# Synopsis: QED passes with flying colors

Precision measurements of the electron magnetic moment in hydrogenlike silicon provide a stringent test of bound-state quantum electrodynamics.

An important early test of quantum electrodynamics (QED)—the theory that quantizes the interaction of light and matter—was predicting the gyromagnetic ratio ($g$ factor) of a free electron. Now, with a precise measurement of the $g$ factor of an electron in a hydrogenlike orbit around the silicon nucleus, scientists in Germany report in Physical Review Letters the most stringent test to date of QED applied to a bound particle.

The $g$ factor is a dimensionless ratio that relates a particle’s magnetic moment in a field to its spin or orbital angular momentum. According to QED, an electron bound to an atom has a $g$ factor that deviates from a value close to $2$ by terms that depend on the ion’s nuclear charge and the fine structure constant ($\alpha \approx 1/137$). The lowest-order corrections have been confirmed in lighter elements, like carbon and oxygen. To see higher-order effects of QED, Sven Sturm at the Max-Planck-Institute for Nuclear Physics in Heidelberg, Germany, and colleagues used a sequence of three magnetoelectric (Penning) traps to strip away all but one of silicon’s fourteen electrons and measured the ratio of the spin precession frequency of the remaining electron and the cyclotron frequency of the ion.

The team established the bound electron’s $g$ factor to an unprecedented certainty—enough to see QED contributions of less than a tenth of a millionth of a percent. – Jessica Thomas

### Announcements

More Announcements »

Optics

Fluid Dynamics

## Related Articles

Atomic and Molecular Physics

### Synopsis: Deep Freezing Molecules

Researchers cooled trapped molecules well below $1\phantom{\rule{2.22198pt}{0ex}}\text{mK}$—a record temperature for molecules that have not been assembled from pre-cooled atoms. Read More »

Particles and Fields

### Viewpoint: Extending an Alternative to Feynman Diagrams

A simplifying technique for calculating scattering amplitudes—the basis for predictions in particle physics experiments—has been extended to cover a class of effective quantum field theories. Read More »

Cosmology

### Synopsis: A Little Empty Inside

A new model has allowed researchers to test a theory for why the centers of dark matter halos are less dense than expected. Read More »