cesiumbeamklok

Cesium beam clock, or cesium beam klok in some languages, is a type of atomic clock that uses a beam of neutral cesium-133 atoms to measure time with very high precision. It was a foundational technology in mid-20th century timekeeping and helped establish the cesium standard for the second. Although more advanced approaches have emerged, cesium beam clocks remain important as robust, well-characterized realizations of the SI second in many national laboratories.

Principle of operation: A heated source emits a beam of cesium atoms that travels through a microwave cavity tuned near the cesium hyperfine transition frequency of 9,192,631,770 Hz. The microwaves induce transitions between the two ground-state hyperfine levels. After the interaction region, a state selector and detector determine how many atoms occupy the excited state. The detected signal is fed back to stabilize the frequency of a local oscillator, which thereby defines the clock’s time base.

Performance and characteristics: Cesium beam clocks rely on meticulous control of vacuum, magnetic fields, and environmental conditions to minimize systematic shifts. They typically achieve high long-term accuracy on the order of 10^-12 to 10^-13 with moderate short-term stability. Compared with newer fountain or optical clocks, their ultimate performance is lower, but they provide a proven and reachable realization of the cesium transition.

Context and use: Since 1967 the second is defined by the cesium-133 hyperfine frequency, and cesium beam clocks serve as realizations of that definition in laboratories around the world. They are increasingly complemented or superseded for primary standards by cesium fountain clocks and optical clocks, but continue to play a role in calibration, time transfer, and as stable transfer standards within timekeeping institutions.