A refined Frauchiger--Renner paradox based on strong contextuality

TL;DR

This paper presents a stronger version of the Frauchiger--Renner paradox using the GHZ--Mermin scenario, highlighting the role of strong contextuality and proposing a resolution based on an extension of Peres's dictum.

Contribution

It introduces a new paradox based on strong contextuality that does not require post-selection or quantum reasoning by observers, advancing understanding of quantum self-reference issues.

Findings

The GHZ--FR paradox is stronger than the original FR paradox.

Strong contextuality is the key ingredient of the paradox.

An extension of Peres's dictum is proposed to resolve these paradoxes.

Abstract

The Frauchiger--Renner paradox derives an inconsistency when quantum theory is used to describe the use of itself, by means of a scenario where agents model other agents quantumly and reason about each other's knowledge. We observe that logical contextuality (\`a la Hardy) is the key ingredient of the FR paradox, and we provide a stronger paradox based on the strongly contextual GHZ--Mermin scenario. In contrast to the FR paradox, this GHZ--FR paradox neither requires post-selection nor any reasoning by observers who are modelled quantumly. If one accepts the universality of quantum theory including superobservers, we propose a natural extension of Peres's dictum to resolve these extended Wigner's friend paradoxes.