Optical atomic clocks are today's most accurate time-keeping devices. Their underlying principle of operation is based on the fact that the difference between two electron energy levels is independent of space and time. The transition energy corresponds (via the Planck constant) to a particular frequency of light which is required to drive the transition. Therefore, an atomic electron transition can be excited with laser light, if the light's frequency exactly corresponds to the energy of the transition. A laser's frequency can be stabilized to match the corresponding atomic electron transition energy by continuous verification of a successful laser excitation of the atomic transition. Once thus stabilized, the frequency of the laser light will always be the same (independent of space and time).