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

Nature Physics volume 16, pages 1199–1205 (2020)Cite this article

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Abstract

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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Fig. 1: Concept of the extended Wigner’s friend scenario.
Fig. 2: A specific bipartite Wigner’s friend experiment.
Fig. 3: A two-dimensional slice of the space of correlations, illustrating the correlations discussed in this work.
Fig. 4: Results for the left-hand sides of Bell and LF inequalities for different quantum states.
Fig. 5: Experimental set-up.

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Data availability

Data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

Code availability

The numerical codes used to determine the inequalities and to choose the measurement settings are available from the corresponding authors upon reasonable request.

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Acknowledgements

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.

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Author notes

  1. These authors contributed equally: Kok-Wei Bong, Aníbal Utreras-Alarcón.

Authors and Affiliations

  1. Centre for Quantum Computation and Communication Technology (Australian Research Council), Centre for Quantum Dynamics, Griffith University, Brisbane, Queensland, Australia

    Kok-Wei Bong, Aníbal Utreras-Alarcón, Farzad Ghafari, Nora Tischler, Geoff J. Pryde & Howard M. Wiseman

  2. Department of Physics and Center for Quantum Frontiers of Research & Technology (QFort), National Cheng Kung University, Tainan, Taiwan

    Yeong-Cherng Liang

  3. Centre for Quantum Computation and Communication Technology (Australian Research Council), Centre for Quantum Dynamics, Griffith University, Southport, Queensland, Australia

    Eric G. Cavalcanti

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Contributions

A.U.-A., E.G.C., Y.-C.L. and H.M.W. performed the theory work. K.-W.B., N.T., H.M.W. and G.J.P. designed the experiment, which was realized by K.-W.B., N.T., F.G. and G.J.P. All authors contributed to the preparation of the manuscript and N.T. and E.G.C. took responsibility for its final form.

Corresponding authors

Correspondence to Nora Tischler or Eric G. Cavalcanti.

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Supplementary information

Supplementary Information

Supplementary discussion, Fig. 1 and Table 1.

Source data

Source Data Fig. 4

Plotted data, experiment (mean and standard deviation) and theory.

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Bong, KW., Utreras-Alarcón, A., Ghafari, F. et al. A strong no-go theorem on the Wigner’s friend paradox. Nat. Phys. 16, 1199–1205 (2020). https://doi.org/10.1038/s41567-020-0990-x

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