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Optical atomic clock with highly charged ions

A clock with entirely new particle species provides new insights into fundamental physics

PTB-News 1.2023
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time and frequency metrology

fundamental research

Optical atomic clocks are the most accurate measurement devices ever built, and they have become a key technology in fundamental and applied research used, for example, in testing the stability of natural constants or for geodetic height measurements. As part of a collaboration within the QuantumFrontiers Cluster of Excellence, researchers from the QUEST Institute at PTB have developed the first-ever optical atomic clock based on highly charged ions.

Laser spectroscopy performed on a highly charged ion

Highly charged ions are widespread in the cosmos (e.g., in the sun and other stars). They have lost many electrons and therefore have a strong positive charge. The bond between their remaining outer-shell electrons and the atomic nucleus is thus particularly strong. For this reason, highly charged ions are less reactive to disturbances caused by external electromagnetic fields, but they can serve as sensitive probes for detecting fundamental effects of the special relativity theory, quantum electrodynamics and the atomic nucleus. Using this knowledge, researchers in a cooperation project between PTB’S QUEST Institute, the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg and TU Braunschweig, were able to clarify an important issue of fundamental physics: Quantum electrodynamic nuclear recoil, an important phenomenon predicted by theory, was demonstrated for the first time in a multi-electron system.

Due to their special atomic structure, highly charged ions cannot be cooled directly using laser light, nor can conventional detection methods be applied. This problem was solved by isolating a single highly charged argon ion from hot plasma and storing it in an ion trap together with a singly charged beryllium ion. This procedure allows the highly charged ion to be cooled indirectly with the aid of the beryllium ion so that it can be studied. Finally, a quantum algorithm was applied to cool the highly charged ion even further, namely nearly down to the quantum mechanical ground state, which corresponds to a temperature of 200 millionths of a kelvin above absolute zero. These experiments were conducted in a cryogenic trap system developed specifically for this purpose.

An atomic clock based on thirteen-fold charged argon ions has now been developed. Its frequency has been compared to that of PTB’s existing ytterbium ion clock. To this end, the system had to be characterized very accurately to recognize aspects such as the motion of the highly charged ion as well as effects caused by external disturbance fields. The relative systematic measurement uncertainty achieved in this experiment was 2 · 10–17, which is comparable to what can be attained with many of the optical atomic clocks that are currently in operation. It is anticipated that further technical improvements will put this new clock into a league with the best atomic clocks.

The methods applied can be universally used and allow many different highly charged ions to be investigated. This also includes atomic systems that can be used for exploring ways to extend the Standard Model of particle physics. Selected highly charged ions are especially sensitive to changes in the fine-structure constant and to specific dark matter candidates that are required in models that go beyond the Standard Model but could not be observed using the methods available to date.


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

Scientific publication

S. A. King, L. J. Spieß, P. Micke et al: An optical atomic clock based on a highly charged ion. Nature 611, 43–47 (2022)
Opens external link in new windowhttps://www.nature.com/articles/s41586-022-05245-4