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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, readout of their internal state via quantum logic spectroscopy, and finally the preparation of the HCI in the ground state of motion of the trap. Here, we present the first operation of an atomic clock based on an HCI (Ar13+ in our case) and a full evaluation of systematic frequency shifts of the employed 2P1/2-2P3/2 fine-structure transition at 442 nm. The achieved uncertainty is almost eight orders of magnitude lower than any previous frequency measurements using HCI and comparable to other optical clocks. One of the main features of quantum logic spectroscopy is the flexibility of the investigated species. This allowed us to perform isotope shift spectroscopy of the 2P0-2P1 fine-structure transition at 569 nm in Ca14+ ions. In a large theory-experiment collaboration, we combined this data with improved measurements of the Ca+ 2S1/2-2D5/2 clock transition at 729 nm, new isotope mass measurements, and highly accurate calculations of the 2nd order mass shift. The resulting King plot allows us to put the currently most stringent bound from isotope shifts on a hypothetical 5th force coupling neutrons and electrons [1], despite a large (~900σ) residual nonlinearity, suspected to be dominated by nuclear polarizability. This demonstrates the suitability of HCI as references for high-accuracy optical clocks and to probe for physics beyond the standard model.
A next-generation HCI optical clock may be based on Ni12+, which offers a long excited state lifetime of >10 s. By developing efficient search strategies with quantum logic techniques [2] and precise atomic structure calculations [3], we identified the logic and clock transitions in this species within just a few hours of searching.
[1] A. Wilzewski et al., arXiv:2412.10277, Phys. Rev. Lett. in print
[2] S. Chen et al., Phy. Rev. Appl. 22, 054059 (2024)
[3] C. Cheung et al., arXiv:2502.05386
