Atom clocks are the most accurate clocks is the world witch can get the time right at the accuracy of loosing only 1 second in 130 million years. This is a remarkable feat of engineering and the fact that we, humans, have created something this precise amazes me. But what is more amazing is just jot sophisticated but easy the mechanism is for this atomic clock.
An atomic clock uses the same technology that your quartz wrist watch uses but it takes it up a notch and makes it so precise that this clock will not loose a second in millions of years. Usually watches work by jolting quartz crystal with electric current and letting it oscillate and by counting those oscillations we can tell what is the precise time that has passed from one measurement to the other. But because Quartz crystals oscillate they slowly loose energy and they start to oscillate a bit less and that is bad for measuring time. That is why in atomic clocks Quartz crystals are kept oscillating at 5 million times every second and a constant electrical jolt is allied to that crystal when it slows down by the tiniest bit. If you would construct a perfect quartz crystal clock it would loose a second in around 90 thousand years but that is not good enough for out very precise time-keeping devices and out global economy and that is why the atomic clock was produced.
Atomic clocks in addition to that quartz crystal uses Cesium atoms to keep that oscillation of that crystal constant and it dies this with the help of Cesium high and low energy particles that can be easily separated with magnets. When a stream of cesium atoms is separated into high and low energy states you can take those low energy atoms and give them just enough energy to bump them to the higher energy state and then by detecting those atoms you can create an electric current and with a feedback loop zap that quartz oscillator at the right frequency so that it will not slow down. When that quartz crystal eventually starts slowing down then that electric current will be lower and that will tell the system to zap that crystal with an additional current and with this mechanism create an enclosed system that will not loose a second in those millions of years.
These amazing atomic clicks are so precise that they can even measure the relativity that Einstein predicted and that is why the atomic clocks that are up in space that are used for your GPS need to be constantly adjusted because their time passes a little bit slower than ours. It is amazing that we have come so far that we can even measure so minute time differences that we need to adjust our reading according to the laws of physics and the fact that time is relative. Without atomic clocks there would be no GPS and many other of out modern inventions would also struggle to work properly starting from out electric grid to the internet connections and data transfers across internet. And because this technology has now been around for decades these atomic clocks can now be made to fit inside a box that is no bigger than 10 centimeter cube and because of that these clocks can be mounted ins servers in cars, planes, ships and on spaceships as well as on satellites to create our modern time keeping world.