How do atoms keep time? In some ways, NIST's-F1 is like an early Chinese water clock. The Chinese clock consisted of a wheel with a series of evenly spaced cups. As each cup was filled with water, it became heavy enough to trip a lever that allowed the next cup to move into place. The wheel revolved in steps, turning gears that moved the clock's hands and kept track of time.

All Atoms are Created Equal

NIST-F1 uses atoms instead of cups. Atoms make better timekeepers than cups because they are all exactly the same and behave exactly the same way.

The atoms are filled with energy instead of water. Inside the NIST-F1, a cloud of cesium atoms is tossed up into a vertical chamber and exposed to microwave energy (like the energy in a microwave oven). The microwaves are tuned to a specific frequency, just as a radio can be tuned to a specific frequency to pick up a particular station. The atoms absorb energy best at this resonance frequency.

Excited Atoms: Ready to Shine

When the atoms absorb the microwave energy, they change to a higher energy, excited state. Just as the water clock cups can hold only a certain amount of water, the atoms can absorb only a certain amount of energy. When a laser beam hits the atoms, only those in the excited state respond by absorbing and re-emitting the laser light. They shine. If the microwave energy in NIST-F1 is at exactly the right frequency, the light given off by the atoms when the laser is shined on them is maximized.

Billions of Ticks per Second

In our water clock, each filled cup advances the clock by one tick. In the NIST-F1 clock, each wave peak of the microwaves at the correct frequency -the resonance frequency of cesium-equals one tick.

One second equals the time it takes for exactly 9,192,631,770 ticks at the resonance frequency of cesium. In order for our water clock to keep up with NIST-F1, it would have to fill more than 9 billion cups in a second!

illustrations by William Welsh