Violation of Bell Inequalities with a Mixture of Separable States in a Multiple-Photon Absorption Attack on Ekert Protocol

TL;DR

This paper reveals a novel attack on the Ekert quantum key distribution protocol where an adversary can induce Bell inequality violations using only separable states by exploiting multiple-photon absorption, challenging the assumption that such violations imply entanglement.

Contribution

It introduces a new attack method on the Ekert protocol that mimics entanglement signatures using separable states and multiple-photon absorption effects.

Findings

Bell violations can be simulated with separable states under this attack.

The attack is effective with two-photon and three-photon absorption scenarios.

The method can produce Bell inequality violations similar to those of entangled states.

Abstract

We propose a new type of attack on Ekert protocol in which, rather surprisingly, Eve drives a violation of Bell inequalities in Alice's and Bob's detectors with a mixture of separable states. She does so by sending correlated pulses containing several photons at frequencies where only multiple-photon absorptions are possible in their detectors. Whenever the photons stemming from a same pulse are dispatched in such a way that the number of photons is insufficient to trigger a multiple-photon absorption in either channel, the pulse remains undetected. We show that this simple feature leads to violations of Bell inequalities that can match closely those predicted for entangled states, even in the simplest cases of two-photon and three-photon absorptions.