Resonance frequency measurement with accuracy and stability at the 10−12 level in a copper microwave cavity below 26 K by experimental optimization

Abstract

Single-pressure refractive index gas thermometry (SPRIGT) is a novel primary thermometry technique, developed jointly by the Technical Institute of Physics and Chemistry (TIPC) of the Chinese Academy of Sciences (CAS) in China and LNE-Cnam in France. To help obtain a competitive uncertainty of 0.25 mK in thermodynamic temperature measurements, high-stability and low-uncertainty of microwave resonance frequency measurements better than 2 ppb have been demonstrated. This article describes how high-stability and low-uncertainty resonance frequency measurements were achieved using a copper microwave cavity. Microwave measurements were carried out under vacuum and isobarically at helium-4 pressures of 30, 60, 90, and 120 kPa over the temperature range of 5–26 K, with good consistency among microwave modes for the thermodynamic temperatures determined. Performance was optimized using an Allan variance analysis. In this way, with an integration time of 3 h, a stability and accuracy at the 10–12 level were obtained, an almost 20-fold improvement upon our previous result (Zhang et al 2019 Sci. Bull. 64 286–8).