TY - JOUR

T1 - A strong no-go theorem on the Wigner’s friend paradox

AU - Bong, Kok Wei

AU - Utreras-Alarcón, Aníbal

AU - Ghafari, Farzad

AU - Liang, Yeong Cherng

AU - Tischler, Nora

AU - Cavalcanti, Eric G.

AU - Pryde, Geoff J.

AU - Wiseman, Howard M.

N1 - Funding Information:
This work was supported by the Australian Research Council (ARC) Centre of Excellence CE170100012, the Ministry of Science and Technology, Taiwan (grant nos. 107-2112-M-006-005-MY2 and 107-2627-E-006-001), ARC Future Fellowship FT180100317 and grant no. FQXi-RFP-1807 from the Foundational Questions Institute and Fetzer Franklin Fund, a donor advised fund of Silicon Valley Community Foundation. A.U.-A., K.-W.B. and F.G. acknowledge financial support through Australian Government Research Training Program Scholarships and N.T. acknowledges support by the Griffith University Postdoctoral Fellowship Scheme. We gratefully acknowledge A. Acín for bringing ref. 33 to our attention, and thank S. Slussarenko for useful discussions. Avatars in Figs. 1 and 2 are adapted from Eucalyp Studio, available under a Creative Commons licence (Attribution 3.0 Unported), https://creativecommons.org/licenses/ by/3.0/, at https://www.iconfinder.com/iconsets/avatar-55.
Publisher Copyright:
© 2020, The Author(s), under exclusive licence to Springer Nature Limited.

PY - 2020/12

Y1 - 2020/12

N2 - 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.

AB - 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.

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U2 - 10.1038/s41567-020-0990-x

DO - 10.1038/s41567-020-0990-x

M3 - Article

AN - SCOPUS:85089475626

VL - 16

SP - 1199

EP - 1205

JO - Nature Physics

JF - Nature Physics

SN - 1745-2473

IS - 12

ER -