Testing the reality of Wigner's friend's observations

Does quantum theory apply at all scales, including that of observers? A resurgence of interest in the long-standing Wigner's friend paradox has shed new light on this fundamental question. Here---building on a scenario with two separated but entangled "friends" introduced by Brukner---we rigorously prove that if quantum evolution is controllable on the scale of an observer, then one of the following three assumptions must be false: "No-Superdeterminism", "Locality", or "Absoluteness of Observed Events" (i.e. that every observed event exists absolutely, not relatively). We show that although the violation of Bell-type inequalities in such scenarios is not in general sufficient to demonstrate the contradiction between those assumptions, new inequalities can be derived, in a theory-independent manner, which are violated by quantum correlations. We demonstrate this 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 quantum reality than Bell's theorem.Comment: In v1, v2 we claimed to give the first rigorous proof of Brukner's theorem, interpreting his "Observer Independent Facts" assumption to be weaker than what he formalized. This was inaccurate (Brukner's theorem follows from his assumptions) and obscured the significantly stronger implications of our theorem. In v3 we name the weaker assumption in our theorem "Absoluteness of Observed Events

Allowing Wigner's friend to sequentially measure incompatible observables

The Wigner's friend thought experiment has gained a resurgence of interest in recent years thanks to no-go theorems that extend it to Bell-like scenarios. One of these, by us and co-workers, showcased the contradiction that arises between quantum theory and a set of assumptions, weaker than those in Bell's theorem, which we named "local friendliness". Using these assumptions it is possible to arrive at a set of inequalities for a given scenario, and, in general, some of these inequalities will be harder to violate than the Bell inequalities for the same scenario. A crucial feature of the extended Wigner's friend scenario in our aforementioned work was the ability of a superobserver to reverse the unitary evolution that gives rise to their friend's measurement. Here, we present a new scenario where the superobserver can interact with the friend repeatedly in a single experimental instance, either by asking them directly for their result, thus ending that instance, or by reversing their measurement and instructing them to perform a new one. We show that, in these scenarios, the local friendliness inequalities will always be the same as Bell inequalities.Comment: 12 pages, 3 figures. No observers were harmed in the conduct of this wor