Robust Quantum Key Distribution Arbitrarily Close to Local Correlations

Abstract

Recently, Wooltorton et al. [Phys. Rev. Lett. 132, 210802 (2024)] and Farkas [Phys. Rev. Lett. 132, 210803 (2024)] have exhibited the mismatch between Bell inequality violations and their cryptographic application in device-independent quantum key distribution, by showing that arbitrarily close to the set of local behaviours there exist quantum correlations guaranteeing a constant rate of secret key. While these results require correlations attaining the maximum quantum value of a suitable Bell observable (aka the Tsirelson bound) and rely on a kind of ideal self-testing of a maximally entangled state and associated Bell measurement, here we show that the effect is robust: for every one of the Bell inequalities considered by Wooltorton \emph{et al.}, a constant rate of secret key ensues if the observed Bell violation is sufficiently close to the respective Tsirelson bound. For these and also the Bell inequalities of Farkas, we furthermore present numerical results based on semidefinite relaxations of the minimum min-entropy consistent with a certain Bell violation, which demonstrate that small but nonzero key rates can be guaranteed (in principle) by practical and efficient means.