Atomic clocks are the most accurate way we have to count the passing hours and seconds. What makes them so much better than digital watches and grandfather clocks?
It all has to do with a quartz crystal and atomic vibrations.
An atomic clock times things the same way almost all other clocks do - by vibration. Most other clocks use the vibration of pendulums, or the extension and contraction of a spring. Some even use the vibration of a quartz crystal. The atomic clock uses a more fundamental vibration, and it uses it as part of a self-regulating system.
The Quartz Crystal
A crystal of quartz has two properties that help it keep time. It retains energy very well, and it is piezoelectric. When a crystal is bent slightly, it generates a charge. Conversely, apply a difference in charge across it, and the crystal will deform slightly. This slight deformation is worth noting, as quartz crystals are chemically and thermodynamically tough. They'll stand up to heat and to chemicals without much change, but electricity will - at least temporarily - change them. When tapped, the crystal will vibrate like a tuning fork, and create an oscillating charge. This charge is used as a timer. It can also be used as a guide, allowing the watch mechanism to tap the crystal at the right time to keep the oscillation going. The crystal's ringing reinforces itself.
So far, we have a conventional quartz crystal watch, which is good enough, but "loses" a second far too often as it slips off frequency. Atomic clocks also use a crystal's oscillations. Instead of self-reinforcement, the oscillations trigger something more elaborate. They control the frequency of microwave radiation. The crystal's oscillations keep the microwave radiation within a certain range of frequencies. Within that range, the clock sweeps through different microwave frequencies. And it sends that range of frequencies shooting towards cesium atoms.
The Cesium Atom
The cesium atoms used in the atomic clock are put in a position (the right temperature and pressure) that allows them to jump between a lower energy state and a slightly higher energy state. Apply a magnetic field, and it's possible to separate out the atoms that are in the low energy state from the slightly higher one. As a stream of cesium atoms runs through the clock, a magnet does just this. It separates the low and high energy atoms and allows the atoms in the lower energy state to flow into the chamber with the microwaves.
If the microwaves are at the wrong frequency, the cesium atoms stay low-energy. They leave the chamber with the microwaves. When they are passed through a second magnetic field, they continue on their way, and are recycled into the cesium stream. If the microwaves are at the right frequency, the cesium atoms hop up a step in energy. When they pass through that second magnetic field, they go to a detector. As the microwaves approach the right frequency, more and more cesium atoms hit the detector. When the proportion of atoms hitting the detector hits its peak, the microwaves are at exactly the right frequency, which happens to be 9,192,631,770 Hertz, or cycles per second. There is our pendulum, although it's a pendulum that goes considerably faster than most.
Whenever the number of cesium atoms hitting the detector drops down, it's a signal that the quartz needs to be tapped, the microwaves need to adjusted, and the clock needs to be brought up to peak efficiency again.
Top Image: Derek Key