Fluctuation thermometry of an atom-resolved quantum gas: Beyond the fluctuation-dissipation theorem

TL;DR

This paper introduces a new in-situ thermometry method for ultracold quantum gases using single-atom resolution, capable of measuring local and global temperatures without relying on the fluctuation-dissipation theorem, thus overcoming limitations of existing techniques.

Contribution

The authors develop a novel thermometry technique based on number fluctuations and density correlations, applicable to homogeneous and trapped systems, and demonstrate its effectiveness on an ideal Fermi gas.

Findings

Able to measure local and global temperatures across a broad range

Detects deviations from fluctuation-dissipation predictions at low temperatures

Works without requiring global thermal equilibrium or precise trap calibration

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 or standard fluctuation thermometry, are limited by the requirement of global thermal equilibrium and inapplicability to homogeneous systems. In this work, 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 with approximately one atom on average, we extract both global and local temperatures over a broad dynamic range. Unlike traditional fluctuation thermometry, 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 sub-extensive fluctuations. Our method is applicable to systems with arbitrary trapping potentials, requiring neither precise trap calibration nor global thermal equilibrium. This nearly universal thermometer for quantum gases overcomes key limitations of existing techniques, paving the way for more accurate and versatile temperature measurements in ultracold quantum systems.