Measurement of the number of cold atoms trapped in cesium magneto-optical trap via probe absorption

The number of cold atoms trapped in magneto-optical trap (MOT) can be derived from probe absorption signal. After the MOT is switched off, the cold cloud freely expands in gravitational field with an initial velocity distribution. So the measurement process can be described with the interaction model between the individual stationary two-level atom and single mode resonate laser. Absorption method can largely improve the measurement precision. In this paper, we introduce the physical principle of measurement and demonstrate the whole experimental procedure in detail. The number of atoms trapped in our MOT, (8 ± 0.3) × 10⁶ is measured in experiment. And the number of steady-state atoms and the lifetime of atom trapped in MOT were used to deduce the capture rate of MOT.