© NIST/Jeffrey Aarons.
The first step in creating an atomic fountain is to trap and cool the atoms using the techniques described in the previous section. Once a cloud of cold atoms is trapped in the region where the six laser beams overlap, the trap lasers are turned off. Two vertical laser beams are then pulsed on and off to give the atoms a gentle upward push. The ball of atoms rises about a meter before it begins to fall under the influence of gravity. As they do this, the atoms pass through a microwave cavity twice: once on the way up, and once on the way down. Depending on the microwave frequency in the cavity, the state of the atoms might be altered as they interact with the microwave signal. If the microwave frequency is perfectly matched to the atoms, the atoms will emit light after interacting with the probe laser beam below the microwave cavity. This light is collected by the detector and the frequency of the microwave cavity is adjusted to maximize the signal on the detector. Once linked to the atomic resonance, the microwave frequency is a periodic signal that can be used for timekeeping just like the pendulum in a grandfather clock. (Unit: 5)