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?
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Nope. It is still way too hot, and the laser light couldn't get
the atoms any colder.
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Why is that?
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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.
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And so they used the "evaporative cooling" you mentioned?
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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.
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Wait a minute. You just said that they used a magnetic field
for the laser trap. Isn't this the same thing?
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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.
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I have seen tops that will spin for a very long time because
they have magnets in them. Is that anything like this?
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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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6th
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