Magnetic evaporative cooling

Magnetic evaporative cooling is a technique for lowering the temperature of a group of atoms. The process uses a magnetic field to put atoms in a magnetic trap, a flask-shaped magnetic field. Collisions mean that over time, individual atoms will become much more energetic than the others, and they will escape the trap, removing energy from the system and reducing the temperature of the group remaining. This process, where particles by collision surmount a barrier, is similar to the familiar process by which standing water becomes water vapor.

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A way to visualize this is to take a bowl and fill it almost all the way up with popcorn. Then shake the bowl enough so that the bouncing isn't enough to spill over the side. Every so often one of the popped corns will jump out, "evaporating" from the bowl.

This technique was developed to study the Bose-Einstein condensate, an exotic state of matter in which multiple atoms enter the same quantum state. This condensation only happens at very low temperatures: around 170 nanokelvins for rubidium atoms.

The use of kinetic evaporation was pioneered in 1995, first by Eric A. Cornell and Carl E. Wieman for rubidium, and then by the group of Randy Hulet for lithium. Since then the technique has been improved upon, notably by Bouyer's team at Groupe d'Optique Atomique Laboratoire at the Charles Fabry Optical Institute.

References

M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Wieman and E. A. Cornell, Observations of Bose-Einstein Condensation in a Dilute Atomic Vapor, Science, 269:198–201, July 14 1995.
J. J. Tollett, C. C. Bradley, C. A. Sackett, and R. G. Hulet, Permanent magnet trap for cold atoms, Phys. Rev. A 51, R22, 1995.
Bouyer et al, RF-induced evaporative cooling and BEC in a high magnetic field, physics/0003050, 2000.

Categories: Thermodynamics | Atomic physics