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Nark
Forums Junkie

Location:
Cabramatta, NSW
Registered:
May 2002

Re: OT: VC rest areas?!

Wed, 04 June 2003 06:28

Yup... 20nK.... 20 billionths (0.000 000 02) of a degree above absolute zero...

They used lasers to cool it. If you're interested, this is a description of how they use the lasers.

Quote:

Laser cooling of neutral atoms was proposed in 1975 by T. W. Hänsch and A. L. Schawlow. The basic principle is to exchange momentum between photons and atoms. Cooling is achieved by arranging that the photons can be absorbed only if they collide head-on with the atom in its flight. The speed is then reduced, primarily to a limit set by the randomness course of spontaneous emission. The 1997 Nobel Laureates showed that what is termed the Doppler limit can be overcome using refined processes, so that considerably lower temperatures can be achieved. However the cloud of cooled atoms must also be held together, and this can take place in what are termed atom traps. These often work on a combination of laser beams and magnetic fields. The magneto-optical trap (MOT) has become specially important. Several research groups have used this technique for approaching BEC conditions. However a further cooling technique proved necessary, evaporative cooling, which was employed in D. Kleppner's and T. J. Greytak's group at MIT. Here the medium is cooled by ensuring that the fastest atoms leave the community. The average temperature among those remaining then is reduced. The coffee in a cup cools in a similar fashion! In an atom trap the atoms are kept in place by magnetic dipole forces. The attractive force can be turned into a repelling force if the atomic magnetic poles are reversed. This can be achieved with a radio-frequency field, an effective method proposed by D. E. Pritchard at MIT. The most rapid atoms move high up at the edge of the potential well, where the magnetic field and hence the conversion frequency for pole switching is high. By initially applying a high frequency and then gradually lowering it, it is possible to successively skim off the hot atoms. In this way the JILA* group, led by Cornell and Wieman, managed, in June 1995, for the first time, to achieve a condensation limit in 87Rb. A final difficulty to overcome was to avoid atom loss at the centre of the trap, where the magnetic field is zero and spontaneous pole-switching is possible. By rotating a magnetic field sufficiently rapidly over the sample, it was possible to prevent the atoms from systematically pouring out of the trap.

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