# Consistent Descriptions of Quantum Measurement

## TL;DR

This paper analyzes the Wigner's friend thought experiment, identifies inconsistencies in previous reasoning, and advocates for a relational quantum measurement framework to ensure consistent descriptions among observers.

## Contribution

It demonstrates the inconsistency in prior reasoning about quantum measurements and applies the relational formulation to resolve observer-dependent discrepancies.

## Key findings

- Inconsistencies arise from outdated information use in quantum reasoning.
- Relational quantum measurement ensures observer synchronization.
- Thought experiments confirm the necessity of relational formulation.

## Abstract

The Wigner's friend type of thought experiments manifest the conceptual challenge on how different observers can have consistent descriptions of a quantum measurement event. In this paper, we analyze the extended version of Wigner's friend thought experiment (Frauchiger and Renner, Nature Comm. 3711, 9 (2018)) in detail and show that the reasoning process from each agent that leads to the no-go theorem is inconsistent. The inconsistency is with respect to the requirement that an agent should make use of updated information instead of outdated information. We then apply the relational formulation of quantum measurement to resolve the inconsistent descriptions from different agents. In relational formulation of quantum mechanics, a measurement is described relative to an observer. Synchronization of measurement result is a necessary requirement to achieve consistent descriptions of a quantum system from different observers. Thought experiments, including EPR, Wigner's Friend and it extended version, confirm the necessity of relational formulation of quantum measurement when applying quantum mechanics to composite system with entangled but space-like separated subsystems.