An electron can jump to another shell within an atom if it gains or loses a specific amount of energy. This transition – known as a quantum jump – releases or absorbs the energy in the form of measurable electromagnetic radiation.
The frequency of these transitions allows an atomic clock to keep an accurate reading of time. Modern-day atomic clocks rely on the caesium atom, which is known to have 9,192,631,770 transitions per second.
In early atomic clocks, magnets were used to control the oscillation of specific atoms inside a beam tube. Now, lasers are used to collect and oscillate millions of atoms within a magnetic field. These atoms are pushed up and down in a fountain to detect their energy changes and take an accurate reading of time.