How large can a Wigner friend be while violating Local Friendliness?
This text mentions work from 2024 on. Those parts were partly written without a live literature search and have not been checked by an editor yet.
In plain words
In Wigner's friend experiments an outside observer treats a friend who has made a measurement as a quantum system. The 2020 test used single photons as the friend; small agent-like programs on a quantum computer gave violations in 2026, and the open question is how large and how isolated the friend can be made while the violation survives.
Precise statement
Local Friendliness (LF) inequalities follow from absoluteness of observed events, no superdeterminism and locality (Bong et al., Nature Physics 2020). Each friend is a reversible quantum circuit that records its outcome in a memory register later undone by the superobserver. Determine the largest friend (qubit count and branch factor, as defined by Zeng, Labib and Russo 2025) for which an LF violation can be certified at given two-qubit gate fidelity F, and whether a violation can be obtained with the two superobservers' setting choices and outcomes spacelike separated.
What would settle it
An LF inequality violation with friends implemented as large reversible circuits, verified outcome reversal and spacelike-separated superobservers.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
LF violations with agent-like friends implemented as reversible circuits were reported on an IBM processor in September 2026 (Laux and Cavalcanti, arXiv:2609.12527); friends remain small and parties are not spacelike separated.
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