementations, memory effects caused by the limited bandwidth of modulators and/or other active devices can leak information about previous setting choices. Security proofs addressing this imperfection require the characterization of pulse correlations, which, in principle, can be of an arbitrary order, even unbounded. Experimentally, this is very hard (if not impossible) to achieve. Here, we solve this pressing problem by introducing a simple linear model to explain pulse correlations. In so doing, we can derive upper bounds on the correlation strength of arbitrary order from the study of the step response of the system. Importantly, this is what is needed to ensure the security of QKD in the presence of pulse correlations of unbounded length. We experimentally characterize short-range correlations and apply the proposed method to account for long-range correlations to an infinite order.