Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 28/05/2016 10:22:13
-
In a recent explanation of the workings of a caesium clock (microwave fountain), it was said that an array of laser beams are used to cool a cloud of atoms to within a whisker of absolute zero.
How does an energetic beam of light actually rob a particle of energy in order to reduce its temperature?
-
Laser cooling is able to cool atoms from their usual velocity of hundreds of meters per second at room temperature, down to cm per second.
Several such techniques were awarded Nobel Prizes in 1997 & 2001.
Laser Cooling works best on atoms which have a particular electron shell structure, where electrons can move between just two energy levels, by emitting or absorbing a photon of a fixed energy, say E.
Crossed laser beams with a photon energy just below E are applied to the test chamber. If the atoms were "stationary" (ie ultracold), they would not absorb this photon, and would remain transparent.
Any photons moving towards one of the laser sources (ie they have a temperature above absolute zero) will see a Doppler shift in the laser which makes the laser photon energy equal to E, which will be absorbed, imparting momentum to the atom which opposes its previous motion, reducing the temperature. The photon is re-emitted soon after, but in a random direction. By a series of absorptions and reemissions, the average temperature of a cloud of atoms can be reduced to a temperature of hundreds of microKelvins. It is a statistical process, rather than deterministic (but then most quantum effects are somewhat statistical in nature...).
This works best with single atoms, where the density of atoms is fairly low so they don't bump into each other very often. This is often achieved by capturing the atoms in a magneto-optical trap.
To reach even lower temperatures, this can be coupled with evaporative cooling, where the atoms with the highest energies are allowed to "evaporate" and escape from the magneto-optical trap. This can achieve temperatures around tens of microKelvins.
It is possible to apply the technique to atoms which have more than 2 energy levels, but it is more complex because you need more lasers to kick any atoms out of the unwanted energy state(s).
For more information, see:
https://en.wikipedia.org/wiki/Doppler_cooling
https://en.wikipedia.org/wiki/Optical_molasses
https://en.wikipedia.org/wiki/Magneto-optical_trap
https://en.wikipedia.org/wiki/Ultracold_atom
PS: I'm sure we discussed this in the NS forum in the not-too-distant past, but Google won't find it for me.