This article explains how NIST-F4, one of the most accurate atomic clocks in the world, works. It is a cesium fountain clock, which uses a special design to measure time with incredible precision. Unlike older beam clocks, fountain clocks cool atoms to near absolute zero and gently toss them upward, allowing for longer and more accurate measurements of their internal oscillations. These oscillations are used to define the official international second, which is based on the frequency of cesium atoms.
Fountain clocks are extremely precise, with NIST-F4 being accurate to within one second in 100 million years. However, they are complex and expensive, so only a few exist worldwide. These clocks are not used continuously but are instead used periodically to calibrate other clocks that keep time for everyday use. They act like tuning forks, helping to ensure that all clocks around the world stay synchronized. Despite their high accuracy, fountain clocks are not the ones that run 24/7 — that job is left to other types of clocks, which are adjusted using the data from fountain clocks.
Source: https://www.nist.gov/atomic-clocks/how-atomic-clocks-work/fountains-atoms-exquisite-timekeepers
Keywords: atomic clock, cesium fountain clock, laser-cooling
