What's the principle behind atomic clocks?

Atomic clocks operate based on the precise oscillation frequency of certain atoms, typically cesium or rubidium.

Atomic clocks are the most accurate timekeeping devices in existence today. They function by measuring the exact frequency of vibration of electrons in certain atoms. This frequency is incredibly consistent, making it an ideal basis for keeping time. The most commonly used atom in atomic clocks is cesium, specifically the cesium-133 isotope.

In a cesium atomic clock, cesium atoms are funnelled down a tube where they pass through radio waves. If the frequency of the radio waves matches the natural frequency of the cesium atoms (approximately 9,192,631,770 oscillations per second), the electrons in the atoms will absorb the radio waves and jump to a higher energy level. When the atoms return to their original energy level, they emit a photon which can be detected and counted. The number of these emissions can be used to accurately measure time.

Rb (rubidium) atomic clocks work on a similar principle. They use a gas cell made from a small amount of rubidium-87. The cell is illuminated by a laser, causing the rubidium atoms to oscillate. The frequency of this oscillation is then used to keep time.

The principle behind atomic clocks is rooted in quantum mechanics. The energy levels of an atom are quantised, meaning they can only take on certain discrete values. When an atom absorbs or emits energy, it does so in discrete 'packets' called quanta. This is why the frequency of oscillation of the electrons in an atom is so consistent and reliable.

The accuracy of atomic clocks is astounding. A high-quality cesium clock will only lose about one second every 1.4 million years. This precision is crucial in a variety of applications, from GPS systems to scientific research. Despite their complexity, the principle behind atomic clocks is elegantly simple: they use the consistent, quantised energy levels of atoms to keep incredibly accurate time.