- Physical Review Journals
All JournalsPhysics Magazine

Physical Review Research

Reuse & Permissions

It is not necessary to obtain permission to reuse this article or its components as it is available under the terms of the Creative Commons Attribution 4.0 International license. This license permits unrestricted use, distribution, and reproduction in any medium, provided attribution to the author(s) and the published article's title, journal citation, and DOI are maintained. Please note that some figures may have been included with permission from other third parties. It is your responsibility to obtain the proper permission from the rights holder directly for these figures.

Export citation

Export citation

Choose format for download:

Download Citation
  • Open Access

Wigner’s friend scenarios: On what to condition and how to verify the predictions

Flavio Del Santo1,2,3,*, Gonzalo Manzano4, and Časlav Brukner1,3

  • *Contact author: delsantoflavio@gmail.com
PDF

Phys. Rev. Research 7, 033279 – Published 24 September, 2025

DOI: https://doi.org/10.1103/c3yq-9mc7

Abstract

Wigner’s friend experiment and its modern extensions display the ambiguity of the quantum mechanical description regarding the assignment of quantum states. While the friend applies the state-update rule to the system upon observing an outcome of her measurement in a quantum system, Wigner describes the friend’s measurement as a unitary evolution, resulting in an entangled state for the composite system of the friend and the system. In this respect, Wigner is often referred to as a “superobserver,” who has the supreme technological ability to keep the friend’s laboratory coherent. As such, it is often argued that he has the “correct” description of the state. Here, we show that the situation is more symmetrical than is usually thought: There are different types of information that each of the observers has that the other fundamentally cannot have—they reside in different “bubbles” (in Cavalcanti’s terminology). While this can explain why the objectivity of the state assignment is only relative to the bubble, we consider more elaborated situations in the form of a game in which the players can switch between bubbles. We find that, in certain circumstances, observers may be entitled to adopt and verify the state assignment from another bubble if they condition their predictions on all information that is in principle available to them.

View figure in article

Physics Subject Headings (PhySH)

Article Text

References (36)

