Physics 2000 Home

Laser Cooling and Trapping: Optical Molasses

    So if I have the laser just the right color, it will let me slow down the fast atoms without pushing the slow ones backwards because of the Doppler shift. But this is just in one direction. The atoms in the box are bouncing around in all directions. How do I slow all of them down?

    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 like sort of 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 them.

    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.

BEC Home How Cold is Cold? What is it? Laser Cooling Optical Molasses Magnetic Trapping Evaporative Cooling What it Looks Like What it's Good For