Atomic Clock -

Cesium atoms are sprayed into a vacuum chamber and hit with microwaves.

Scientists use these clocks to detect tiny changes in gravity (General Relativity) and to search for dark matter. Precision Limits Atomic Clock

At the heart of the clock is a physical constant: the "resonant frequency" of an atom. When atoms are exposed to specific frequencies of microwave or light energy, their electrons jump between energy levels. In a standard cesium atomic clock: Cesium atoms are sprayed into a vacuum chamber

Since 1967, the International System of Units (SI) has defined one second as exactly 9,192,631,770 oscillations of the radiation produced by this transition in a cesium-133 atom. Why It Matters Atomic Clock