The electron’s magnetic moment is expressed in units of the Bohr magneton μB with a g-factor that the Dirac equation predicts to be exactly 2. Combining an atomic-beam apparatus with magnetic-resonance techniques, Kusch found g = 2.0023, a small but significant deviation. This deviation, called the anomalous magnetic moment, arises from quantum corrections due to the electron’s interaction with the electromagnetic vacuum. Kusch’s measurement played a decisive role in confirming QED and laid the groundwork for later g-2 experiments and tests of the Standard Model. The magnetic-resonance techniques he refined also evolved into NMR and MRI, which are widely used in medical diagnostics and materials science.

Kusch upgraded Rabi’s atomic-beam magnetic-resonance apparatus and independently modulated external electric and magnetic fields to measure electron spin-flip frequencies with high precision. The resulting g-factor, 2.002319 ± 0.000012, supported Schwinger’s one-loop QED correction of α/2π ≈ 0.00116, providing the first experimental proof of the effect. The experiment introduced innovations in frequency calibration, magnetic-field homogeneity and beam collimation, becoming the technological ancestor of today’s Fermilab Muon g-2 and J-PARC E34 projects. Precision magnetic resonance has since been applied to hyperfine structure measurements, atomic clocks and the control of qubits in quantum computing. Kusch’s achievement exemplifies the mutual advancement of fundamental theory and precision instrumentation.

In Kusch’s atomic-beam experiment, Zeeman transitions in Rb and Ga atoms were used as probes, and the spin-dependent Landé g-values were converted to the free-electron g. An RF field along the beam path generated π/2 pulses, and resonance frequencies were extracted from patterns similar to Ramsey fringes. Nelson spin-echo techniques were applied to cancel residual magnetic fields and motional second-order shifts, achieving calibration at the 10^−5 level. The measured value hinted at (α/π)^2 higher-order terms beyond Schwinger’s correction and stimulated analyses of the Petermann factor and hadronic photon-exchange contributions. Kusch’s data were built into the design parameters of g-2 experiments and used in QED tests for bound vector states. The road to today’s 10^−13 precision single-electron Penning-trap measurements traces back to the benchmark set by his work.