Laser Cooling and Trapping: Optical Molasses
You need to take your laser and send the beam at the atom from all the
different directions. Then if you adjust it to exactly the right color all the
atoms will get cold.
This is great! By choosing the right laser color, I can get big bunches of
very cold atoms.
Physicists call this "optical molasses", and it is just what Cornell and
Wieman did. In their little glass cell they had rubidium atoms that they
cooled down by laser light, much as you are doing.
But if I wait long enough, the atoms wander out of the laser beams. Won't they
hit the walls and warm up?
Good point. To prevent the atoms from wandering away, physicists arranged the
laser beams so that atoms that tried to move away from the center (where the
laser beams crossed) would be pushed back into the middle by more light
hitting them from other laser beams. This is known as a laser trap.
You mean it acts sort of like a "laser thermos bottle". The light keeps the
atoms from touching anything hot and warming up. Sounds neat, but how can you
make the light do that?
You are right, it works just like a thermos bottle, but is many times better
than one made out of glass. It turns out it is pretty easy to make the light
push the atoms to where you want them to be. You just put a couple of small
coils of wire around the cell and run current through them in opposite
directions. That makes a magnetic field that shifts the color of light the
atom wants to absorb slightly. The magnetic field is smallest right in the
middle of the cell and gets bigger at the edges.
I guess I understand. The magnetic field shift is sort of like the Doppler
shift, except while the Doppler shift makes the amount of push depend on how
fast an atom is going, the magnetic field shift makes the push depend on where
in the cell the atom is.
You've got it. By using the magnetic field shift and the Doppler shift
together, the light will slow the atoms down and push them into the middle of
the cell where it will hold them. Here is an actual picture of what these
laser cooled and trapped atoms look like when they are sitting in the cell.
They look bright to us because of all the laser light that is bouncing off
Even if the atoms never touch the wall, won't they heat up just from other
atoms bumping into them like we saw on the temperature page?
Ah, you are thinking of everything today. That is why there is a vacuum pump
attached to the BEC apparatus. This pump sucks almost all of the air out of
the glass cell, so that, except for the few rubidium atoms they want to trap,
there are no other atoms remaining to bump into the cold atoms.