During the first months of the project, an Ar13+ highly charged ion (HCI) optical clock has been operated and a detailed analysis of systematic frequency shifts has been performed. The relative systematic frequency uncertainty was limited to 2.2e-17 by uncompensatable micromotion from a defect of the employed ion trap. Frequency comparisons against a high accuracy clock at PTB based on an octupole transition in Yb+ yielded a frequency ratio with a relative uncertainty of 1e-16 and an absolute frequency of the Ar13+ transition with a relative uncertainty of 1.7e-16. These results demonstrated the first optical clock based on a highly charged ion and confirmed the potential of these species for high accuracy clocks [King et al., Nature 611, 43-47 (2022)].
In parallel, preparations for isotope shift spectroscopy of the stable even Ca14+ isotopes started. A clock laser system was setup and stabilized to a stable reference cavity, transfer-locked to a Si-cavity stabilized laser at PTB. An extension of the electron beam ion trap was designed, built and commissioned to allow ablation loading of metal targets such as Calcium. Precision spectroscopy of 40, 42, 44, 46 and 48-Ca14+ at a level of 1e-16 (corresponding to <100 mHz resolution) was performed against the Yb+ octupole clock at PTB and the corresponding isotope shifts were extracted and combined with existing data on singly charged Calcium. Together with several other experimental and theory groups the quality of the previously existing data is currently being improved and an analysis in terms of exclusion of possible 5th force candidates is under way.
In preparation for the next HCI clock species, several search strategies for finding mHz narrow transitions within a frequency range of THz have been investigated and benchmarked [Chen et al., arXiv:2406.04015]. Of the investigated techniques based on Rabi spectroscopy, oscillating dipole forces and linear continuous sweep, the latter is by far the most efficient if a logic transition is available. Laser systems for spectroscopy of the next optical clock candidate, Ni12+, have been procured and set up. This includes a fiber laser-based logic laser system at 512 nm and a frequency doubled Ti:Sa laser at 495 nm, which offers the broad scan range required to cover the uncertainty in current calculations of the clock transition frequency of on the order of a terahertz. The logic transition has recently been found using the optical dipole force search technique. We are currently preparing the search for the 8 mHz narrow clock transition using the linear continuous sweep technique.
We have also started investigating the use of multiple HCI. Original attempts to prepare those using our standard technique failed due to loss of the HCIs when heating out the excess Be+ ion. We have therefore developed a splitting & discarding technique in another setup [Zawierucha et al., Phys. Rev. A, accepted] and implemented them successfully with HCI. In a next step we will investigate the preparation of decoherence-free substates in HCI to eliminate the linear Zeeman effect and take advantage of the larger signal-to-noise ratio of having two HCI in the trap.
