Ordinary atomic clocks tick when an electron jumps between energy levels. But external fields and jolts can disrupt this rhythm. Highly charged ions, atoms stripped of many electrons, barely notice such disturbances: their electrons sit deep and tight. That makes them ultra-stable 'pendulums.'
To make such clocks work, you need to know the exact transition frequency. The authors calculated the 5f_5/2 – 6p_1/2 transition in Cf17+. They accounted for how electrons interact with each other (correlations), including triple excitations, and added quantum electrodynamics corrections — the influence of virtual particles that constantly pop in and out of existence. It turned out that correlations between the outer electron and the inner ones significantly shift the frequency. Without them, the calculation would be inaccurate.
Spectroscopy of highly charged ions is like tuning a string: you need to hit the right note. The calculation gives experimentalists a guide: at what frequency to look for the transition. Then the ion can be trapped, cooled, and used as an ultra-precise clock. Such clocks could help test whether fundamental constants change over time and aid navigation in deep space.
as written by the authors arXiv:2602.13884
Highly charged ions (HCIs) combine compact electronic structure with strong relativistic effects, offering both robustness against external perturbations and enhanced sensitivity to variations of the fine-structure constant. Recent advances in sympathetic cooling and trapping enable precision measurements of highly charged ions; however, fully exploiting their potential requires accurate theoretical predictions. In particular, reliable calculations of clock wavelengths are essential for experimentally locating HCI clock transitions. Here, we treat Cf17+ as a univalent ion and perform calculations within the relativistic coupled-cluster framework, iteratively including nonlinear single-double contributions and valence and core triple excitations. We also assess quantum-electrodynamic corrections and basis-set and partial-wave truncation effects. Our results establish the impact of different correlation contributions on the low-lying energy spectrum and provide a quantitatively reliable prediction of the 5f_5/2 - 6p_1/2 clock transition, highlighting the critical role of core-valence correlations and iterative triples for precision spectroscopy and optical clock development.
Highly charged ions are so insensitive to external fields that their transitions could be more stable than those of the best current clocks based on neutral atoms.