How do the atomic clocks work?

From the Ask a Scientist Physics Archive

“In a Cesium clock, you tune a microwave oscillator around until it has just the right
frequency to excite particular electrons of a Cesium atom from one well-defined energy level to another well-defined level. (The reason you use Cesium is that it has some very well-defined levels. Later, you will understand that this means the levels are relatively long lived—i.e., not damped.) When you have tuned the oscillator to maximize the excitation of Cesium electrons, you know that the
microwave frequency is the same as the microwave frequency of any other Cesium clock—so you have a standard that someone else can duplicate. Now all you have to do is declare that umpty-ump cycles of such an oscillator is one second. You choose umpty-ump so that it agrees with all other (mostly less precise) definitions of a second.

Here is more detail from the U.S. Naval Observatory”

Welcome to the collective.

An atomic clock is a type of clock that uses an atomic resonance frequency standard as its timekeeping element. They are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, and to control the frequency of television broadcasts and GPS satellite signals.

Atomic clocks do not use radioactivity, but rather the precise microwave signal that electrons in atoms emit when they change energy levels. Early atomic clocks were masers with attached equipment. Currently the most accurate atomic clocks are based on absorption spectroscopy of cold atoms in atomic fountains such as the NIST-F1.

National standards agencies maintain an accuracy of 10^-9 seconds per day (approximately 1 part in 10^14), and a precision set by the radio transmitter pumping the maser. The clocks maintain a continuous and stable time scale, International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but synchronized, by using leap seconds, to UT1, which is based on actual rotations of the earth with respect to the mean sun.

Source

The cesium atom in the clock bounces up and down at a very stable rate, about 9 billion cycles per second. The clock counts this frequency as the basis for determining one second, one minute, one hour, etc.

These clocks are so accurate that they have been used to prove Einstein’s theory of relativity. One clock was left on the ground, while the second clock was flown around the world in a jet. When the jet landed, the two clocks did not show the same time, proving that the moving clock counted time more slowly than the grounded clock.