Optical determination of the Boltzmann constant

The Boltzmann constant k_B is a fundamental physical constant in thermodynamics. The present CODATA recommended value of k_B is 1.3806488(13) × 10^–23 J/K (relative uncertainty 0.91 ppm), which is mainly determined by acoustic methods. Doppler broadening thermometry (DBT) is an optical method which determines k_B T by measuring the Doppler width of an atomic or molecular transition. The methodology and problems in DBT are reviewed, and DBT measurement using the sensitive cavity ring-down spectroscopy (CRDS) is proposed. Preliminary measurements indicate that CRDS-based DBT measurement can potentially reach an accuracy at the 1 ppm level. (topical review)