Evaporative Cooling

img_lrg/BEC_cooling.jpg not found

© Sean Garner and Lene V. Hau.

After the atoms are laser-cooled to microkelvin temperatures, the laser beams are blocked, the electromagnet is turned on, and the atoms are trapped in the spatially varying magnetic field. Collisions between these trapped atoms will cause some of the atoms to lose energy and others to gain energy. The latter will be lost from the trap, and the remaining trapped atoms have now lowered their energy—they have cooled—and eventually, as the cooling continues to very low temperatures and into the nanokelvin regime, atoms start to pile into the quantum mechanical ground state (which is the lowest energy state allowed by quantum mechanics), and a Bose-Einstein condensate is formed. (Unit: 7)