Absolute measurement of the diameter of a fused quartz hemisphere at 6 K

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Abstract
We have measured by interferometry the diameter of a fused‐quartz hemispherical rotor to 200 nm, or an accuracy of 4 parts‐per‐million (ppm), at both room temperature and at liquid helium temperature. We placed the rotor in an optically contacted fused‐quartz etalon, and used a continuous‐wave dye laser to measure the lengths of the optical cavities. The laser frequency was referenced to Doppler‐free absorption lines in molecular tellurium and molecular iodine. These measurements were done in order to determine the area of a superconducting ring used in a precision determination of h/me. Our results confirm earlier length measurements at room temperature performed by the National Bureau of Standards (NBS).