Atomic clocks

“An atomic clock is known as such because the way it works is based on the individual properties of atoms”, Thierry Bastin explains. "A pendulum clock is based on the properties of the pendulum but not on the individual properties of the atoms comprising the pendulum. The same is true of quartz clocks based on the oscillation of a quartz crystal, thus on the whole crystal and not on the atoms comprising it. However, in an atomic clock, we use the atoms’ properties individually, even if we use a large number of atoms (1).”

How do these clocks work?  They are actually improved quartz systems. They do indeed involve a quartz... but one that could be said to be placed under surveillance! By compressing a quartz crystal, oscillating electrical charges appear on its surface; this is how we obtain an electric oscillator (a clock) at a very stable frequency... as long as the dimensions of the quartz crystal don’t change. As we have seen, these possible deformations are of no importance in the majority of applications. But we know that in the long term, a quartz crystal will undergo modifications that won’t be detected. The clock will therefore drift without the user realising it. Atomic clocks allow this defect to be – almost – eliminated since they detect the slightest variations in the quartz’s frequency in real time: we can therefore “recalibrate” it and constantly maintain it at the desired value.

How can atoms play this role? We know that they can emit or absorb electromagnetic radiation. But only some of the radiation’s frequencies will be absorbed (or emitted) by the atoms. And, of course, each type of atom emits (absorbs) in its own particular frequencies: hydrogen and caesium atoms don't behave in the same way in this respect... but all hydrogen (and caesium) atoms behave identically among themselves. Clearly, this is of great value because a caesium atom in Liège is absolutely identical to a caesium atom anywhere else in the world, thus ensuring perfect replication of the phenomena. When atoms emit (or absorb) radiation, their state changes (for instance, they become excited) and it is possible to know, at any given moment, the probability of an atom being in such or such a state. If the frequency of the radiation that causes this change of state varies slightly, the probability of the atom changing state decreases. And if we subject a cloud of atoms, rather than one atom, to radiation at a precise frequency, we can therefore count how many atoms change state in real time. The number reaches a maximum at the optimal frequency; once we start to move away from this frequency, the number of atoms changing state decreases. Therefore, we have the means to constantly readjust a frequency, an oscillation... in this case, that of the quartz which is part of the atomic clock. “Since atoms are highly sensitive to the frequency of the radiation to which they are subjected", Thierry Bastin explains, “they are capable of measuring this frequency with extraordinary accuracy. As soon as the slightest variation is detected, the servo-control system corrects the frequency of the quartz’s oscillation. We thus have an oscillating system – a clock – of phenomenal regularity!"

(1)  Here, it might be useful to remind ourselves that an atomic clock has nothing to do with radioactivity! The chosen atoms aren’t radioactive, there is no phenomenon of disintegration of atoms or emission of particles. Only the size of these clocks and their cost prevents them from being worn on a wrist!