# Eli5: mechanically speaking. How does an atomic clock work and where do they store the atoms to use it

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I saw a YouTube short about atomic clocks and no where I look do they explain how the scientist store atoms to use for the clock. This topic fascinates me and I’m just curious

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The ELI5 is that specific molecules vibrate in consistent and measurable ways. We’ve defined “1 second” as “the time it takes for cesium-133 to vibrate 9,192,631,770 times”, so an atomic clock is a device that measures that.

> how the scientist store atoms to use for the clock

Well the entire clock and the table it’s on is made of atoms, it’s not as much of a challenge as you might think. It’s not exactly antimatter that would annihilate itself and whatever it touches in a spectacular boom, or some plasma that must be kept at a temperature that would vaporize the container itself.

For a Cs133 clock it’s just a little can of cesium-133 that gets slowly boiled into a gas with a heating element. For a Rubidium clock, it’s a little “neon” lamp bulb filled with gaseous rubidium (and often some other noble gas, usually argon or xeon and not neon) that microwaves are aimed at. [It’s the small blue glowing glass thing in the center of the second image](https://www.thinksrs.com/products/prs10.html)

The currently accepted definition of the SI second is “9192631770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133”.

this means if you can excite the atom to one of these special states, then its eventual relaxation will release light with a frequency of the above number…call it 9GHz.

essentially some cesium metal source is heated till atoms evaporate from it. then that atomic gas it is is bombarded by microwaves coming from a piece of quartz when a voltage is applied to it (a crystal oscillator). that piece of quartz has certain dimensions that when applied across with a certain voltage, will produce microwaves roughly about 9GHz and sweeping around that sweet number. when it is exactly on point, a nearby detector will detect a huge spike in energy from the excitation-deexcitation of the Cs-133 atoms. the oscillator has hit the atom’s resonant frequency. similar to how rubbing your finger on the rim of a wineglass just at the right speed will cause it to ring. you don’t even have to be that accurate with your finger; if you are close enough, the glass will tell you and that same glass always sings at the same frequency, similar to how Cs-133 will always sing at 9192631770Hz when exposed to roughly 9GHz of microwave energy.

then the settings used to activate the oscillator at that specific sweet spot are locked and we simply count 9192631770 cycles of the emitted radiation and call it a day. i mean a second.