An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms where each atom that is used has naturally occurring oscillations. For any clock to function efficiently, oscillations produce a steady reproducible beat regularly. Clocks for displaying time predominantly use a quartz crystal that produces a low frequency (32,767 Hz) which will eventually result in an error in milliseconds with prolonged usage and will require to be serviced frequently.

Quartz crystal-based clocks are also temperature dependent which makes the clock run faster in warm conditions and slower under colder conditions. Most importantly, when compared to atoms, no two quartz crystals that are used in making such quartz-based clocks are the same. They might differ in shape, size, and density which will result in an irregular production of frequency required for oscillations.

For atomic clocks, atoms are used predominantly because they possess naturally occurring oscillations with very high frequency (9.192 GHz), and isotopes used of elements in making atomic clocks will be the same which will always provide the constant frequency required for oscillations. Cesium-133 is the most common element that is used for making atomic clocks.

For an atomic clock to function, the atoms of an element used are excited by using a laser. This results in atoms reaching a higher level with increased energy (excited state) and will naturally try to drop their excited state by releasing energy. The energy released is in the form of a photon and the frequency that is generated is known as the electron transition frequency. 

The frequency emitted by Cesium generates oscillation and about 9,192,631,770 oscillations amount to a second where the number of oscillations will always be constant irrespective of any environmental changes which guarantees the output of an accurate time with no maintenance required and practically no scope for manufacturing defects.

Atomic clocks are self-adjusting in nature, which means that even if the frequency changes even by a minute scale, a detector placed at the receiving end of the Cesium will act like a Tuner and will adjust the intensity at which the Cesium is being excited and is emitting frequency in form of oscillations.

Atomic clocks are predominantly used in GPS satellites and other space missions. For A GPS satellite to work out the precise position of a user on earth, the timing of the signals that are being sent & received must be highly accurate. These signals travel with the speed of light which suggests that an error of even a microsecond in timing will give an error of about 300m on the ground due to latency. Hence regular clocks made up of quartz are rendered unusable for space applications and atomic clocks are used.