Question: How do atomic clocks work?

Iain Bethune answered on 17 Jun 2015:

In an atomic clock, the ‘tick’ is provided by the electromagnetic waves that are created when electrons in an atom jump from one energy level to another. Because of Quantum Mechanics, the energy levels that are allowed are very precisely defined (actually what quantised means is that something can only have discrete values like 1 or 2 rather than any possible value in between e.g. 1.12, 1.13… So when electrons jump between the energy levels then they emit radiation (EM waves) with a very precise frequency, which is used to keep the clock in time.