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Spin systems as quantum field theories in inflationary universe: A study with Unruh-DeWitt detectors

Shunichiro Kinoshita1,*,†, Keiju Murata1,‡, Daisuke Yamamoto1,2,§, and Ryosuke Yoshii3,4,‖

  • *Present address: Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japan.
  • †Contact author: kinoshita@neptune.kanazawa-it.ac.jp
  • ‡Contact author: murata.keiju@nihon-u.ac.jp
  • §Contact author: yamamoto.daisuke21@nihon-u.ac.jp
  • ‖Contact author: ryoshii@rs.socu.ac.jp
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Phys. Rev. Research 7, 043135 – Published 5 November, 2025

DOI: https://doi.org/10.1103/s3qb-n94w

Abstract

We propose a method to probe the thermal properties of quantum field theory (QFT) in an inflationary universe simulated by spin systems. Our previous work [Phys. Rev. Res. 7, 023197 (2025)] has demonstrated that QFT of Majorana fermions in an arbitrary two-dimensional spacetime can be mapped onto a spin system. In this study, we apply this mapping to investigate the thermal properties of an inflationary universe. An interaction between a quantum field and a detector allows one to extract information about the quantum field from the excitation probability of the detector, known as the Unruh-DeWitt detector. In an inflationary universe with Hubble constant H, the excitation probability of an Unruh-DeWitt detector follows a thermal distribution with temperature H/(2π), indicating that a static observer in the inflationary universe perceives a thermal field. We consider a spin system corresponding to QFT in an inflationary universe and introduce a single spin interacting with this system as an Unruh-DeWitt detector. We demonstrate that the detector response asymptotically approaches the result of QFT with an appropriate power of the number of spin sites. Since the dynamics of spin systems can be implemented on programmable quantum simulation platforms, our study offers a concrete route toward experimentally probing the thermal properties of an inflationary universe in controlled quantum settings. This highlights the potential of quantum technologies to emulate and investigate aspects of quantum field theory in curved spacetimes.

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Phys. Rev. Research 7, 043135– Published 5 November, 2025
  • Received 29 May 2025
  • Accepted 28 September 2025
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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.

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