When it comes to the most tested and precise scientific theories, quantum electrodynamics (QED) ranks at or near the top of the list. The theory of light–matter interactions has predicted, for example, the value of the electron’s magnetic dipole moment to 12 decimal places, consistent with observations published last year. Yet despite QED’s superlative predictions, it’s incomplete.
Perturbation theory can precisely determine the quantum behavior of electrons and photons in hydrogen atoms and other low-mass bound systems. Quantum perturbations in a high-mass element, however, cannot be well approximated. By the late 2000s, after decades of work, theorists succeeded in developing nonperturbative methods for predicting QED effects in heavy atoms, including 90-fold ionized uranium. It’s also known as helium-like uranium because of its two remaining electrons. But the corresponding spectroscopy measurements reported in 2009 lacked accuracy because of technical limitations and other uncertainties.
After designing a more robust experiment, a team of 34 researchers, including Martino Trassinelli (the CNRS in Paris) and Robert Loetzsch (Friedrich Schiller University Jena in Germany), now present energy measurements of an atomic-orbital transition that are eight times as precise as previous results. In Darmstadt, Germany, at the GSI Helmholtz Centre for Heavy Ion Research’s experimental storage ring, excited helium-like uranium ions are generated when hydrogen-like uranium ions each capture one electron from a gas-jet target. Then two x-ray spectrometers can observe QED effects by measuring the transition energy as one of the electrons moves from an excited state in the 2p atomic orbital to a lower-energy state in the 2s orbital.
To precisely measure the energy transition, the researchers had to design a better spectrometer. Instead of one, they used two spectrometers to generate redundant data and collect more measurements. The new instruments incorporate high-quality curved germanium crystals, which reflect incoming x-ray photons much more predictably than the 2009 version. By developing better correction for the relativistic Doppler shift and incorporating some previous spectral results, the researchers also improved the precision of the spectrometer’s viewing angle.
The new measurements of highly ionized uranium show that QED theory still works, even for high-mass systems. One quantum effect on the transition energy is the so-called one-loop QED contribution, in which virtual particles interacting with an electron slightly modifies the transition energy. The improved measurements are consistent with not only the one-loop contribution but also a second, weaker two-loop contribution from local fluctuations of the electromagnetic field. Now that such details have been precisely tested in the lab, it should be possible to better discriminate between nonperturbative QED calculations. (R. Loetzsch et al., Nature 625, 673, 2024.)
