Source of 4σ discrepancy between optical thermometry and diode sensor at deep cryogenic temperatures

Determine whether the observed 4σ discrepancy at the highest temperature point (approximately 7.7–7.8 K) between the optically determined temperature using Doppler broadening of the H₂ 1-0 S(0) transition at 2.22 µm in the cryogenic high-finesse cavity and the reference temperature measured with the Lake Shore Cryotronics DT-670 diode sensor arises from systematic error in the optical thermometry method or from error in the diode sensor measurement or calibration.

Background

The paper demonstrates optical realization of SI units in the deep cryogenic regime (approximately 4–20 K), including thermometry based on Doppler broadening of a molecular line. The authors compare the optically determined temperature (derived from the Doppler width of the H₂ 1-0 S(0) transition) with a reference temperature measured by a calibrated diode sensor.

Across most temperatures, the two methods agree within combined uncertainties. However, at the highest temperature point in the paper (around 7.7–7.8 K), the difference between the optical method and the diode sensor reaches 4σ, and the authors explicitly state that they do not know which measurement is responsible for the discrepancy. Resolving this uncertainty is necessary to validate the optical realization of the kelvin at deep cryogenic temperatures and to assess the reliability of the diode sensor reference under these conditions.