Ultrastable and ultra-accurate clock transitions in open-shell highly charged ions

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Highly charged ions (HCIs) are less sensitive to external perturbations and are therefore attractive for the development of ultrastable clocks. However, only a few HCI candidates are known to provide optical clock transitions. In this work, we discover a large family of HCI clocks, with more than 100 suitable optical clock transitions hidden in the fine-structure terms of open-shell ions over 70 elements. Their projected instabilities and accuracies are στ~10−17/τ and δν/ν < 10−20, respectively, surpassing state-of-the-art optical clocks by several orders of magnitude. This indicates that having a high-performance optical clock transition is not a property of particular elements, but a virtue host by most elements from the periodic table. Furthermore, at given configurations, the clock transitions in heavy ions scale up to the XUV and soft-x-ray region, and thus enable the development of ultrastable clocks based on shorter wavelengths. The existence of multiple clock transitions in different charge states of a single element, as well as in a whole isoelectronic sequence would significantly enrich the search for new physics and the test of nuclear theories via high-precision spectroscopy.