How does a clock laser tick?

We previously understood that basically anything that 'ticks' at regular intervals can be considered a clock. Those ticks can be the periodic, unhurried oscillation of a pendulum or the swift, electromagnetic oscillations of a light field.

But how to make a regular tick? With the pendulum clock there is the intricate escapement mechanism that disciplines the rotation of the clockwork gears to the oscillations of the pendulum. Each full swing becomes one tick. It turns out that the time between two ticks is defined by the length of the pendulum alone. (Strictly speaking, gravity also has a word in this but we will treat that as constant.) The better the lengths of the pendulum is kept under control, the more regular the ticks, the better and more useful the clock.

Now, how to bring some discipline into the oscillations of light? One starts with a laser that by itself already produces light with quite regular electromagnetic oscillations. This light is coupled into a so-called optical resonator which consists of two highly reflective mirrors set at a fixed distance from each other. A clever electronic feedback circuit is then used to fine-tune the frequency of the laser to have it constantly match a resonance frequency of the resonator. This is not unlike a piano wire, an acoustic resonator that can only vibrate at very specific frequencies, dictated by the length of the wire. The same holds true for the optical resonator where the resonance frequencies are defined by the distance between the mirrors, that is the length of the resonator, alone.

It thus follows that the stability of a high-tech clock laser, required for state-of-the-art quantum computers as well as the most precise clocks the human mind has ever devised, is defined by our ability to precisely control the physical distance between two mirrors, bringing us back to the length of the pendulum in an old-school mechanical clock.