Allowing Wigner’s friend to sequentially measure incompatible observables
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Cavalcanti, Eric G
Wiseman, Howard M
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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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Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
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480
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2288
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Engineering
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Utreras-Alarcón, A; Cavalcanti, EG; Wiseman, HM, Allowing Wigner’s friend to sequentially measure incompatible observables, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2024, 480 (2288), pp. 20240040