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Laser spectroscopy of highly charged ions

Pioneering experiment in quantum logic enables use of highly charged ions in various fields of research in physics

PTBnews 1.2020
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fundamental research in physics


atomic clock development

In cooperation with the Max Planck Institute for Nuclear Physics in Heidelberg, the QUEST Institute at PTB has succeeded for the first time in performing precision spectroscopy on highly charged ions. This pioneering experiment makes the field of highly charged ions accessible for research on novel atomic clocks and tests of fundamental physics.

Artistic representation of the ion pair: laser cooled Be+ (top right) and highly charged Ar13+ (bottom left)

Highly charged ions are widespread in the cosmos, for example in the form of matter in the sun and all other stars. Due to its high positive charge, the electron shell of an atom is more strongly bound to that atom's nucleus. Thus, disturbances caused by external elec t romagnet i c fields are strongly attenuated, whereas the fundamental effects of the special relativity theory, of quantum electrodynamics and of the nucleus play a more prominent role. This makes them promising candidates for high-precision atomic clocks that can be used to test fundamental physics in particular.

But measurements such as those that have long been established in optical atomic clocks were previously inconceivable on highly charged ions. Generating such ions requires a large amount of energy, and the ions then exist in the form of plasma as hot as the sun itself. However, the most precise experiments require the exact opposite – ultra-low temperatures and easily controllable ambient conditions in order to reduce the shifts and broadening mechanisms of the spectral lines to be measured. Due to their atomic structure, it is virtually not possible for highly charged ions to be cooled with laser light, nor can conventional detection methods be applied.

In an experiment that is unique in the world at the QUEST Institute for Experimental Quantum Metrology in Braunschweig, Germany, in collaboration with the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, the problem was solved using quantum logic spectroscopy. A single highly charged ion was isolated from hot argon plasma and stored in an ion trap together with a singly charged beryllium ion. Using this so-called logic ion, the two-ion crystal was cooled down to the quantum-mechanical ground state of motion. This state is usually assigned a temperature of only a few millionths of a degree above absolute zero. The spectral structure of the highly charged ion was then resolved by means of an ultra-stable laser.

A beryllium ion can be used as a logic ion for most highly charged ions, and the production process and extraction of a highly charged ion are completely unspecific with regard to the atomic species. Thus, the demonstrated experiment grants access to an extremely extensive and previously inaccessible range of atomic systems for use in precision spectroscopy and for future clocks with special properties. A promising approach to fundamental questions thus becomes possible for basic research. It will, for example, be possible to answer the questions as to whether our standard model of particle physics is complete, what dark matter is, or whether fundamental constants are really constant at all.


Piet O. Schmidt
QUEST Institute at PTB
Phone: +49 531 592-4700
Opens window for sending emailpiet.schmidt(at)ptb.de

Scientific publication

P. Micke, T. Leopold, S. King, E. Benkler, L. Spieß, L. Schmöger, M. Schwarz, J. C. López-Urrutia, P. Schmidt: Coherent laser spectroscopy of highly charged ions using quantum logic. Nature 578, 60–65 (2020)