Generalized probability rules from a timeless formulation of Wigner's friend scenarios

Veronika Baumann; Flavio Del Santo; Alexander R. H. Smith; Flaminia Giacomini; Esteban Castro-Ruiz; Caslav Brukner

doi:10.22331/q-2021-08-16-524

Generalized probability rules from a timeless formulation of Wigner’s friend scenarios

Veronika Baumann^1,2,3, Flavio Del Santo^1,2, Alexander R. H. Smith⁴, Flaminia Giacomini^1,2,5, Esteban Castro-Ruiz^1,2,6, and Caslav Brukner^1,2

¹Institute for Quantum Optics and Quantum Information (IQOQI-Vienna) of the Austrian Academy of Sciences, Boltzmanngasse 3, A-1090, Vienna, Austria
²Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria
³Faculty of Informatics, Università della Svizzera italiana, Via G. Buffi 13, CH-6900 Lugano, Switzerland
⁴Department of Physics and Astronomy, Dartmouth College, Hanover, New Hampshire 03755, USA
⁵Perimeter Institute for Theoretical Physics, 31 Caroline St. N, Waterloo, Ontario, N2L 2Y5, Canada
⁶QuIC, Ecole polytechnique de Bruxelles, C.P. 165, Université libre de Bruxelles, 1050 Brussels, Belgium

Published:	2021-08-16, volume 5, page 524
Eprint:	arXiv:1911.09696v4
Doi:	https://doi.org/10.22331/q-2021-08-16-524
Citation:	Quantum 5, 524 (2021).

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Abstract

The quantum measurement problem can be regarded as the tension between the two alternative dynamics prescribed by quantum mechanics: the unitary evolution of the wave function and the state-update rule (or "collapse") at the instant a measurement takes place. The notorious Wigner's friend gedankenexperiment constitutes the paradoxical scenario in which different observers (one of whom is observed by the other) describe one and the same interaction differently, one –the Friend– via state-update and the other –Wigner– unitarily. This can lead to Wigner and his friend assigning different probabilities to the outcome of the same subsequent measurement. In this paper, we apply the Page-Wootters mechanism (PWM) as a timeless description of Wigner's friend-like scenarios. We show that the standard rules to assign two-time conditional probabilities within the PWM need to be modified to deal with the Wigner's friend gedankenexperiment. We identify three main definitions of such modified rules to assign two-time conditional probabilities, all of which reduce to standard quantum theory for non-Wigner's friend scenarios. However, when applied to the Wigner's friend setup each rule assigns different conditional probabilities, potentially resolving the probability-assignment paradox in a different manner. Moreover, one rule imposes strict limits on when a joint probability distribution for the measurement outcomes of Wigner and his Friend is well-defined, which single out those cases where Wigner's measurement does not disturb the Friend's memory and such a probability has an operational meaning in terms of collectible statistics. Interestingly, the same limits guarantee that said measurement outcomes fulfill the consistency condition of the consistent histories framework.

Featured image: The circuit representation of the Wigner’s friend experiment as encoded in the (timeless) physical state. This can be thought of as the perspective of a hypothetical higher level observer whose experimental powers are even beyond that of Wigner.

Popular summary

The measurement problem plagues a common sense interpretation of quantum theory. It arises from the tension between two different dynamics describing the evolution of a quantum system: the first (unitary evolution) resembles the deterministic evolution of classical physics, while the second one ("collapse") applies whenever a measurement takes place and is a discontinuous, probabilistic process. Despite an ambiguity between which of these two dynamics to apply when, physicists know well which description to use in their daily practice. However, since there is today no strong evidence that quantum mechanics has a limit of applicability—in principle it holds at any scale, from subatomic particles to chairs, from human bodies to planets, etc.—one can think that a whole laboratory including the physicist carrying out her experiments can be regarded as a quantum system, and therefore observed by another physicist outside the laboratory. This scenario is called a Wigner's friend experiment, and it shows that the physicist inside the lab (the friend) and the one measuring them from outside (Wigner) both using the same textbook of quantum mechanics arrive at different conclusions about the probabilities of certain measurements, because they both apply one of the two different dynamics.

In our work, we have argued that a way to clarify the conceptual issues of the measurement problem in a Wigner's friend scenario is to eliminate time (in a classical sense, that is, a parameter which always increases) from the description. We have shown that in this way, Wigner and the friend remove the which-dynamics ambiguity because they always agree on a single timeless state of the whole physical situation. However, even if they always agree, they can use different rules to predict probabilities for measurement results. We have shown that some probability rules give the probabilities provided by unitary dynamics, while others lead to probabilities equal to those given by "collapse“ dynamics. This means, depending on which rule Wigner and his friend use, they will either agree on the probabilities for unitary dynamics or those for collapse dynamics. In our framework, the ambiguity between the two dynamics is thus pushed back to the choice of the probability-assignment rule, but once Wigner and the friend agree on a rule (ideally based on some physical arguments or evidence), there cannot be disagreement on the probabilities.

► BibTeX data

@article{Baumann2021generalized,
  doi = {10.22331/q-2021-08-16-524},
  url = {https://doi.org/10.22331/q-2021-08-16-524},
  title = {Generalized probability rules from a timeless formulation of {W}igner's friend scenarios},
  author = {Baumann, Veronika and Santo, Flavio Del and Smith, Alexander R. H. and Giacomini, Flaminia and Castro-Ruiz, Esteban and Brukner, Caslav},
  journal = {{Quantum}},
  issn = {2521-327X},
  publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}},
  volume = {5},
  pages = {524},
  month = aug,
  year = {2021}
}

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Cited by

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The above citations are from Crossref's cited-by service (last updated successfully 2023-11-29 18:10:19) and SAO/NASA ADS (last updated successfully 2023-11-29 18:10:20). The list may be incomplete as not all publishers provide suitable and complete citation data.

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