Allowing Wigner's friend to sequentially measure incompatible observables

• URL: http://arxiv.org/abs/2305.09102v1
• Date: Tue, 16 May 2023 02:01:12 GMT
• Title: Allowing Wigner's friend to sequentially measure incompatible observables
• Authors: An\'ibal Utreras-Alarc\'on, Eric G. Cavalcanti and Howard M. Wiseman
• Abstract summary: 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. We show that, in these scenarios, the local friendliness inequalities will always be the same as Bell inequalities.
• Score: 0.0
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: 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.

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