How does a cesium atomic clock turn resonance into time?

A cesium atomic clock uses a repeatable transition between two internal atomic energy states as a frequency reference. It does not wait for atoms to decay. An electronic oscillator generates microwaves near the frequency associated with the cesium-133 clock transition. Prepared atoms interact with that radiation, and detectors measure how their states have changed.

By comparing measurements taken at carefully chosen frequencies, the system obtains information about whether the oscillator is above or below the resonance. Feedback adjusts the oscillator toward the reference. The atoms supply the comparison; electronics generate the oscillation that can be continuously counted. A fountain clock improves the observation by cooling atoms and launching them through a microwave region before gravity brings them back.

The SI second is defined by assigning the unperturbed cesium-133 hyperfine transition frequency the exact value 9,192,631,770 hertz. In an ideal realization, counting that many reference cycles marks one second. Our simplified feedback picture starts with an oscillator 2 hertz high and applies a correction of minus 2 hertz. Counting 18,385,263,540 corrected cycles would then mark two seconds.

The definition is exact, but every physical realization has uncertainty. Magnetic fields, thermal radiation and interactions between atoms can shift a measured resonance. Clock comparisons also account for motion and gravitational potential. Evaluating and correcting such effects is part of making an accurate standard. The remarkable feat is a chain from a controlled quantum transition to a measured, corrected frequency and finally to a count of elapsed cycles.