Highly Charged Ion Clocks to Test Fundamental Physics

The extreme electronic properties of highly charged ions (HCI) render them highly sensitive probes for testing fundamental physical theories. The same properties reduce systematic frequency shifts, making HCI excellent optical clock candidates. The technical challenges that hindered the development of such clocks have now all been overcome, starting with their extraction from a hot plasma and sympathetic cooling in a linear Paul trap, and readout of their internal state via quantum logic spectroscopy (QLS). This led to the first operation of an atomic clock based on an HCI based on Ar 13+ with an almost eight orders of magnitude lower uncertainty than any previous frequency measurements using HCI and comparable to other optical clocks. Taking advantage of the flexibility of QLS allowed us to perform isotope shift spectroscopy of the 2 P 0 - 2 P 1 fine-structure transition at 569 nm in Ca 14+ ions. In a large theory-experiment collaborations, we combined this data with improved measurements of a narrow transition in Ca + , new isotope mass measurements, and accurate calculations of the 2 nd order mass shift. A careful analysis of a so-called King plot allowed us to put the currently most stringent bound on a hypothetical 5 th force coupling neutrons and electrons. More accurate calculations of the 2 nd order mass shift and better models for the nuclear polarization would further improve this bound. This demonstrates the suitability of HCI as references for high-accuracy optical clocks and to probe for physics beyond the standard model.