National Physical Laboratory

How do Atomic Clocks work?

Atoms as clocks

Every atom is composed of a nucleus, which contains the atom’s protons and neutrons (collectively known as nucleons). Orbiting that nucleus are the atom’s electrons, which occupy different orbits, or energy levels.

By absorbing or releasing exactly the right amount of energy, the electrons can ‘jump’ from one energy level to another. This is called a transition. The electrons absorb energy to move to a higher energy level (away from the nucleus), and release energy to move down an energy level (towards the nucleus).

The energy released or absorbed in these transitions takes the form of electromagnetic radiation (e.g. visible light or microwaves). The same amount of energy is released every time the same transition occurs, no matter where or how many times it is measured.

As with all waves, the radiation has a certain frequency (i.e., it completes a certain number of full waves in a second, similar to the way a pendulum completes a certain number of swings in a minute) and this frequency can be measured. This means that a clock can be based on the wave frequency of an electron’s transition energy in an atom, in a similar way to a clock based on the swinging of a pendulum.

Why do we use caesium?

The caesium atom defines the SI second. The second is 9 192 631 770 periods of the electromagnetic radiation emitted or absorbed by the ground state hyperfine transition of the caesium atom. This means that a second is the amount of time it takes for the radiation from this transition to complete 9 192 631 770 full waves.

As with all atoms, no matter where or how it is measured this number will never change, meaning that it’s a far more reliable method of timekeeping than the Sun’s movement in the sky.

Measuring the second in a caesium fountain atomic clock

An atomic fountain clock has three stages:

  1. Six lasers placed at right angles to each other (aimed above, below, left of, right of, in front of and behind the target) are fired at a group of caesium atoms. This is known as an optical trap: the light from the six lasers pushes the caesium atoms closer together, stopping them moving to the point where they almost stop vibrating at all. As both a particle and a wave, light has momentum (just like any other object that is moving), and is able to push very small objects such as atoms. Since atomic vibrations are what we feel as heat, the caesium atoms become ultra-cold, reaching temperatures of around one microKelvin - a tiny fraction of a degree above absolute zero (-273.15 °C).
  2. Once the atoms have been cooled down, the lasers above and below them are used to launch them upwards inside the fountain’s microwave chamber, and the atoms then fall back down under gravity. This launch-and-fall movement is why the clock is referred to as a ‘fountain’. The chamber uses microwave radiation to cause the caesium atoms’ electrons to move between two specific energy levels as they fly up and fall down through it.
  3. Finally, once the atoms have completed their flight, the energy levels of the electrons can be measured through fluorescence – atoms with electrons in different energy levels will emit different radiation patterns when probed with a laser.

This whole process takes about a second, and is repeated over and over with different microwave frequencies until the frequency that causes the maximum number of caesium electrons to change energy levels is found. This frequency is the resonant frequency, and this is the frequency that is used to define the SI second. As caesium fountain clocks are improved, the microwave frequency can be more finely tuned and the SI second can be even more accurately defined.

Last Updated: 26 Oct 2011
Created: 26 Oct 2011

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