Just as a quartz crystal vibrates inside a quartz watch to give us a constant frequency from which to measure time, so to, do atoms. The first quartz clocks built in the 1920s taught us that the earth didn't actually rotate at a constant speed from day to day. Prior to the laboratory quartz clock we were unable to keep time accurately enough to be able to make such a measurement. So now we knew that to define one second as being 1/86,400 of a solar day was nonsense. We needed something that was more consistent than the turning of the earth on which to base our new standard unit of time, we needed an atomic clock.
Each of the atoms of the periodic table emit electromagnetic radiation at its own characteristic frequency. Scientists believe that these frequencies are constant over time and that the frequency of a cesium atom today is still the same as it would have been millions of years ago. Measuring the atomic vibration of particles would be our replacement for the quartz crystal, resulting in a clock many millions of times more accurate.
War time experimentation and development in radar and microwave radio transmissions had advanced our knowledge to the level required to build the first atomic clock. Microwaves would be needed to excite the atoms of an element and in1949, early American experiments were focused on the ammonia molecule. Sadly the results were not much better than for laboratory quartz clocks and so attention quickly moved to the cesium clocks being tested in Europe.
In 1955, the world's first atomic clock, based on cesium, was built in England by the National Physical Laboratory in conjunction with the US Naval Observatory. They were able to successfully measure the resonant frequency of cesium-133 or more accurately measure the frequency relative to astronomical time. More cesium testing devices were built in America for cross-referencing the results and finally, in 1967, the world was ready to change their reference of time from the rotation of the earth to the resonant frequency of cesium-133. The second was redefined as being exactly 9,192,631,770 oscillations of the cesium atom's resonant frequency. These new generation atomic clocks are reported to keep time to within 30 billionths of a second per year.
How does the cesium atomic clock work?
Cesium-133 atoms are first heated to a gas inside the clock. A high velocity beam of cesium-133 atoms escape from the super-heated clock and are directed towards a powerful magnetic field. This magnetic field separates absorption cesium atoms from their opposite state, emission cesium atoms. The absorption cesium atoms are then beamed into a microwave oven emitting microwaves at a frequency very close to that of cesium-133. Some of these atoms then absorb microwaves and are redirected towards a detector. By amplifying this frequency and feeding it back to the microwave oven the microwaves can be tuned to the exact frequency of cesium-133, concentrating the number of energized atoms that reach the detector. Once the exact frequency of cesium-133 has been achieved by the microwave oven the frequency can be downloaded and reduced in frequency and used to keep time to immense accuracy.
The theory of the atomic clock, simply expressed, all atoms of cesium-133 are exactly the same and vibrate at the same frequency and therefore, collectively, make an ideal clock, more accurate than we have ever seen before.