Abstract

By measuring the ballistic motion of sodium atoms released from optical molasses¹ we determine their temperature to be as low as 43 ± 20 μK. Four different measurement techniques are used, giving temperatures ranging from 43 to 80 μK. These observations are inconsistent with the accepted theory of laser cooling,² which sets a lower limit on the temperature of k_BT = ħγ/2. This Doppler cooling limit is achieved for a detuning of Δ = – γ/2. For sodium cooled on the D2 line, γ = 2π × 10 MHz, and the Doppler cooling limit is 240 μK. We have measured the detuning dependence of the temperature and found it to be in strong disagreement with theoretical predictions. The temperature is minimized for |δ| much larger than γ/2 and is always below the cooling limit except at small detunings. These results are remarkable and unexpected; they raise important questions about the adequacy of present theories of laser cooling.

© 1988 Optical Society of America