Highly charged ions are promising candidates for developing extremely precise optical clocks due to their low sensitivity to external disturbances. These clocks could outperform the current generation of atomic clocks in terms of accuracy. Precise measurements of ion properties enable stringent tests of fundamental physics, such as quantum electrodynamics (QED), and comparisons between measured and theoretically predicted values could provide hints of new physics.
Researchers, led by Lukas Spieß from the Quantum Logic Spectroscopy group of the QUEST Institute at the Physikalisch Technische Bundesanstalt (PTB) in Braunschweig have now measured the magnetic properties of highly charged ions (specifically carbon-like Ca14+) in their excited state with unprecedented precision. They determined the excited state g-factor with a relative uncertainty of 10-6, as well as the first measured second-order Zeeman coefficient for such a system, finding it to be the smallest reported value for an atomic transition to date.
These experimental results are in excellent agreement with the state-of-the-art theory calculations, provided by collaborators from the University of Delaware. The study reveals that subtle quantum effects—such as the frequency-dependent Breit interaction and contributions from negative-energy states and QED effects—play a significant role in shaping the energy levels and magnetic behaviour of highly charged ions. The findings also underscore the reduced sensitivity of highly charged ions to higher-order Zeeman effects, reinforcing their suitability for use in high-accuracy frequency metrology. The researchers have published their results in the latest issue of the journal Physical Review Letters.
The work is a yet another result of the outstanding collaboration between PTB, Leibniz University Hannover and TU Braunschweig in the framework of the Cluster of Excellence QuantumFrontiers, the Collaborative Research Centre DQ-mat and the ERC project FunClocks. Additional partners were the University of Delaware, GSI Helmholtzzentrum für Schwerionenforschung and the Max-Planck-Institut für Kernphysik.
Original publication
Excited-state magnetic properties of carbon-like Ca14+
Lukas J. Spieß, Shuying Chen, Alexander Wilzewski, Malte Wehrheim, Jan Gilles, Andrey Surzhykov, Erik Benkler, Melina Filzinger, Martin Steinel, Nils Huntemann, Charles Cheung, Sergey G. Porsev, Andrey I. Bondarev, Marianna S. Safronova, José R. Crespo López-Urrutia, and Piet O. Schmidt
Phys. Rev. Lett. 135, 043002 – Published 22 July, 2025
DOI: https://doi.org/10.1103/p88p-brnx
