Laser cooling of neutral atoms

The past year has seen major advances and surprises in laser cooling of neutral atoms in optical molasses— an arrangement of counterpropagating laser beams that strongly damps the motion of atoms. For atoms at rest in equal intensity, oppositely propagating laser beams, the total force is, of course, zero. But for atoms moving in light tuned to a frequency below the atomic resonance, the traditional view of optical molasses was that the Doppler shift causes atoms to absorb light more strongly from the beams that oppose their motion. This produces a damping force that both cools the atoms and provides a viscous confinement within the intersecting laser beams (although there is no restoring force). In particular, the simple theory for two level atoms predicted that the lowest atomic kinetic energy would be hΥ/4, where Υ is the decay rate of the atomic state excited by the laser. For sodium atoms cooled on the yellow D-line transition, this corresponds to a temperature of 240 μK.

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