Quantum electrodynamics tested 100 times more accurately than ever

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Using a newly developed technique, scientists at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg have measured the very small difference in the magnetic properties of two isotopes of highly charged neon in an ion trap with previously unattainable accuracy. Comparison with equally highly accurate theoretical calculations of this difference enables a record-level test of quantum electrodynamics (QED). The agreement of the results is an impressive confirmation of the Standard Model of physics, allowing conclusions to be drawn about the properties of nuclei and setting limits for new physics and dark matter.

Electrons are some of the most fundamental building blocks of matter we know. They are characterized by some very distinctive properties such as their negative charge and the existence of a very specific intrinsic angular momentum known as spin. As a charged particle with spin, each electron has a magnetic moment that aligns itself in a magnetic field similar to a compass needle. The strength of this magnetic moment, given by the so-called g-factor, can be predicted with extraordinary accuracy by quantum electrodynamics. This calculation corresponds to the experimentally measured g-factor to 12 digits, one of the most accurate correspondences of theory and experiment in physics to date. However, the magnetic moment of the electron changes as soon as it is no longer a “free” particle, ie unaffected by other influences, but is bound to, for example, an atomic nucleus. The small changes of the g-factor can be calculated using QED, which describes the interaction between electron and nucleus in terms of an exchange of photons. Highly accurate measurements allow a sensitive test of this theory.

“With our work, we have now succeeded in examining these QED predictions with unprecedented resolution, and partly for the first time,” reports group leader Sven Sturm. “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.” These are similar to hydrogen, but with a 10 times higher nuclear charge, amplifying the QED effects. Isotopes differ in the number of neutrons in the nucleus only if the nuclear charge is the same. 20no9+ and 22no9+ with 10 and 12 neutrons, respectively.

The ALPHATRAP experiment at the Max Planck Institute for Nuclear Physics in Heidelberg offers a specially designed Penning trap to store single ions in a strong 4 Tesla magnetic field in a near-perfect vacuum. The purpose of the measurement is to determine the energy required to reverse the direction of the “compass needle” (spider) in the magnetic field. To this end, the exact frequency of the microwave excitation that is required for this is sought. However, this frequency also depends on the exact value of the magnetic field. To determine this, the researchers use the movement of ions in the Penning trap, which also depends on the magnetic field.

Despite the very good temporal stability of the superconducting magnet used here, unavoidable small fluctuations of the magnetic field limit previous measurements to an accuracy of about 11 digits.

The idea of ​​the new method is to store the two ions to be compared, 20no9+ and 22no9+ simultaneously in the same magnetic field in a coupled motion. In such a movement, the two ions always rotate opposite each other on a common circular path with a radius of only 200 micrometers,” explains Fabian Heiße, Postdoc at the ALPHATRAP experiment.

As a result, the fluctuations of the magnetic field have almost identical effects on both isotopes, so that there is no influence on the difference of the energies sought. Combined with the measured magnetic field, the researchers were able to determine the difference of the g-factors of both isotopes with a record accuracy of up to 13 digits, an improvement by a factor of 100 compared to previous measurements and thus the most accurate comparison of two g-factors worldwide. . The resolution achieved here can be illustrated as follows: If the researchers had measured Germany’s highest mountain, the Zugspitze, with such precision instead of the g-factor, they would be able to recognize individual extra atoms at the summit by the height from the mountain.

The theoretical calculations were performed with comparable accuracy in Christoph Keitel’s department at MPIK. “Compared to the new experimental values, we confirmed that the electron does indeed interact with the atomic nucleus through the exchange of photons, as predicted by QED,” explains group leader Zoltán Harman. This has now been solved for the first time and successfully tested by the difference measurements on the two neon isotopes. Alternatively, assuming the QED results are known, the study makes it possible to determine the nuclear radii of the isotopes more accurately than previously possible by a factor of 10.

“Conversely, the agreement between the results of theory and experiment allows us to limit new physics beyond the well-known Standard Model, such as the strength of the ion’s interaction with dark matter,” says postdoc Vincent Debierre.

“In the future, the method presented here could enable some new and exciting experiments, such as the direct comparison of matter and antimatter or the ultra-precise determination of fundamental constants,” said first author Dr. Tim Sailer.


Investigating the magnetic properties of helium-3


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

Provided by Max Planck Institute of Microstructure Physics

Quote: Quantum electrodynamics tested 100 times more accurately than ever (2022, June 15) retrieved June 16, 2022 from https://phys.org/news/2022-06-quantum-electrodynamics-accurately.html

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