Measuring an Electron’s Magnetism in a Molecule
Physics beyond the standard model could lie in a possible discrepancy between the calculated and measured values of the electron’s g factor, which characterizes its magnetic moment. The g factor for a free electron is known to one part in 1013. Achieving a similar precision for an electron bound within a molecule could be revealing, since the value is sensitive to interactions between the electron and other particles in its vicinity. Now Charlotte König at the Max Planck Institute for Nuclear Physics in Germany and colleagues have determined g for the electron in an ionized molecule of hydrogen and deuterium with record precision [1].
The researchers trapped a single ion and subjected it to a microwave signal at a certain frequency. Then they recorded whether its spin had flipped by observing how it oscillated within a magnetic field. The electron’s spin-transition energy depends on the configuration of the proton and deuteron spins. This configuration changes spontaneously, but König and colleagues could also trigger a reconfiguration with the microwave signal. They repeated the flip-and-measure procedure hundreds of times for each nuclear-spin configuration while varying the signal frequency. This procedure yielded the electron-spin spectrum for all possible configurations and with it a value for g with a relative uncertainty of 2 × 10–10. This measurement is about 1000 times more precise than the previous best. Although the value itself was consistent with quantum electrodynamics, two related quantities, the electron–proton and electron–deuteron coupling strengths, deviated tantalizingly from theory.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- C. M. König et al., “High-precision Penning trap spectroscopy of the ground state spin structure of HD+,” Phys. Rev. Lett. 136, 143002 (2026).