Forbidden extreme-ultraviolet transitions in highly charged ions for frequency metrology

Forbidden transitions of various multipolarities appear in highly charged ions (HCI) belonging to many different isoelectronic sequences. Moreover, these transitions span an energy range extending from the optical up to the X rays. The outermost electron of an HCI cannot be photoionized by photons with energies below the threshold given by its high binding energy. This enables laser excitation and, consequently, frequency metrology in the extreme ultraviolet (XUV) region, where electronic transitions exhibit larger relativistic, quantum electrodynamic, and nuclear-size effects than at optical frequencies. This also makes them more sensitive to new physics than neutral or singly ionized species. Furthermore, the predicted low sensitivity of HCI to higher-order Zeeman shifts and other external perturbations is advantageous for high-precision frequency metrology and optical clock operation. Electronic transitions in HCI near degeneracies arising from charge-state-dependent orbital level crossings become even more sensitive to hypothetical beyond-the-standard-model phenomena. These level crossings are rare but more accessible in the XUV range than in the optical domain. For this purpose, we are developing a method to excite cold, trapped highly charged ions using an extreme ultraviolet frequency comb based on high harmonic generation. Our experiment uses an electron beam ion trap to generate highly charged ions and a decelerating beamline to transfer them to a superconducting RF trap. There, the HCI undergo sympathetic cooling within a Coulomb crystal of laser-coolable singly charged ions. Then, the HCI will be excited by an XUV frequency comb produced in an enhancement cavity where high-harmonic generation starts from an infrared frequency comb. This approach will allow us to apply the same quantum logic interrogation methods which are already used for HCI optical clocks to extend the desired high-accuracy frequency determinations into the XUV domain, opening new possibilities for investigating nuclear properties and searching for hypothetical fifth forces.