Precise determination of h/meusing a rotating, superconducting ring

A precise determination of h/$m_e$ (Planck’s constant divided by the free-electron mass) using a rotating, superconducting ring is in progress. The measurement is based on flux quantization and the London moment—two manifestations of the macroscopic quantum nature of superconductivity. Here we report the first results from a precision, thin-film niobium ring. These data have a statistical error of 10 parts per million (ppm). A systematic error exists at the level of 2500 ppm, due to the presence of electric charge on the rotor. A model for the charge distribution is suggested and used to adjust the results and obtain a value of h/$m_e$. This work has determined h/$m_e$ directly to an accuracy of 100 ppm, a higher level of precision than previously reported. Recent theoretical calculations have predicted a relativistic mass increase of 150 ppm for electrons in superconducting niobium, but the uncertainty in our final results prevented the clear identification of this effect.