Cesium atoms are sprayed into a vacuum chamber and hit with microwaves.
The best modern atomic clocks are so stable that they wouldn't lose or gain a single second in —a span of time longer than the age of the universe. Atomic Clock
GPS satellites carry atomic clocks. Because radio signals travel at the speed of light, even a billionth of a second of error could result in your GPS being off by several miles. Cesium atoms are sprayed into a vacuum chamber
High-speed data transfers and cellular networks require perfectly synchronized "stamps" to ensure packets of data arrive in the correct order. Atomic Clock
Scientists use these clocks to detect tiny changes in gravity (General Relativity) and to search for dark matter. Precision Limits