Frequency comb referenced displacement interferometry for the NIST calculable capacitor

This paper describes the ongoing work on measuring the variable spacing between two electrodes in the next-generation NIST calculable capacitor. The spacing measurement is based on a high finesse Fabry-Perot interferometer system where displacements are inferred from optical frequency measurements. Using a frequency comb referenced to an atomic clock as an optical frequency standard, we achieve a fractional uncertainty of δν/ν ~ 2·10^-11 at 2 seconds. This corresponds to a stabilization of the spacing at the picometer level.