Precision laser absorption spectroscopy for primary thermometry

There is currently a global effort to refine high-accuracy primary thermometry techniques driven by a call from the Bureau des Internationale de Poid et Measures to remeasure the Boltzmann constant, k_B, in preparation for the upcoming redefinition of the kelvin. We have used quantitative laser spectroscopy to precisely measure the Doppler broadening of atomic transitions in rubidium and cesium vapors. By using a conventional platinum resistance thermometer and the Doppler thermometry technique, we were able to determine k_B with a relative uncertainty of 1.4 × 10^-4. Our experiment, using an effusive atomic vapor, departs significantly from other Doppler-broadened thermometry (DBT) techniques, which rely on weakly absorbing molecules in a diffusive regime. In these circumstances, the dominant systematic effects strongly differ from those of the molecular experiments and thus can lend further confidence if the outcomes of the experiments are all in agreement.