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  The Nobel Prize in Physics 2001     
 
 
The final step to reach BEC is the evaporative cooling.
The edge is successively lowered by a varying radio-frequency field: the hottest atoms escape. Only the colder atoms remain.
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…colder…

When the atom trap is filled, the next step in the cooling process begins. One of this year’s Nobel Laureates, Carl E. Wieman, proposed around 1990 that, after laser cooling, the light should be turned off and what is termed evaporative cooling should be applied to the magnetically confined cloud. In this technique, the edge of the ”pit” in which the atoms are confined is lowered so that the fastest ones can escape. Those that remain then have a lower temperature. This is like what takes place in a cup of hot coffee: the fastest water molecules leave the surface as steam.

 
 
 
 
 


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