Our optical clock is based on rubidium atoms. Rubidium is an atom with the symbol Rb and atomic number 37. Rubidium is a very soft, silvery-white metal. It’s called “rubidium” from the Latin word rubidus, or ruby-red, because when put into fire it causes the flame to burn red.
Rubidium is an alkali metal – this means that of its 37 electrons, 36 are paired up in closed atomic shells leaving a single electron to have all the fun. Rubidium behaves a lot like a very simple atom with a big nucleus and a single electron, much like hydrogen which is the simplest of all atoms.
Like all atoms and molecules, rubidium has a number of energy levels. Figuring out exactly what all those energy levels are is the job of quantum mechanics. But that’s why we like rubidium – because it basically looks like an atom with just one electron, the math and physics is significantly simpler.
The energy level with the smallest amount of energy is called the “ground state”, while all of the other energy levels are called “excited states”. An individual atom can “jump” from one energy level to another if it absorbs some photons of just the right amount of energy (see our post here for more information). An atom in an excited state will eventually fall back down to the ground state, emitting photons in the process to get rid of the excess energy. In our clock, the rubidium atoms absorb two infrared photons (778 nm) to jump to an excited state, and when the atoms fall back down to the ground state they emit a blue photon (at 420 nm). We know that our infrared lasers are tuned to just the right energy when the rubidium atoms start glowing blue. As long as the atoms keep glowing blue, we know exactly what frequency that infrared laser is and we can use it to keep time!