# A strong no-go theorem on the Wigner's friend paradox

## TL;DR

This paper presents a new no-go theorem demonstrating that if quantum evolution is controllable at the observer scale, then at least one of the assumptions of No-Superdeterminism, Locality, or Absoluteness of Observed Events must be false, providing stronger constraints than Bell's theorem.

## Contribution

The authors derive new, theory-independent inequalities related to the Wigner's friend paradox and experimentally demonstrate their violation, strengthening the constraints on physical reality.

## Key findings

- Quantum correlations violate derived inequalities in the experiment.
- At least one of the assumptions (No-Superdeterminism, Locality, Absoluteness) must be false.
- The theorem imposes stricter constraints than Bell's theorem.

## Abstract

Does quantum theory apply at all scales, including that of observers? New light on this fundamental question has recently been shed through a resurgence of interest in the long-standing Wigner's friend paradox. This is a thought experiment addressing the quantum measurement problem -- the difficulty of reconciling the (unitary, deterministic) evolution of isolated systems and the (non-unitary, probabilistic) state update after a measurement. Here, by building on a scenario with two separated but entangled friends introduced by Brukner, we prove that if quantum evolution is controllable on the scale of an observer, then one of 'No-Superdeterminism', 'Locality' or 'Absoluteness of Observed Events' -- that every observed event exists absolutely, not relatively -- must be false. We show that although the violation of Bell-type inequalities in such scenarios is not in general sufficient to demonstrate the contradiction between those three assumptions, new inequalities can be derived in a theory-independent manner, that are violated by quantum correlations. This is demonstrated in a proof-of-principle experiment where a photon's path is deemed an observer. We discuss how this new theorem places strictly stronger constraints on physical reality than Bell's theorem.