Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jmode on 15/11/2005 00:33:53
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how do lasers cool matter and what is up with Adiabatic nuclear Demagnitization.
please help
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Don't know about the Adiabatic nuclear Demagnitization, but I have heard reports about cooling with lasers, but it only works with matter that is already very close to absolute zero.
My understanding is that temperature is proportional to the speed of movement of atoms, so to cool things down, you want to slow the atoms down. What a laser can do is give an atom a kick. Normally, if you give an atom a kick, you would expect it to speed up, but if the atom is moving towards you, and you give it a very small kick away from you, the effect is to slow down the speed at which it approaches you.
What you do is tune the laser to a frequency just below the frequency at which the atom will absorb the light. Then, any atom that is stationary will largely ignore the laser light, but an atom moving towards the laser, because of the dopler shift due to its motion, will actually tune into the laser light, and will receive a small kick that will slow it down.
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The amount of entropy in the world must allways increase, however it can move about. The entropy in a system can be thermal, or organisational. If water evaporates the organisational entropy increases so much, as the water molecules can wander all over the shop, that the thermal entropy actually decreases - so it gets colder.
Adiabatic nuclear Demagnitization works similarly, you start off wit a paramagnetic salt, this behaves like a load of bar magnets. If you start off with a strong magnetic field applied all the bar magnets will align. You then cool this down as far as you can with another means, then turn off the field. All of a sudden the magnets are not constrained so can point in whatever direction you like - a bit like water evaporating the organisational entropy increases so the whole thing can get colder.
Here is another related explanation which probably makes less sense:
Temperature on a very small scale is proportional to the energy of each degree of freedom - something that moves in 3D has 3 degrees of freedom, something that moves in 3D and it's nucleus can rotate in a couple of directions has 5 degrees of freedom - if you apply a very strong magnetic field you can constrain the direction the nucleus points in, so reducing the number of degrees of freedom. If you cool this thing down as far as possible, then turn off the magnetic field you all of a sudden get 5 degrees of freedom so the energy is spread around so the temperature can decrease.
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Thanks Dave and Someone, this is interesting.
Dave - have you got any links/references for what you cover above? I'm interested in finding out more. (I thought I understood entropy, but now I'm not so sure. And I've not found many useful references to entropy via google searches - the links it gives tend to be rather basic and short.)
Also, I'm sure I've read somewhere that they can now trap individual atoms using lasers (3 or more lasers circling the atom) - so effectively holding a single atom at "absolute zero". Does anyone know anything more about this?
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quote:
Originally posted by Solvay_1927
Thanks Dave and Someone, this is interesting.
Also, I'm sure I've read somewhere that they can now trap individual atoms using lasers (3 or more lasers circling the atom) - so effectively holding a single atom at "absolute zero". Does anyone know anything more about this?
As you say, the same thing as the laser cooling technique.
http://nobelprize.org/physics/laureates/1997/press.html
http://www.colorado.edu/physics/2000/bec/lascool4.html
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Just put the atoms in bed with my ex wife. They'd soon freeze!