A gas of cesium atoms enters the clock's vacuum chamber. Six lasers slow the movement of the atoms, cooling them to near absolute zero and force them into a spherical cloud at the intersection of the laser beams. The ball is tossed upward by two lasers through a cavity filled with microwaves. All of the lasers are then turned off.
Gravity pulls the ball of cesium atoms back through the microwave cavity.
The microwaves partially alter the atomic states of the cesium atoms. Cesium atoms that were altered in the microwave cavity emit light when hit with a laser beam. The entire process is repeated until the maximum fluorescence of the cesium atoms is determined. This point defines the natural resonance frequency of cesium, which is used to define the second.