Development of an Electron Beam Ion Source Setup for a Ba⁴⁺-based Optical Clock

Highly charged ions (HCIs) greatly expand the collection of atomic species that can be used for advancing research in areas such as high-accuracy optical clocks, testing for a time variation of the fine structure constant, α, and exploring spectroscopic studies of quantum electrodynamics (QED) effects. The ionization energies required for creating such species are generally well below 1keV. Recently, ¹³⁸Ba⁴⁺ has been identified as a potential high-accuracy optical clock candidate, with the added benefit of co-trapping with a singly charged Ca⁺ ion for sympathetic cooling and quantum logic spectroscopy[1,2]. The availability of high field magnets such as neodymium iron boron (NdFeB) and samarium cobalt (SmCo), coupled with the inefficiency of traditional electron beam ion traps for HCI species with lower ionization thresholds has brought smaller room temperature HCI sources into focus. We present the development of a compact electron beam ion source (EBIS) optimized to generate Ba⁴⁺ ions and the transport of Ba⁴⁺ ions to a radiofrequency Paul trap, as part of the development of an optical clock based on Ba⁴⁺. We present the relevant physics of electron beam ion sources and traps, along with the processes of extracting, identifying, selecting, pre-cooling, and loading ions into a radiofrequency Paul trap where quantum logic spectroscopy with a Ca⁺ ion is to be performed.

[1] K. Beloy, V. A. Dzuba, and S. M. Brewer, Phys. Rev. Lett. 125, 173002 (2020).

[2] P. O. Schmidt, T. Rosenband, C. Langer, W. M. Itano, J. C. Bergquist, D. J. Wineland, Science. 309, 749-52 (2005).