Scientists tested quantum electrodynamics more accurately than ever

The agreement of the results is an impressive confirmation of the standard model of physics.


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Each electron has a magnetic moment that aligns itself in a magnetic field. The strength of this magnetic moment, given by the so-called g-factor, can be predicted with extraordinary accuracy by quantum electrodynamics. When studying the differences between isotopes, many common QED contributions cancel owing to the identical electron configuration, making it possible to resolve the intricate effects stemming from the nuclear differences. Experimentally, however, this quickly becomes limited, particularly by the precision of the ion masses or the magnetic field stability.

Scientists from the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg have reported a new measurement technique that overcomes these limitations. Using their technique, they measured the very small difference in the magnetic properties of two isotopes of highly charged neon in an ion trap with previously inaccessible accuracy.

Group leader Sven Sturm said, “With our work, we have now succeeded in investigating these QED predictions with unprecedented resolution, partially, for the first time. To do this, we looked at the difference in the g-factor for two isotopes of highly charged neon ions that possess only a single electron.”

For this study, scientists used two isotopes: 20Ne9+ and 22Ne9+. Both isotopes differ only in the number of neutrons in the nucleus but have the same nuclear charge. They have 10 and 12 neutrons, respectively.

A specifically built Penning trap is used in the ALPHATRAP experiment at the Max Planck Institute for Nuclear Physics in Heidelberg to store single ions in a strong magnetic field of 4 Tesla in a nearly perfect vacuum. The experiment’s goal is to figure out how much energy it takes to flip the “compass needle’s” (spin) orientation in a magnetic field.

This requires the exact frequency of microwave excitation, which depends on the precise value of the magnetic field. Scientists determined this by exploiting the motion of ions in the Penning trap, which also depends on the magnetic field.

Despite the superconducting magnet’s excellent temporal stability, unavoidable small variations in the magnetic field limit prior observations to roughly 11 digits of precision.

Fabian Heiße, Postdoc at the ALPHATRAP experiment, said, “The idea of the new method is to store the two ions to be compared, 20Ne9+ and 22Ne9+, simultaneously in the same magnetic field in a coupled motion. In such a motion, the two ions always rotate opposite each other on a common circular path with a radius of only 200 micrometers.”

As a result, magnetic field changes have nearly identical effects on both isotopes, implying that the difference in energies sought has no influence. Scientists also determined the difference in the g-factors of both isotopes with a record accuracy of 13 digits when combined with the measured magnetic field, an improvement of a factor of 100 over prior measurements and hence the most exact comparison of two g-factors in the world.

The resolution achieved here can be illustrated as follows: If, instead of the g-factor, the scientists had measured Germany’s highest mountain, the Zugspitze, with such precision, they would be able to recognize additional individual atoms on the summit by the height of the mountain.

Group leader Zoltán Harman said, “In comparison with the new experimental values, we confirmed that the electron does indeed interact with the atomic nucleus via the exchange of photons, as predicted by QED. This has now been resolved and successfully tested for the first time by the different measurements on the two neon isotopes. Alternatively, assuming the QED results are known, the study allows the nuclear radii of the isotopes to be determined more precisely than previously possible by a factor of 10.”

Postdoc Vincent Debierre said, “Conversely, the agreement between the results of theory and experiment allows us to constrain new physics beyond the known standard model, such as the strength of the interaction of the ion with dark matter.”

First author Dr. Tim Sailer said“In the future, the method presented here could allow for several novel and exciting experiments, such as the direct comparison of matter and antimatter or the ultra-precise determination of fundamental constants.”

Journal Reference:

  1. Tim Sailer et al., Measurement of the bound-electron g-factor difference in coupled ions, Nature (2022). DOI: 10.1038/s41586-022-04807-w