A simple method for counting the number of trapped ions in an ion trap

Abstract.

The number of stored Ca$^+$ ions in an ion trap was measured optically by utilizing the metastable states. All the ions trapped are first pumped into the metastable $D$ states. The ions in the metastable $D$ states are transferred to the ground $S$ state via the $P$ state by exciting a $D\rightarrow P$ transition. Each ion then emits one photon through a subsequent $P\rightarrow S$ spontaneous emission. Thus, the number of photons is the same as the number of trapped ions initially in the metastable states. When a fraction of all the stored ions are pumped into the metastable states, the method is still applicable if the fraction of the ions is known.