# Experimental test of local observer-independence

## TL;DR

This paper reports an experimental test of local observer-independence in quantum mechanics using a 6-photon setup, violating a Bell-type inequality and suggesting that quantum observations are inherently observer-dependent.

## Contribution

The first empirical test of the extended Wigner's friend scenario demonstrating violation of local observer-independence assumptions in quantum mechanics.

## Key findings

- Violation of Bell-type inequality by 5 standard deviations
- Supports observer-dependent interpretation of quantum theory
- Extends experimental tests of quantum objectivity

## Abstract

The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics, the objectivity of observations is not so clear, most dramatically exposed in Eugene Wigner's eponymous thought experiment where two observers can experience seemingly different realities. The question whether these realities can be reconciled in an observer-independent way has long remained inaccessible to empirical investigation, until recent no-go-theorems constructed an extended Wigner's friend scenario with four observers that allows us to put it to the test. In a state-of-the-art 6-photon experiment, we realise this extended Wigner's friend scenario, experimentally violating the associated Bell-type inequality by 5 standard deviations. If one holds fast to the assumptions of locality and free-choice, this result implies that quantum theory should be interpreted in an observer-dependent way.