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There’s an interesting article on Scientific American that tells how to cool individual atoms to within a very tiny fraction of Absolute Zero.

It uses two laser beams acting on a very cold rarified gas in a magnetic trap. Initially, all of the atoms are in one state. The first laser beam (call it red) only affects atoms in a second stable state. A second laser (call it orange), offset and parallel to the first of a different color is turned on. This laser switches atoms to the previously mentioned second stable state when it hits an atom.

These excited atoms cannot pass the first laser beam (the excited atoms rebound from this beam). This leads to a concentration of the excited atoms on the side of the second beam. All of the atoms will eventually cross the two beams and move into the smaller area, being compressed without raising their temperature (supposedly). The lasers are then turned off, the gas is allowed to expand (and cool) into its original volume and you end up with a lower temperature gas.

I’ve left the following question there, but haven’t gotten an answer (I keep getting notices that the comments have been added to, but even when I clear my browser cache, or use a different browser, I still don’t see them). So here is my question. Can you help me understand?

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The question as I currently read it is 'Can you help me understand?'. Maybe! But only if you edit your question to be more specific. What do you want to know? –  Andrew Mar 9 '11 at 4:12
Read this: physics.stackexchange.com/questions/5851/… –  Georg Mar 9 '11 at 11:57
I think Michael means to ask why the energy from the excitation beam (the orange one) doesn't heat up the atoms. My guess would be that they in fact do heat it up, but by an amount that is much less than the amount the gas is cooled down by the subsequent expansion. My question is how can the excited state be stable? –  yatima2975 Mar 9 '11 at 12:48
The actual article (behind a pay wall) is here: scientificamerican.com/… . There might be more information in it. –  Peter Shor Mar 9 '11 at 14:02
I thought the video was pretty clear. Another potential question is: how do the atoms get back from the red state to the blue one? I would guess that the answer is that the red state is an excited state of the atoms, and that they slowly transition back to the blue state. This will raise their energy as well, but again, by a relatively small amount. This answers yatima2975's question as well: the excited state is not stable, but it's stable on the timescale of one step of the cooling process. –  Peter Shor Mar 9 '11 at 14:05

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