When cooling atoms (e.g. to produce a Bose-Einstein Condensate, or BEC) there are at least three different methods of laser cooling employed to cool
atoms. They are 1) simple radiation pressure, 2) Doppler cooling, and 3) evaporative cooling by selective removal of atoms. As I understand it, these
are the three steps that are currently used in making BECs, and I'll go over them as three steps in cooling a bunch of atoms down to extremely low
Before I start, though, it's important to be familiar with the idea that light carries momentum, and can exert pressure on things. For the purpose of
this explanation, I think it's easiest to think of light as a sort of "wind" of photons (or "light-particles"). So a laser would be like a stream
of photons. It's also important to know that these photons 1) carry momentum along with them, and 2) can get absorbed by atoms. And when they do get
absorbed by atoms, the momentum has to go into the atom (since the photon is no longer there). So if a photon and an atom have a head on collision
where the photon gets absorbed, the atom gets slowed down a bit.
Okay, now to make a BEC, first you need to accumulate a relatively small group of atoms for the condensate, and you've got to get them away from
other atoms (or the other atoms would just heat them right back up), so you want to get them off of a lump of material and into a vacuum where you can
isolate them. The general way of doing this is by heating a lump of the material in a vacuum, so that a bunch of atoms are essentially boiled off the
surface. Then you channel them all into a jet and point it towards the place where you want to make your BEC.
Okay, because you just “boiled” these atoms, they are very hot (and moving very fast - remember temperature is proportional to the speed of the atoms
squared), so first you want to just kinda stop them from shooting away from the oven at high speeds. This can be done simply by shining a laser head
on at the atoms. As they're flying towards the laser, they absorb photons traveling in the opposite direction and are slowed down, and since they are
going slower, they by definition have a lower temperature. Now there's a slight problem with this, which you may have spotted: once they stop, the
photons will keep getting absorbed, and the atoms will start to accelerate back the way they came! So this only works for a little while, and then you
need to switch to the second cooling method, Doppler cooling.
For Doppler cooling, we need another detail from quantum mechanics, and a bit of relativity. First the quantum mechanics. Basically, the atoms we're
looking at don't absorb every photon that comes near them - they absorb photons somewhat randomly, depending on the wavelength of the photon. Also,
each atom has a specific wavelength it "likes", or is more likely to absorb. So if you shine a laser of light at an atom, it has a probability of
absorbing that light that is basically a Gaussian (bell) curve that peaks at the wavelength the atom likes. If the light is at a wavelength the atom
doesn't like as much, it's less likely to absorb photons that pass by. As for the relativity part, we just need to know about the Doppler Effect,
which says that if you are traveling towards a wave, it will seem to be a higher frequency (shorter wavelength) than if you're standing still, and if
you're traveling away from a wave, it will seem to have a lower frequency (longer wavelength) than if you were standing still.
Right, so now for the Doppler cooling part. Imagine we've got one atom sitting in a vacuum, in what we'll call our "trap", which is really just a
region of space. Now imagine we've got two laser beams hitting the atom, one coming from the left, the other from the right. At first glance, it
seems like overall nothing would happen - whatever effect the beam from one side had, the beam from the other side would cancel it out. But there's a
trick! The trick is to make the lasers operate at a frequency that is just below the frequency of light that the atom likes to absorb (this is known
as "red-detuning"). That way something really neat happens. Say the atom is going moving to the right. Since it's moving into the beam coming from
the right, that beam looks like it's got a slightly higher frequency (due to the Doppler shift) - and this means the atom sees photons that are
closer to the frequency it likes to absorb, so it's more likely to absorb photons from the right, which means it will get a bunch of little pushes
from the right (pushing it back left). What about the beam from the left? Well, the atom is moving away from it, so the photons from the left are
shifted to a lower frequency - away from the frequency that the atom likes, so it's not absorbing them nearly as much, and not getting pushes towards
the right. The same sort of thing happens when the atom moves towards the left, and voila! You've got a system where whichever direction the atom
moves, it gets slowed down. (In a 3-D trap, you really have lasers coming from 6 directions - right, left, up down, and front and back - so that the
atom truly does get slowed down whichever direction it's going).
Unfortunately, this method can’t cool the atoms down to cold enough temperatures and high enough densities to produce a BEC. The reason for this is
that when the atoms get too dense, the blob of them becomes opaque to the Doppler cooling lasers – in other words, the clump of atoms becomes so thick
that the lasers can only hit the atoms on the surface, and can’t cool the ones in the middle anymore. The limits of Doppler cooling are densities of
around 10^11 atoms per centimeter, and temperatures around 10 to 100 microKelvin.
The final method of cooling is really sneaky. The basic idea is that you want to get rid of a few of the hottest atoms (i.e. the most energetic ones).
When you do this, they take a bunch of energy out of the system, and when the rest of the atoms settle, they are at a lower temperature. Okay, so the
idea in itself isn’t that sneaky – you do it all the time when you blow on a cup of coffee or tea to cool it down. The way you do it with atoms is the
To understand how to accomplish evaporative cooling with atoms, we need to first take a look at how these atoms are being held in the trap. The basic
trapping mechanism is a magnetic field. So the atoms all have a magnetic moment (say for the sake of example, they are all pointing “up”), and you set
up a magnetic field so that atoms with magnetic moments pointing upwards feel a force towards the center of the trap, wherever they are. For those who
are familiar with magnetic potential wells, you basically just set up a potential well that is concave up for upwards magnetic moments, and the atoms
with magnetic moments pointing up all congregate around the minimum potential. You might ask, though, how we know that all the atoms have their
magnetic moments pointing upward? Well, we actually enforce that when we set up our trap; if you think about it, if we are forcing “up” atoms towards
the center of the trap, atoms with opposite magnetic moments (“down”) would be forced away from the center of the trap – kicked out, in effect.
Here’s the sneaky part – we use that fact that atoms with the wrong moments are kicked out of the trap. Remember when I said that atoms absorb some
wavelengths of light more than others? Well it turns out (to go more in depth with this, you need a course in quantum mechanics, or a couple…), that
this absorption depends many things, including what happens to the atoms when they absorb the light. And there’s a very special case which physicists
can exploit, where if you shoot exactly the right frequency of light into your trap, it will take atoms with a specific energy (for instance the atoms
with the highest energy in your trap – the ones we want to kick out), and flip their magnetic moments! This is great, because once we’ve flipped the
moments of these atoms, they are kicked out of our trap! And they take energy with them, which leaves the rest of the atoms cooled down! And then you
re-tune your beam to take off the next layer of most energetic atoms, and repeat this until you have your BEC. Using these methods, physicists have
achieved temperatures on the order of nanoKelvins (10^-9 degreed Kelvin) above absolute zero. And there are all sorts of neat things you can do with
matter that cold, but I’ll leave that for the reader to explore! (You might want to search topics such as Bose-Einstein Condensates,
superconductivity, superfluidity, and atom lasers to start with, and see where you go from there!)