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Video 3. How an atomic clock works, and its use in the global positioning system (GPS) by EngineerGuy (2012) Source. Shows how conceptually an atomic clock is based on a feedback loop of two hyperfine structure states of caesium atoms (non-radioactive caesium-133 as clarified by the Wikipedia page). Like a quartz clock, it also relies on the piezoelectricity of quartz, but unlike the quartz clock, the quartz is not shaped like a tuning fork, and has a much larger ressonating frequency of about 7 MHz. The feedback is completed by producing photons that resonate at the right frequency to excite the caesium.