Tickon

Meet Tickon, a caesium atom who keeps time so well that he defines the second.

Atoms of caesium-133 have two closely spaced energy levels in their ground state. Flipping between them absorbs or gives out microwaves of one extremely precise frequency. An atomic clock tunes a microwave signal until the caesium atoms respond most strongly, then counts the cycles.

Since 1967 the second has been defined as exactly 9 192 631 770 cycles of this caesium radiation. That frequency, about 9.2 GHz, corresponds to a wavelength of about 3.3 cm, similar to the microwaves in radar. Since the 2019 SI revision this frequency is one of the seven defining constants, fixed exactly, so it carries no measurement uncertainty.

The best caesium fountain clocks are so stable that they would drift by less than a second in tens of millions of years.

Your phone's clock depends on them too. Satellite navigation uses atomic clocks in orbit, and an error of a single microsecond in timing would shift your position by about 300 metres.