Observation of Atoms Laser Cooled below the Doppler Limit

Abstract
We have measured the temperature of a gas of sodium atoms released from "optical molasses" to be as low as 43±20 μK. Surprisingly, this strongly violates the generally accepted theory of Doppler cooling which predicts a limit of 240 μK. To determine the temperature we used several complementary measurements of the ballistic motion of atoms released from the molasses.