Figure 1: Schematic overview and electronic structure of ⁴⁰Ca⁺.

(a) Image of 42 ions. The red arrow indicates the position of the laser beam exciting the probe ion on the blue motional sidebands of the transverse motion with light at 729 nm. (b) Ramsey sequence with a free evolution time t governed by the Coulomb interaction between the ions. (c) relevant electronic levels of ⁴⁰Ca⁺. The quadrupole transition between the and states is driven by a narrow linewidth 729 nm laser beam tightly focused on the probe ion. The Zeeman degeneracy is lifted by applying a magnetic field of 323 μT. Cooling and readout are performed on the transition at 397 nm.