cesiumklokker

Cesium clocks, or caesium clocks, are a class of atomic clocks that use the cesium-133 atom as the basis for a highly stable frequency reference. The SI second is defined by the radiation corresponding to the cesium-133 ground-state hyperfine transition, specifically 9,192,631,770 cycles. In practice, a cesium clock measures microwaves near 9.2 GHz and uses a feedback loop to lock a local oscillator to the cesium transition, producing a precise time signal.

Most cesium clocks today are cavity-based devices that interrogate a sample of cesium atoms. Historically, there were cesium beam clocks, which used hot atoms passing through a microwave region, but modern designs increasingly employ cesium fountain configurations. In a fountain clock, cold atoms are launched upward, interact with the microwave field, and are detected, providing high stability and accuracy.

Role and applications: Cesium clocks serve as primary frequency standards for realizing and maintaining International Atomic Time (TAI) and Coordinated Universal Time (UTC). National metrology institutes operate cesium clocks and compare them to each other as part of time scale realizations, often in combination with other standards such as hydrogen masers to improve short-term stability.

History and context: The first practical cesium clock was developed in the 1950s by Louis Essen at the National Physical Laboratory in the United Kingdom. In 1967–1968 the cesium frequency defined the second, replacing older astronomical definitions. While optical clocks have surpassed cesium clocks in ultimate accuracy, cesium standards remain central to international timekeeping and are still used as primary references for disseminating time.