2 Electron shelving
Trapped-ion optical clocks use narrow transitions in an ion to which the frequency of a reference laser can be accurately stabilized. However, it is difficult to monitor the absorption of the probe light because the transition is so weak. One solution is the technique of "electron shelving", in which the ion is laser-cooled using the strong transition to the short-lived state. The ion can be detected by monitoring the fluorescence from this transition. When the probe laser induces the weak reference transition, the ion is shelved in the long-lived state and the fluorescence disappears until the ion decays to the ground state.