New Measurement of the Electron Magnetic Moment Using a One-Electron Quantum Cyclotron

A new measurement resolves cyclotron and spin levels for a single-electron quantum cyclotron to obtain an electron magnetic moment, given by $g/2=1.00115965218085(76)[0.76\mathrm{ppt}]$. The uncertainty is nearly 6 times lower than in the past, and $g$ is shifted downward by 1.7 standard deviations. The new $g$, with a quantum electrodynamics (QED) calculation, determines the fine structure constant with a 0.7 ppb uncertainty—10 times smaller than for atom-recoil determinations. Remarkably, this 100 mK measurement probes for internal electron structure at 130 GeV.