Fig. 5: Atomic energy levels and laser wavelengths.

Blue arrows in a and b indicate the lasers used for ionizing Ca and Yb, respectively. The electron energy is given by the difference between the ionization limit and the laser photon energy. c, d show levels used in Ca⁺ and Yb⁺ spectroscopy, respectively. The blue arrows indicate the laser-driven transitions while the red dashed arrows indicate the spontaneous emission with the associated branching fractions. In this experiment, we use a Ti:sapphire laser to repump the 854 nm transition in Ca⁺. Repumping the transition at 850 nm does not change the measured results. We do not repump the 935 nm transition in Yb⁺.