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

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...

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Veröffentlicht in:Nature physics Jg. 16; H. 12; S. 1199 - 1205
Hauptverfasser: Bong, Kok-Wei, Utreras-Alarcón, Aníbal, Ghafari, Farzad, Liang, Yeong-Cherng, Tischler, Nora, Cavalcanti, Eric G., Pryde, Geoff J., Wiseman, Howard M.
Format: Journal Article
Sprache:Englisch
Veröffentlicht: London Nature Publishing Group UK 01.12.2020
Nature Publishing Group
Schlagworte:
639/766/400/482
639/766/483/481
Atomic
Bell's inequality
Classical and Continuum Physics
Complex Systems
Condensed Matter Physics
Evolution
Experiments
Mathematical and Computational Physics
Molecular
Observers
Optical and Plasma Physics
Paradoxes
Physics
Physics and Astronomy
Quantum theory
Theorems
Theoretical
ISSN:1745-2473, 1745-2481
Online-Zugang:Volltext
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Zusammenfassung: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. For a scenario of two separated but entangled observers, inequalities are derived from three fundamental assumptions. An experiment shows that these inequalities can be violated if quantum evolution is controllable on the scale of an observer.
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ISSN:1745-2473
1745-2481
DOI:10.1038/s41567-020-0990-x