Atomic thermometry in optical lattice clocks

Abstract

Accurate measurement of atomic temperature is fundamental for a wide range of applications, from quantum sensing to precision metrology. In optical lattice clocks, precise characterization of atomic temperature is required to minimize systematic uncertainties at the $10^{-18}$ level. In this work, we investigate atomic temperature measurements in the ytterbium optical lattice clock developed at INRIM, IT-Yb1, employing sideband and Doppler spectroscopy across a wide range of trapping conditions. By implementing clock-line-mediated Sisyphus cooling [Phys. Rev. Lett. 133, 053401 (2024)], we reduce the atomic temperature and enable operation at shallower lattice depths down to $D = 50E_{R}$. We compare temperature estimates obtained from the harmonic oscillator model with those derived using a Born-Oppenheimer-based approach [Phys. Rev. A 101, 053416 (2020)], which is expected to provide a more accurate description of atomic motion in both longitudinal and radial directions, especially for hotter atoms whose motion deviates from the harmonic regime. Discrepancies up to a factor of two in extracted temperatures are observed depending on the chosen model. We assess the impact of these modeling differences on the evaluation of lattice shifts and find relative frequency deviations up to $8\times10^{-17}$. Even though extended Sisyphus cooling reduces these inconsistencies to the $1\times10^{-18}$ level, residual biases may still limit the accuracy of optical lattice clocks.