= Sub-Doppler cooling discovery thermometry
The 1988 experiment loaded <sodium> <optical molasses> from a slowed <atomic beam>. After stopping loading, a shutter extinguished the six cooling beams. Released atoms crossed a lower probe; a <photomultiplier> recorded <fluorescence> versus arrival time. Calculations integrated the measured initial cloud distribution and an assumed three-dimensional <Maxwell-Boltzmann distribution>, including <gravity> and detection geometry. The minimum was $43\pm20\ \mathrm{\mu K}$, versus the $240\ \mathrm{\mu K}$ <Doppler cooling limit>.<Observation of Atoms Laser Cooled below the Doppler Limit by Paul D. Lett and others>[pp. 169–170, preparation, TOF apparatus, Fig. 1 and uncertainty]{ref}
Probe separation, probe intensity, cloud density and shutter timing were varied. Fountain-height, horizontal-spreading and release/recapture measurements also gave below-limit results. Geometry remained an uncertainty; agreement among complementary thermometers made a detector artifact less plausible. These measured released atoms, rather than an in-situ <velocity> distribution.<Observation of Atoms Laser Cooled below the Doppler Limit by Paul D. Lett and others>[pp. 170–171, controls, four thermometers and release limitation]{ref}
<Steven Chu>'s account connects the explanation to <optical pumping>, <Stark effect> shifts and spatially changing <electromagnetic polarization>; <Claude Cohen-Tannoudji>'s lecture separates this mechanism from subrecoil schemes. Their lectures and <William D. Phillips>'s describe complementary experimental and theoretical routes.<The manipulation of neutral particles by Steven Chu>[pp. 693–695, Fig. 6]{ref}<Nobel Lecture: Manipulating atoms with photons by Claude Cohen-Tannoudji>[pp. 710–713, §§II–III]{ref}
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