Atomic Clocks

A clock is simply an apparatus that counts a repetitive event, e.g. in a mechanical clock, a pendulum might swing once every second and by counting the number of swings, one knows how many seconds have passed. Atomic clocks use atoms, more precisely the electronic transition from one state to another as their ‘pendulum’. They take advantage of the fact that all atoms of a particular element (e.g. Caesium and Rubidium) have exactly the same electron configuration. Different atomic clocks using the same element thus use exactly the same ‘pendulum’, unlike e.g. different mechanical clocks which each deploy a slightly different pendulum (due to inaccuracies in the manufacturing process). For this reason, the second is actually defined as exactly 9 192 631 770 cycles of a Caesium atomic clock.
The best atomic clocks today are so accurate that they would go wrong less than a second if they were turned on at the birth of the universe 13.8 billion years ago. We cannot measure anything more precisely than time.
Here is how an atomic clock works in principle:
Electrons moving around an atomic nucleus can only occupy a small number of well-defined ‘states’ which have well-defined energies – one of the key discoveries that led to the formulation of quantum mechanics. In the simple Bohr model, you can imagine that electrons can only move in a small number of trajectories around the nucleus, unlike e.g. planets in a solar system which in principle can rotate around the central star at any distance.
In an atomic clock, light (or generally electro-magnetic radiation) of a certain frequency is shone on an atom (or more commonly a cooled cloud of atoms). If the frequency of the light is just right (the energy of its photons is exactly equal to the energy difference between two states) the atom absorbs a photon and changes into an excited state. In an atomic clock, this process is repeated over and over again, the efficiency of exciting the atom is constantly monitored and the frequency of the light constantly corrected to yield the highest efficiency. The atoms thus constantly “watches” over the frequency of the light source, which in turn can be counted and used to measure the duration of an event.
Strictly speaking, the energy difference between ground state and excited state is constant only for individual, completely isolated atoms. External influences, such as electromagnetic fields, can influence the energy levels of the states and thus also the transition frequency. Atomic clocks are therefore shielded at great expense, and special pairs of electrical configurations are selected that are as insensitive as possible to external interference.