cesiumklok

A cesium clock, or cesium clock, is an atomic clock that uses the microwave frequency associated with the hyperfine transition of cesium-133 atoms to define the SI second. It is one of the most precise timekeeping devices available and serves as a primary reference for national and international time standards.

In operation, cesium atoms are prepared and exposed to microwave radiation near 9,192,631,770 hertz. The probability of cesium atoms changing their hyperfine state is detected, and this resonance is used to discipline a local oscillator. The oscillator is continuously adjusted so that its output matches the exact transition frequency, ensuring an accurate and stable time signal.

Two common implementations are cesium beam clocks and cesium fountain clocks. Cesium beam clocks direct a beam of atoms through a microwave cavity, while cesium fountain clocks use laser cooling to launch cold atoms upward in a fountain-like trajectory, allowing longer interaction times and improved stability and accuracy.

Role and significance. The SI second is defined as exactly 9,192,631,770 cycles of the cesium-133 hyperfine transition, a definition realized worldwide by cesium clocks. These clocks underpin international time scales such as International Atomic Time (TAI) and Coordinated Universal Time (UTC) and provide calibration references for satellite navigation systems and telecommunications networks. While optical clocks have surpassed cesium clocks in potential precision in laboratory settings, cesium clocks remain the primary standard for the second.

History and status. Practical cesium clocks emerged in the mid-20th century, with rapid advances in the 1950s–1960s. The definition of the second in 1967 anchored timekeeping to cesium-133, a status maintained since, even as newer optical clocks push the boundaries of accuracy for scientific applications.