Accurate determination of the Boltzmann constant by Doppler spectroscopy: Towards a new definition of the kelvin

TL;DR

This paper demonstrates that high-precision Doppler spectroscopy of ammonia can determine the Boltzmann constant with uncertainties comparable to acoustic methods, supporting a potential redefinition of the kelvin based on fundamental constants.

Contribution

It presents new spectroscopic measurements and models that enable a ppm-level determination of the Boltzmann constant, advancing the precision of fundamental constant measurements.

Findings

Achieved a few ppm uncertainty in Boltzmann constant measurement

Validated models including Dicke narrowing and collisional effects

Supports redefining the kelvin based on fundamental constants

Abstract

Accurate molecular spectroscopy in the mid-infrared region allows precision measurements of fundamental constants. For instance, measuring the linewidth of an isolated Doppler-broadened absorption line of ammonia around 10 $\mu$m enables a determination of the Boltzmann constant k B. We report on our latest measurements. By fitting this lineshape to several models which include Dicke narrowing or speed-dependent collisional effects, we find that a determination of k B with an uncertainty of a few ppm is reachable. This is comparable to the best current uncertainty obtained using acoustic methods and would make a significant contribution to any new value of k B determined by the CODATA. Furthermore, having multiple independent measurements at these accuracies opens the possibility of defining the kelvin by fixing k B, an exciting prospect considering the upcoming redefinition of the International System of Units.