Highly charged ions are a common form of matter in the cosmos. They are so-called because they have lost many electrons and have a high positive charge. This is why the outermost electrons are more strongly bound to the atomic nucleus than in neutral or weakly charged atoms.
As a result, highly charged ions exhibit less reactions to electromagnetic interference from the outside world but develop greater sensitivity to the fundamental effects of quantum electrodynamics, special relativity, and the atomic nucleus.
Now, researchers at the QUEST Institute at the Physikalisch-Technische Bundesanstalt (PTB), in collaboration with the Max Planck Institute for Nuclear Physics (MPIK) and the TU Braunschweig and the scope of the QuantumFrontiers Cluster of Excellence, have realized for the first time an optical atomic clock based on highly charged ions. This type of ion lends itself to such an application because it has extraordinary atomic properties and low sensitivity to external electromagnetic fields.
PTB physicist Lukas Spieß said, “Therefore, we expected that an optical atomic clock with highly charged ions would help us to test these fundamental theories better. This hope has already been fulfilled: We could detect the quantum electrodynamic nuclear recoil, an important theoretical prediction, in a five-electron system, which has not been achieved in any other experiment before.”
Beforehand, the team had to work for years to find solutions to specific fundamental issues, such as detection and cooling: For atomic clocks, one needs to chill the particles significantly to stop them as much as possible and then read out their frequency at rest. But producing highly charged ions requires the production of very hot plasma. High-charged ions cannot be directly cooled with laser light due to their extraordinary atomic structure, nor can they be detected using conventional techniques.
A collaboration between MPIK in Heidelberg and the QUEST Institute at PTB solved this problem by isolating a single highly charged argon ion from a hot plasma and storing it in an ion trap with a singly charged beryllium ion.
As a result, the highly charged ion can be indirectly cooled and analyzed using the beryllium ion. Then, for the subsequent experiments, an upgraded cryogenic trap system was developed at MPIK and finished at PTB, which was carried out partly by students switching between the institutions. Subsequently, a quantum algorithm developed at PTB succeeded in cooling the highly charged ion even further, close to the quantum mechanical ground state. This corresponded to a temperature 200 millionths of a kelvin above absolute zero.
Scientists now took a step forward: they have realized an optical atomic clock based on thirteen-fold charged argon ions and compared the ticking with the existing ytterbium ion clock at PTB. To accomplish this, they had to thoroughly analyze the system to comprehend things like the highly charged ion’s motion and the impacts of outside interference fields. They attained measurement inaccuracy of 2 parts in 1017, equivalent to several optical atomic clocks now in use.
Thus, in addition to the optical atomic clocks now in use, the researchers have developed a new method based on neutral strontium atoms or individual ytterbium ions, for example. The techniques employed enable the study of a wide variety of highly charged ions and are globally applicable.
The Standard Model of particle physics can be extended using atomic systems. Other highly charged ions are especially sensitive to variations in the fine structure constant and to some dark matter candidates that are needed in theories outside the Standard Model but were undetectable with earlier techniques.