A slower cloud is a measurement question
Light transfers momentum. With suitable detuning, opposing laser beams can exert a force that resists atomic motion. But slowing the average drift of a cloud is not enough to demonstrate cooling: temperature concerns the spread of velocities around that drift.
A useful record therefore needs a thermometer, not just a photograph of a compact bright cloud. Expansion after release can reveal a velocity spread; interpreting that spread requires control of the initial cloud size, observation time and forces acting after release.
Read the evidence: NBS (1988), Center for Atomic, Molecular, and Optical Physics technical activities, cooling below the cooling limit
Friction competes with randomness
In the simplest low-intensity, two-level Doppler model, cooling competes with momentum diffusion from photon scattering. The expression below gives the minimum temperature in its usual idealized limit. The linewidth is an angular-frequency spontaneous decay rate, and the other constants are the reduced Planck and Boltzmann constants.
The limit belongs to that model. It is not a universal minimum for laser cooling, and it does not mean motion stops. A calculation that leaves out internal sublevels and polarization-dependent optical pumping cannot settle what those mechanisms do.
Read the evidence: Lett et al. (1988), Observation of atoms laser-cooled below the Doppler limit
The surprising result survived a new thermometer
Phillips’s group re-examined sodium optical molasses and found temperatures near 40 microkelvin, far below the roughly 240 microkelvin Doppler benchmark. NIST’s contemporary report describes confirmation with four independent thermometry methods, as well as sensitivity to magnetic fields and beam polarizations.
The discrepancy exposed missing internal-state physics. Polarization gradients and optical pumping between magnetic sublevels provide additional cooling routes. The 1997 award recognized Chu, Cohen-Tannoudji and Phillips for developing cooling and trapping methods; it did not declare every optical trap equivalent or every cold cloud a condensate.
Read the evidence: NBS (1988), Center for Atomic, Molecular, and Optical Physics technical activities, cooling below the cooling limitNIST, The Nobel Moment: Bill PhillipsNobel Prize in Physics 1997 — award record
Cooling and confinement answer different questions
Optical molasses damps velocity but does not, by itself, provide the restoring force of a magneto-optical trap. In a MOT, the field gradient and polarized light make the scattering force position dependent. A cloud can cool and still escape; confinement can coexist with heating.
Use the existing Lab to compare friction, diffusion and capture under its stated approximations. Its pedagogical force and temperature readouts do not reproduce the historical four thermometers. A cold-looking animation cannot establish sub-Doppler physics unless the implemented internal-state model actually includes it.
Read the evidence: NBS (1988), Center for Atomic, Molecular, and Optical Physics technical activities, cooling below the cooling limitLett et al. (1988), Observation of atoms laser-cooled below the Doppler limit