Physics 2000 The Atomic Lab Bose Einstein Condensation

Magnetic Trapping

You said that the laser cooled the atoms to less than 1/10,000 of a degree above Absolute zero. Isn't that cold enough for BEC?

Nope. It is still way too hot, and the laser light couldn't get the atoms any colder.

Why is that?

Because, as you saw, each photon gives a certain kick to an atom, so when the atoms are moving as slowly as possible, they are still always getting jostled around by the amount of a single photon kick, and this motion is what limited the temperature. Cornell and Wieman had to figure out how to get much colder than that temperature.

And so they used the "evaporative cooling" you mentioned?

Yes, but first they had to find a new kind of thermos bottle that would keep the atoms from hitting the hot walls, but did not use photons. For this they used a magnetic trap.

Wait a minute. You just said that they used a magnetic field for the laser trap. Isn't this the same thing?

No, then they were holding the atoms just by using a small magnetic field to control the laser push. Here they are using a very strong magnetic field that pulls directly on the atoms because there is a tiny bar magnet attached to each atom. If you make the right kind of magnetic field, it will pull on those little bar magnets to keep the atoms in the center of the cell without any light around.

I have seen tops that will spin for a very long time because they have magnets in them. Is that anything like this?

It is exactly like this. Those tops are held by magnetic fields in just the same way as the cold atoms are held. Such a magnetic trap makes the world's best thermos bottle!

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