  1. V. Baumann and S. Wolf, On formalisms and interpretations, Quantum 2, 99 (2018).
  2. E. P. Wigner, Remarks on the mind-body question, in Philosophical Reflections and Syntheses, edited by J. Mehra (Springer, Berlin, 1995), pp. 247–260.
  3. Č. Brukner, On the quantum measurement problem, in Quantum [Un]Speakables II, edited by R. Bertlmann and A. Zeilinger (Springer, Cham, 2017), pp. 95–117.
  4. Č. Brukner, A no-go theorem for observer-independent facts, Entropy 20, 350 (2018).
  5. D. Frauchiger and R. Renner, Quantum theory cannot consistently describe the use of itself, Nat. Commun. 9, 3711 (2018).
  6. R. Healey, Quantum theory and the limits of objectivity, Found. Phys. 48, 1568 (2018).
  7. V. Baumann, F. Del Santo, and Č. Brukner, Comment on Healey’s “quantum theory and the limits of objectivity”, Found. Phys. 49, 741 (2019).
  8. V. Baumann and Č. Brukner, Wigner’s friend as a rational agent, in Quantum, Probability, Logic, edited by M. Hemmo and O. Shenker (Springer, Cham, 2020), pp. 91–99.
  9. V. Baumann, F. Del Santo, A. R. Smith, F. Giacomini, E. Castro-Ruiz, and C. Brukner, Generalized probability rules from a timeless formulation of Wigner’s friend scenarios, Quantum 5, 524 (2021).
  10. M. Proietti, A. Pickston, F. Graffitti, P. Barrow, D. Kundys, C. Branciard, M. Ringbauer, and A. Fedrizzi, Experimental test of local observer independence, Sci. Adv. 5, eaaw9832 (2019).
  11. K.-W. Bong, A. Utreras-Alarcón, F. Ghafari, Y.-C. Liang, N. Tischler, E. G. Cavalcanti, G. J. Pryde, and H. M. Wiseman, A strong no-go theorem on the Wigner’s friend paradox, Nat. Phys. 16, 1199 (2020).
  12. E. G. Cavalcanti, The view from a Wigner bubble, Found. Phys. 51, 39 (2021).
  13. M. Żukowski and M. Markiewicz, Physics and metaphysics of Wigner’s friends: Even performed premeasurements have no results, Phys. Rev. Lett. 126, 130402 (2021).
  14. P. A. Guérin, V. Baumann, F. Del Santo, and Č. Brukner, A no-go theorem for the persistent reality of Wigner’s friend’s perception, Commun. Phys. 4, 1 (2021).
  15. G. Leegwater, When Greenberger, Horne and Zeilinger meet Wigner’s friend, Found. Phys. 52, 68 (2022).
  16. N. Ormrod and J. Barrett, A no-go theorem for absolute observed events without inequalities or modal logic, arXiv:2209.03940.
  17. V. Vilasini and M. P. Woods, A general framework for consistent logical reasoning in Wigner’s friend scenarios: Subjective perspectives of agents within a single quantum circuit, arXiv:2209.09281.
  18. M. Haddara and E. G. Cavalcanti, A possibilistic no-go theorem on the Wigner’s friend paradox, New J. Phys. 25, 093028 (2023).
  19. H. M. Wiseman, E. G. Cavalcanti, and E. G. Rieffel, A “thoughtful” local friendliness no-go theorem: A prospective experiment with new assumptions to suit, Quantum 7, 1112 (2023).
  20. V. Baumann and Č. Brukner, Wigner’s friend’s memory and the no-signaling principle, Quantum 8, 1481 (2024).
  21. V. Baumann, Classical information and collapse in Wigner’s friend setups, Entropy 25, 1420 (2023).
  22. A. Utreras-Alarcón, E. G. Cavalcanti, and H. M. Wiseman, Allowing Wigner’s friend to sequentially measure incompatible observables, Proc. R. Soc. A 480, 20240040 (2024).
  23. A. P. Polychronakos, Quantum mechanical rules for observed observers and the consistency of quantum theory, Nat. Commun. 15, 3023 (2024).
  24. R. Healey, Quantum theory: A pragmatist approach, Br. J. Philos. Sci. 63, 729 (2012).
  25. K. Fine, The reality of tense, Synthese 150, 399 (2006).
  26. M. A. Lipman, On Fine's fragmentalism, Philos. Stud. 172, 3119 (2015).
  27. D. Dieks, Perspectival quantum realism, Found. Phys. 52, 95 (2022).
  28. J. L. Pienaar, A single space-time is too small for all of Wigner’s friends, arXiv:2312.11759.
  29. W. H. Zurek, Decoherence, einselection, and the quantum origins of the classical, Rev. Mod. Phys. 75, 715 (2003).
  30. W. H. Zurek, Quantum Darwinism, Nat. Phys. 5, 181 (2009).
  31. W. H. Zurek, Quantum reversibility is relative, or does a quantum measurement reset initial conditions? Philos. Trans. R. Soc. A 376, 20170315 (2018).
  32. M. Schlosshauer, Quantum decoherence, Phys. Rep. 831, 1 (2019).
  33. T. Breuer, The impossibility of accurate state self-measurements, Philos. Sci. 62, 197 (1995).
  34. A. Wald, Sequential tests of statistical hypotheses, Ann. Math. Stat. 16, 117 (1945).
  35. A. Sudbery, Single-world theory of the extended Wigner’s friend experiment, Found. Phys. 47, 658 (2017).
  36. F. Giacomini, E. Castro-Ruiz, and Č. Brukner, Quantum mechanics and the covariance of physical laws in quantum reference frames, Nat. Commun. 10, 494 (2019).

Outline

Information

Phys. Rev. Research 7, 033279– Published 24 September, 2025
  • Received 10 June 2025
  • Accepted 3 September 2025
Crossmark - Check for updates button
Creative Commons Logo

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Sign In to Your Journals Account

Filter

Filter

Article Lookup

Enter a citation