HELMHOLTZ PRIZE 2012 (award ceremony on 28 March 2012)
Anke Wagner, Sven Sturm, Prof. Klaus Blaum for their work "Der g-Faktor des gebundenen Elektrons in wasserstoffähnlichem Silizium − Der empfindlichste Test der Quantenelektrodynamik gebundener Zustände" (The g-factor of the bound electron in hydrogen-like silicon − The most sensitive test of the quantum electrodynamics of bound states)

2012 Prizewinners: The electron g-factor

(from left to right) Anke Wagner, Dr. Nathalie von Siemens, Sven Sturm, Prof. Dr. Klaus Blaum, Prof. Dr. Joachim Ullrich

Anke Wagner was born in Mainz in 1983. She studied physics at the Johannes Gutenberg University Mainz and received her doctorate under Klaus Blaum in Heidelberg in 2013. From 2013 to 2014, she was a postdoc at Florida State University in Tallahassee, where she performed high-precision mass measurements. Since then, she has been working in industry. Anke Wagner is the first female recipient of the Helmholtz Prize. The Helmholtz Prize was also the highlight of her excellent collaboration with Klaus Blaum and Sven Sturm.

Sven Sturm, who was born in Koblenz in 1981, studied physics in Heidelberg and completed his German "Diplom" in 2007 based on his work at CERN. Afterwards, he went to Mainz, where he received his doctoral degree under Klaus Blaum in 2012. He has been a group leader at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg since 2012.

Klaus Blaum was born in Bad Sobernheim in 1971. He studied physics in Mainz and completed his studies in 1997 with a "Diplom" thesis under Ernst Otten. He also received his doctorate under Ernst Otten in 2000. After a period of being a postdoc at CERN, among others in H.-J. Kluge's research group, he was the head of a Helmholtz Young Researchers Group from 2004-2008. He became the Director and a scientific member of the MPIK in 2007. In addition, he has been an honorary professor at Heidelberg University since 2008.

Precision measurements of the magnetic properties of the electron have played an important role in the development of quantum electrodynamics (QED). In 1947, for example, a value had been measured which was slightly higher than the value that had been predicted by the Dirac theory. It was only by means of QED that this discrepancy between the measured value and the predicted value could be resolved. The magnetic moment μ and the spin s of the electron fulfil the equation μ = g ∙ μB∙s/ħ, whereby μB is the Bohr magneton. For the dimensionless electron g-factor, the measurements carried out since that time resulted in increasingly accurate values which were in agreement with the results of ever more comprehensive QED calculations.

As known from QED, the interaction of the electron with the vacuum already has an influence on the g-factor. If, however, the electron is bound to an atomic nucleus, the strong electric field of the nucleus leads to the numerical value of the g-factor changing distinctly. Sven Sturm, Anke Wagner and Prof. Klaus Blaum of the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg measured the electron g-factor in such a hydrogen-like and highly charged ion with high precision. In 2012, they were awarded the Helmholtz Prize for their work.

The experiments carried out by Anke Wagner and Sven Sturm within the scope of their dissertations were based on a procedure developed by Heinz-Jürgen Kluge and Günter Werth in Mainz. For this purpose, a cryogenically cooled, 13-fold-charged silicon-28 ion, which had only one electron left, was kept in a Penning trap with electric and magnetic fields in a very good vacuum. 

Whereas the ion, which had the mass M and the charge q, carried out a cyclotron movement with the frequency fC = q ∙ B/(2πM) in the magnetic field B, the magnetic moment of the electron precessed in the B field with the Larmor frequency fL = g ∙ µB ∙ B/h. By measuring the cyclotron frequency and the Larmor frequency, the researchers could determine the g-factor from their quotient: g = (2fLfc) ∙ q ∙ m/ (e ∙ M). The cyclotron frequency was measured with the aid of image currents caused by the circling ion in the electrodes of the Penning trap. In order to determine the Larmor frequency, the researchers irradiated the ion with microwaves whose frequency they changed.

In doing so, they observed at which frequency a resonance occurred and the electron spin reversed. If such a spin flip had taken place, this could be seen from the fact that the longitudinal oscillations which the ion carried out in the Penning trap parallel to the magnetic field lines changed their frequency slightly. The resonance frequency determined in this way agrees with the Larmor frequency. The three scientists received the result g = 1.995 348 958 for the g-factor (with an uncertainty of a few 10–10) which agreed perfectly with the result of the QED calculations. The measurements were thus so sensitive that they could detect the influence of the nucleus size on the magnetic moment of the electron. As a result, the QED has also successfully passed this most stringent test so far.


S. Sturm et al.: g Factor of Hydrogenlike 28Si13+. Phys. Rev. Lett. 107, (2011), 023002

Sven Sturm, Günter Werth and Klaus Blaum: Electron g-factor determinations in Penning traps. Ann. Phys. (Berlin) 525, (2013), 620