Towards a quantum standard for absolute pressure measurements

The work concerns the development of a novel absolute gas pressure standard in a range of 200 Pa to 20 kPa (0.002 atm to 0.2 atm). The method used, refractive index gas manometry, involves the determination of the refractive index of a pure noble gas (helium-4 or argon) using a microwave resonator. The refractive index is determined via the ratio of the resonant frequencies measured with the resonator under vacuum and under pressure at a constant temperature. Given the thermodynamic temperature of the gas and its refractive index, it is possible to calculate its pressure. This principle is the reciprocal of that used for refractive index gas thermometry (RIGT) in which thermodynamic temperature is extracted from the measured refractive index of gas at a known pressure. The frequency scanning pattern and fitting algorithms take advantage of earlier work in the laboratory on the determination of the Boltzmann constant prior to the 2019 redefinition of the kelvin, the SI unit of thermodynamic temperature. For Helium-4, measurements were made at 5.4 K using a quasi-spherical copper resonator coated on its inner surface with a 3 μm thick layer of superconducting niobium. Superconductivity improves the quality factor Q of the resonator and thereby the frequency resolution of the system. The pressure determination benefits from the very high accuracy of the dielectric and density virial coefficients of helium-4 obtained by ab initio calculations. Measurements on Argon were performed at a temperature of 90.4 K with an uncoated copper resonator. At this temperature, the apparatus is less demanding in terms of cryogenics than its helium counterpart and so easier to replicate.

ab initio calculation, pressure standard, metrology, manometry, thermometry, microwave cavity, superconductivity, refractive index, virial coefficient, helium, argon

PhD Thesis (EN) : https://tel.archives-ouvertes.fr/tel-03652344