The definition of a second has since officially been the time it takes a cesium atom to make 9,192,631,770 vibrations.

Their clock was based on a transition in the cesium atom.

Since 1967 the second has been defined as the time during which the cesium atom makes 9,192,631,770 of these oscillations.

The clock separates cesium atoms of different energy levels by magnetic field.

The standard for one second is the amount of time that it takes cesium atoms to vibrate 9,192,631,770 times.

The cesium atoms in the atomic clock at Boulder did not stop vibrating.

Since 1967, an international agreement has defined a second as the period in which a cesium atom vibrates 9,192,631,770 times.

This frequency is closest to the natural frequency of the cesium atoms.

This makes me wonder: Do cesium atoms oscillate in synch?

Well, so long as they have a cesium 133 atom lying around.