Device-independent quantum key distribution with asymmetric CHSH inequalities

TL;DR

This paper enhances device-independent quantum key distribution by using asymmetric Bell inequalities, improving robustness and error thresholds in practical scenarios.

Contribution

It introduces an extended family of asymmetric Bell expressions, providing tighter bounds on entropy and increasing noise tolerance in quantum key distribution.

Findings

Tolerable error rate increased from 7.15% to 7.42%.

Adding noise raises threshold to over 8.34%.

Provides a tight bound on von Neumann entropy for asymmetric CHSH expressions.

Abstract

The simplest device-independent quantum key distribution protocol is based on the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality and allows two users, Alice and Bob, to generate a secret key if they observe sufficiently strong correlations. There is, however, a mismatch between the protocol, in which only one of Alice's measurements is used to generate the key, and the CHSH expression, which is symmetric with respect to Alice's two measurements. We therefore investigate the impact of using an extended family of Bell expressions where we give different weights to Alice's measurements. Using this family of asymmetric Bell expressions improves the robustness of the key distribution protocol for certain experimentally-relevant correlations. As an example, the tolerable error rate improves from 7.15% to about 7.42% for the depolarising channel. Adding random noise to Alice's key before the postprocessing pushes the threshold further to more than 8.34%. The main technical result of our work is a tight bound on the von Neumann entropy of one of Alice's measurement outcomes conditioned on a quantum eavesdropper for the family of asymmetric CHSH expressions we consider and allowing for an arbitrary amount of noise preprocessing.