Towards a quantum-logic clock based on quadruply ionized barium

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 THz. This clock will employ a single 40Ca⁺ ion for sympathetic cooling and quantum-logic readout operations. In this talk, we present progress towards the development of an optical atomic clock based on Ba⁴⁺. As a preliminary milestone, we have demonstrated optical clock operation on the 729 nm |2S_1/2〉−|2D_5/2〉 clock transition in 40Ca⁺ as well as sub-Doppler dark-resonance cooling and sideband cooling for minimization of the time-dilation shift due to the ion's secular motion in the trap.