How to cool off atoms (by shooting them with lasers)

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Scientists also use lasers to cool atoms, nearly to absolute zero. Over time, a laser can take almost all the energy out of an atom. And it does this in the most contrary way possible: forcing energy into the atom.

Lasers are most often used for burning things. No, that's not true. Lasers are most often used for getting cats to run around like idiots, but after that, they are most often used for burning things. Whether it's a person's corneas, sheets of metal, or James Bond (crotch first), lasers generally work by getting things so hot, so fast, that they make a clean cut.

Lasers heat atoms by throwing photons at them. Humans feel the heat of most laser beams because their bodies are made up of many different atoms, willing to accept the heat of many different photons. Individual atoms, though, are more discerning when it comes to what kind of photons they will absorb. The electrons of an atom have certain levels at which they orbit the nucleus. When the atom absorbs a photon, one of the electrons uses the photon's energy to jump up a level. If the photon doesn't have the exact right amount of energy, the electron can't jump up, and the photon doesn't get absorbed.


In light, photon energy corresponds with wavelength. If scientists adjust a laser beam's wavelength, they can make it so that it can either be absorbed by an atom, or pass through without affecting the atom's energy. When using a laser to cool an atom, they adjust it precisely enough so that if the atom is moving towards the beam, it will absorb photons. If it is moving away, it won't.


At first glance, that doesn't seem to help. If anything, it means that scientists are providing a steady feed of energy to a charging atom.

The key to laser cooling is that when an electron jumps up a level, it doesn't stay there for long. It jumps back down, emitting a photon as it goes. When it emits that photon, it does so in a random direction. In laser cooling, when it takes on a photon, it does so only when it moves in a specific direction. This is the imbalance that allows scientists to cool the atom.


As the atom comes toward the beam, it is hit with, and absorbs a photon. This slows it progression. If it emitted the photon in the opposite direction, its original velocity would be restored. Instead, it emits the photon randomly. Since the atom emits photons in all directions, the average change in its speed due to emission is nothing. Since it keeps getting hit with photons from the beam from one direction, it loses a little speed from that direction each time. Repeat the process enough and the atom slows. Since the heat of an atom is a product of its speed, the laser will be reducing the atom's heat with every photon absorbed and emitted.

[Via Physics Central]