Doppler cooling

Doppler cooling is like when you hear a train coming and then going away. When it's coming towards you, the sound has a higher pitch (frequency) than when it's going away from you. It's the same with atoms. Doppler cooling is a way to make atoms from a hot gas slow down and become cooler.

First, we use lasers to shine on the atoms in the gas. The laser light helps the atoms move around. Then, we adjust the laser frequency (the pitch or the sound). When the frequency of the laser is higher than the natural frequency of the atoms, it pushes the atoms away. This makes the atoms slower and cooler. When the frequency of the laser is lower, the atoms are pulled towards the laser and they speed up. By changing the laser frequency, we can make the atoms slow down and cool down.