Build the World’s Smallest Atomic Clock, Trap a Nitrogen Atom in a Carbon Cage. By@ ieee.org:
An atomic clock begins with an oscillator [see diagram], which creates a frequency close to the energy level of the atom being used. If the oscillator deviates from the reference frequency, the atom’s absorption pattern changes, the change is detected by a laser, and the laser’s signal is used as feedback to tune the oscillator. For the very best performance, the atoms must be electromagnetically isolated, which requires equipment that can take up entire rooms.
Build the World’s Smallest Atomic Clock – [Link]