Relating Wigner's Friend Scenarios to Nonclassical Causal Compatibility, Monogamy Relations, and Fine Tuning

Yìlè Yīng; Marina Maciel Ansanelli; Andrea Di Biagio; Elie Wolfe; David Schmid; Eric Gama Cavalcanti

doi:10.22331/q-2024-09-26-1485

Relating Wigner’s Friend Scenarios to Nonclassical Causal Compatibility, Monogamy Relations, and Fine Tuning

Yìlè Yīng^1,2, Marina Maciel Ansanelli^1,2, Andrea Di Biagio^3,4, Elie Wolfe^1,2, David Schmid⁵, and Eric Gama Cavalcanti⁶

¹Perimeter Institute for Theoretical Physics, Waterloo, Ontario, Canada, N2L 2Y5
²Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1
³Institute for Quantum Optics and Quantum Information (IQOQI) Vienna, Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria
⁴Basic Research Community for Physics e.V., Mariannenstraße 89, Leipzig, Germany
⁵International Centre for Theory of Quantum Technologies, University of Gdańsk, 80-309 Gdańsk, Poland
⁶Centre for Quantum Dynamics, Griffith University, Yugambeh Country, Gold Coast, Queensland 4222, Australia

Published:	2024-09-26, volume 8, page 1485
Eprint:	arXiv:2309.12987v4
Doi:	https://doi.org/10.22331/q-2024-09-26-1485
Citation:	Quantum 8, 1485 (2024).

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Abstract

Nonclassical causal modeling was developed in order to explain violations of Bell inequalities while adhering to relativistic causal structure and $faithfulness$---that is, avoiding fine-tuned causal explanations. Recently, a no-go theorem that can be viewed as being stronger than Bell's theorem has been derived, based on extensions of the Wigner's friend thought experiment: the Local Friendliness (LF) no-go theorem. Here we show that the LF no-go theorem poses formidable challenges for the field of causal modeling, even when nonclassical and/or cyclic causal explanations are considered. We first recast the LF inequalities, one of the key elements of the LF no-go theorem, as special cases of monogamy relations stemming from a statistical marginal problem. We then further recast LF inequalities as causal compatibility inequalities stemming from a $nonclassical$ causal marginal problem, for a causal structure implied by well-motivated causal-metaphysical assumptions. We find that the LF inequalities emerge from this causal structure even when one allows the latent causes of observed events to admit post-quantum descriptions, such as in a generalized probabilistic theory or in an even more exotic theory. We further prove that $no$ nonclassical causal model can explain violations of LF inequalities without violating the No Fine-Tuning principle. Finally, we note that these obstacles cannot be overcome even if one appeals to $cyclic$ causal models, and we discuss potential directions for further extensions of the causal modeling framework.

Presentation Relating Wigner’s Friend Scenarios to Nonclassical Causal Compatibility, Monogamy Relations, and Fine Tuning by Yìlè Yīng Perimeter Institute for Theoretical Physics, September 16, 2024

► BibTeX data

@article{Yng2024relatingwigners,
  doi = {10.22331/q-2024-09-26-1485},
  url = {https://doi.org/10.22331/q-2024-09-26-1485},
  title = {Relating {W}igner's {F}riend {S}cenarios to {N}onclassical {C}ausal {C}ompatibility, {M}onogamy {R}elations, and {F}ine {T}uning},
  author = {Yīng, Y{\`{i}}l{\`{e}} and Ansanelli, Marina Maciel and Biagio, Andrea Di and Wolfe, Elie and Schmid, David and Cavalcanti, Eric Gama},
  journal = {{Quantum}},
  issn = {2521-327X},
  publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}},
  volume = {8},
  pages = {1485},
  month = sep,
  year = {2024}
}

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