In 2011, the General Conference on Weights and Measures (CGPM) resolved to find a better definition of the kelvin. Currently, the unit is defined as 1/273.16 of the triple point of water, a standard that works well for thermometers operating near room temperature but causes inaccuracies in those designed for extreme conditions (such as a ceramic furnace or a liquid helium bath). CGPM argued that the kelvin should instead be defined in terms of the Boltzmann constant, a fundamental constant that relates the average mechanical energy in a particle to temperature. Researchers have since sought new ways to measure the constant with a high level of accuracy. Now, Luigi Moretti at the Second University of Naples, Italy and colleagues report in Physical Review Letters a sixfold reduction in the uncertainty of the Boltzmann constant when it is determined using laser spectroscopy.
An atom at rest absorbs light at sharp, well-defined frequencies, but these frequencies shift if the atom moves toward or away from the light source. An absorption line measured in a gas of warm, moving atoms will therefore be smeared out in frequency—an effect called Doppler broadening that varies with the square root of the Boltzmann constant. Moretti et al. used a pair of frequency-stabilized lasers to carefully measure this broadening around an infrared absorption line in water held at the triple point, and they determined the constant to be 1.380631±0.000024×10-23 joules/kelvin. Though this value has an uncertainty 20 times greater than the best measurement reported to date, Moretti et al. foresee improvements by summing up larger number of spectra and using more refined models to fit the shape of the atomic absorption line.