Explicit attacks on BB84 with distinguishable photons

TL;DR

This paper investigates how photon distinguishability affects the security of BB84 quantum key distribution, presenting explicit attack strategies that exploit side channels to reduce security thresholds.

Contribution

It introduces new attack strategies on BB84 with distinguishable photons, demonstrating improved eavesdropping methods considering side channels.

Findings

Side channels enable better eavesdropping strategies.

Critical error rate for secure key distribution is reduced.

Upper bounds on secret key rate are established.

Abstract

Distinguishability of photons in non-operational degrees of freedom compromises unconditional security of quantum key distribution since an eavesdropper can improve attack strategies by exploiting this distinguishability. However, the optimal eavesdropping strategies in the presence of light source side channels are not known. Here we provide several explicit attack strategies on the BB84 protocol with partially distinguishable photons. In particular, we consider the phase-covariant cloning attack, which is optimal in the absence of side channels, and show that there are even better strategies in the presence of side channels. The improved strategies exploit a measurement of the side channel state followed by an attack on the signal photon. Our results explicitly demonstrate reduction of the critical error rate, and set an upper bound on the practical secret key rate.