You may have heard about the atomic clocks. The idea to use atomic particles to measure time was first suggested by Lord Kelvin back in 1879. But the first atomic clock was built not before 1949 at the U.S. National Bureau of Standards (NBS, now NIST). It was not that accurate, not better than the already existing quartz clocks, but it was the proof of concept.
The atomic clock took its modern engineering look at 1955 at the National Physical Laboratory in the UK, by Louis Essen. It was based on caesium-133 atom calibrated by the use of the astronomical time scale ephemeris time, which eventually led to the international agreement of the second duration on the SI system...
The most accurate atomic clocks lose only one second over a period of 138 million years... If such a clock began measuring time when Earth was created some 4 billion years ago, this clock would have missed less than half a minute today... So it is accurate.
In the following video there is Bill Hammack, the engineering guy, trying to explain how the atomic clock works.
So, let's see if i got it correct... caesium-133 atoms are being created in an oven. Cesium atoms can be in high-energy or low-energy state, and both of them are created in the oven. The first trick is that lower energy atoms can be separated by high energy atoms using a magnetic field. The second trick is that low energy atoms can jump to high energy if they are bombarded with a very precise wavelength radiation.
So, what we now have is a stream of both high and low energy atoms coming out of an oven. Low energy atoms are separated by the high energy atoms with magnets. Those low energy atoms are then bombarded with a precise wavelength radiation to jump to high energy. The trick to understand the atomic clock is to see how this wavelength radiation is generated. As a matter of fact, there is a precision quartz oscillator that generates this radiation. But this oscillator may slow down a little. If this happens, the low energy atoms will NOT be converted into high energy, because the radiation frequency will be lower than the one required to excite them!
And that is the trick! Right after the radiation chamber there is another set of magnets which deflects the atoms coming out of the chamber. If these atoms are high-energy atoms, then they are deflected onto a detector. As long as the detector detects these atoms, everything works pretty well. But if the atoms that are coming out of the chamber are still in low energy state, they are deflected to another direction. The detector will then sense this lack of particles and it will send a pulse to the quartz oscillator. This pulse is just enough to speed it up a little bit so that it oscillates back again to its precise frequency required to excite the atoms!