X-Message-Number: 31969
From: John Clark <>
Subject: Laser cooling
Date: Sun, 13 Sep 2009 13:07:37 -0400

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In #31952 On Wed, 09 Sep 2009 hrhirsch <> Wrote

> Laser cooling has taken an important step toward such practical
> applications as reducing temperatures rapidly to values near absolute
> zero according to an article in "Wired Science":
> www.wired.com/wiredscience/2009/09/lasercooling/. Although I used to
> be a physicist, I really don't understand how this works. Can someone
> enlighten me?

The idea is to tune a Laser beam to a frequency slightly below the  
quantum
absorption  frequency of an atom, if the atom is motionless or moving  
away
from the Laser it will not be able to absorb a photon from the Laser,  
but if
the atom is moving toward the Laser the frequency of the photon that  
the atom
sees will be shifted slightly higher due to the Doppler effect. The  
photon is
now right at the quantum absorption frequency, the atom will swallow  
it whole
including the momentum of the photon, this will slow the atom down.

Yes, now that the atom is excited and no longer in its ground state
eventually the atom will re emit the photon, but the direction is  
random,
it's just as likely to cool the atom further as heat it, so this has  
no net
effect on the cooling process. If you have 6 such Laser beams one for,
forward, backward, right, left, up, down, you can cool things to very  
close
to absolute zero.

> If practical, it might be just what we need for vitrification.

It's overkill, you don't need things THAT cold.

  John K Clark
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