Towards a quantum-logic ion clock based on Ba⁴⁺

Highly charged ions (HCIs) greatly expand the collection of viable atomic species that can be used to build high-accuracy optical clocks. Recently, ¹³⁸Ba⁴⁺ has been identified as a potential high-accuracy optical clock candidate, with a laser accessible ³P₂ - ³P₀ electric quadrupole transition at 338.8 THz¹. We present progress towards the development of an optical atomic clock based on Ba⁴⁺. The clock will employ a single Ca⁺ ion for sympathetic cooling and quantum-logic readout operations². We also discuss the development of a compact electron beam ion source (EBIS) optimized to generate Ba⁴⁺ ions, along with the initial design of an ion optics beamline used to extract and transfer Ba⁴⁺ ions to a radiofrequency Paul trap for clock operation. As a preliminary milestone, we have demonstrated sideband cooling as well as optical clock operation on the 729 nm ²S_1/2 - ²D_5/2 clock transition in ⁴⁰Ca⁺.

[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, and D. J. Wineland, Science. 309, 749-52 (2005).