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Fluctuation Thermometry of an Atom-Resolved Quantum Gas: Beyond the Fluctuation-Dissipation Theorem

Maxime Dixmerias1,*, Joris Verstraten1,*, Cyprien Daix1, Bruno Peaudecerf2, Tim de Jongh1, and Tarik Yefsah1

  • *These authors contributed equally to this work.
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Phys. Rev. Lett. 137, 123401 – Published 15 September, 2026

DOI: https://doi.org/10.1103/45fw-r1cp

Abstract

Thermometry is essential for studying many-body physics with ultracold atoms. Accurately measuring low temperatures in these systems, however, remains a significant challenge due to the absence of a universal thermometer. Most widely applicable methods, such as fitting of in situ density profiles, are limited by the requirement of global thermal equilibrium and inapplicability to homogeneous systems. In this Letter, we introduce a novel in situ thermometry for quantum gases, leveraging single-atom-resolved measurements via quantum gas microscopy, and demonstrate it on an ideal Fermi gas. By analyzing number fluctuations in probe volumes containing as little as one atom on average, we extract both global and local temperatures over a broad dynamic range. Our method does not rely on the fluctuation-dissipation theorem and is based instead on the exact relationship between number fluctuations and density-density correlations. In the low-temperature regime, it allows us to observe significant deviations from fluctuation-dissipation predictions, uncovering subextensive fluctuations. Our method is applicable to systems with arbitrary trapping potentials, requiring neither precise trap calibration nor global thermal equilibrium. This thermometer for quantum gases overcomes key limitations of existing techniques, paving the way for more accurate and versatile temperature measurements in ultracold quantum systems.

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Phys. Rev. Lett. 137, 123401– Published 15 September, 2026
  • Received 24 February 2025
  • Revised 23 February 2026
  • Accepted 12 June 2026
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