# 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

More Features »

### Announcements

More Announcements »

Optics

Fluid Dynamics

## Related Articles

Atomic and Molecular Physics

### Viewpoint: Quantum Phase Transitions Go Dynamical

The observation of dynamical quantum phase transitions in an interacting many-body system breaks new ground in the study of matter out of thermal equilibrium. Read More »

Particles and Fields

### Synopsis: Filling in a Tetraquark’s Profile

An analysis of electron-positron collision data has determined the spin and parity of a particle thought to consist of four quarks. Read More »

Atomic and Molecular Physics

### Focus: The Coolest Molecular Ion Beams

Two research teams have created the coldest molecular ion beams ever, putting molecules in their ground states of rotation and providing improved experimental stand-ins for interstellar gas clouds. Read More »