A SIMPLE METHOD FOR COUNTING THE NUMBER OF TRAPPED IONS IN AN ION-TRAP

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