A precise determination of the mass of a Cooper pair of electrons in superconducting niobium

We have used a superconducting, thin-film niobium ring deposited on the equator of a precision quartz hemispherical rotor to measure the ratio of Planck's constant to the mass of a Cooper pair of electrons, {dollar}h/msp*{dollar}. We attained a precision of 5 ppm (statistical) and an accuracy of 30 ppm (systematic) for a combined, root sum of squares error of 30 ppm.; As a result of two macroscopic quantum phenomena--flux quantization and the London moment, the flux through a rotating, superconducting ring is a multivalued function of its rotation frequency. The flux goes to zero at certain equally spaced frequencies. The ratio {dollar}h/msp*{dollar} is proportional to this frequency spacing. It is also proportional to the cross sectional area of the niobium ring. The frequency spacing is measured by spinning the quartz hemisphere in a precision helium gas bearing at 6 K, and using a SQUID magnetometer to monitor the flux through the niobium ring as a function of rotation rate. The area of the ring is obtained by measuring two perpendicular equatorial diameters of the quartz rotor at 6 K. This measurement is interferometric, using a continuously tunable dye laser whose frequency is referred to precisely calibrated, Doppler-free absorption lines in molecular tellurium and iodine. The roundness of the quartz rotor is determined at room temperature.; Using the values for Planck's constant and the rest mass of the electron, {dollar}msb{lcub}rm e{rcub}{dollar}, recommended in the most recent fundamental constants revision, we find that the mass measured in this experiment is larger than twice the free electron mass by 84 {dollar}pm{dollar} 30 ppm. Our result disagrees with theoretical predictions that this experiment would observe a mass which is smaller than twice the free electron mass by 8 ppm.