Optimal eavesdropping on noisy states in quantum key distribution

TL;DR

This paper analyzes how added quantum noise affects eavesdropping in the six state quantum key distribution protocol, showing that noise can enhance the protocol's robustness against individual attacks.

Contribution

It provides the first analysis of Eve's optimal information gain in noisy six state QKD, revealing that noise can improve security.

Findings

Added noise increases Eve's mutual information threshold.

Quantum noise can make QKD more robust against eavesdropping.

Optimal mutual information depends on the qubit error rate.

Abstract

We study eavesdropping in quantum key distribution with the six state protocol,when the signal states are mixed with white noise. This situation may arise either when Alice deliberately adds noise to the signal states before they leave her lab, or in a realistic scenario where Eve cannot replace the noisy quantum channel by a noiseless one. We find Eve's optimal mutual information with Alice, for individual attacks, as a function of the qubit error rate. Our result is that added quantum noise can make quantum key distribution more robust against eavesdropping.