Cesiumatomikellot

Cesiumatomikellot, or cesium atomic clocks, are highly precise timekeeping devices that rely on the microwave transition between hyperfine levels of cesium-133 atoms to realize the SI second. In 1967 the second was defined as exactly 9,192,631,770 cycles of this cesium-133 transition, establishing a universal reference for timekeeping.

Most cesium clocks operate by preparing a sample of cesium atoms, exposing them to a microwave field near 9.192 GHz, and detecting the transition probability between hyperfine states. The observed signal is used to stabilize a local oscillator, whose output frequency serves as the clock’s standard. Implementations include vapor-cell clocks and cesium fountain clocks, where atoms are launched upward and interact with the microwaves multiple times to improve stability and accuracy.

Cesium clocks underpin national time standards and time scales such as International Atomic Time (TAI) and Coordinated Universal Time (UTC). They are typically part of ensembles that include hydrogen masers to balance long-term accuracy with short-term stability. While newer optical clocks offer higher precision, cesium clocks remain the defining standard for the second due to the SI definition and the mature metrology infrastructure surrounding them.