# Evaluation of Counterfactuality in Counterfactual Communication Protocols

## TL;DR

This paper introduces an information-theoretic framework to evaluate the counterfactuality of quantum communication protocols, revealing limitations of existing schemes under realistic conditions.

## Contribution

It develops a methodology using Fisher information and probability measures to assess counterfactuality, providing a new way to analyze quantum communication protocols.

## Key findings

- Salih et al.'s protocol only satisfies counterfactuality with perfect channels
- Arvidsson-Shukur and Barnes's protocol satisfies their counterfactuality definition
- Realistic quantum channels cause violations in Salih et al.'s protocol

## Abstract

We provide an in-depth investigation of parameter estimation in Nested Mach-Zehnder interferometers (NMZIs) using two information measures: the Shannon mutual information and the classical Fisher information. Protocols for counterfactual communition (CFC) have, so far, been based on two different definitions of counterfactuality. In particular, some schemes have been based on NMZI devices, but have recently been subject to criticism. We provide a methodology for evaluating the counterfactuality of these protocols, based on an information theoretical framework. More specifically, we make the assumption that any realistic quantum channel in MZI structures will have some weak uncontrolled interaction, and we use the Fisher information to measure counterfactual violations. The measure is used to evaluate the suggested counterfactual communication protocol by Salih et al. \cite{Salih13}. The protocol of Arvidsson-Shukur and Barnes \cite{ArvShukur16}, based on a different definition, is evaluated with a probability measure. Our results show that the definition of Arvidsson-Shukur and Barnes is satisfied by their scheme, whilst that of Salih et al. is only satisfied by perfect quantum channels. For realistic devices the latter protocol does not achieve its objective.