How do atomic clocks keep such precise time?

Atomic clocks achieve their incredible precision by using cesium atoms to regulate the oscillations of a quartz crystal. While a quartz crystal oscillator is already quite accurate, it tends to slow down over time. The atomic clock corrects for this by creating a feedback loop involving cesium atoms.

Here’s how it works: Cesium atoms exist in two different energy states, a low-energy form and a high-energy form. In the clock, a stream of low-energy cesium atoms is bombarded with radiation. The wavelength of this radiation is tied to the vibrations of the quartz crystal. This radiation causes the low-energy atoms to jump to a higher energy state. These high-energy atoms are then directed to a detector, which generates an electrical current.

When the quartz oscillator’s vibrations begin to slow down, the wavelength of the bombarding radiation changes. This change means fewer cesium atoms are converted to the high-energy state, causing the current from the detector to decrease or stop. This drop in current signals the clock’s electronics to deliver an electrical jolt to the quartz crystal, correcting its oscillation. This constant, highly accurate feedback loop ensures the clock’s timekeeping remains exceptionally precise, losing less than a second over many millions of years.