File:Doppler Laser Cooling.webm

English: When researchers laser cool atoms in the lab, there isn’t just one atom but a stream of atoms traveling down a steel tube. A laser beam coming from the opposite direction is like an endless stream of ping-pong balls, all tuned to be exactly the right energy to slow the atoms down. As the atoms lose bits of energy, the stream becomes a tiny bit colder.

As atoms move toward this stream of photons and begin to collide with them, they start to slow down. But once they slow down enough, a funny thing happens: The frequency of the photons starts to change (at least from the perspective of the atoms)! Photons that were once well matched to the atoms’ energy ladder no longer do the trick.

There are two ways around this. Scientists can either continuously tune the color of the photons coming from the laser, or they can adjust the atom’s energy rungs using magnetic coils. Either way, it is possible to take atoms moving at the speed of a bullet to almost a standstill using only laser light and magnets!

Once the atoms are slow (and therefore cold), they can be captured for science experiments. Laser cooling only works with certain metal atoms, like sodium, and a handful of molecules. Even with this limitation this invention was a profound step that led to today’s quantum computing experiments with atoms, ultra-precise clocks, and lots of investigations into the nature of both quantum matter and quantum interactions with light.