Laser cooling

Laser cooling slows atoms using the momentum carried by photons. Tune a laser slightly below an atomic resonance and aim beams from all six directions. An atom moving toward a beam sees that light Doppler-shifted up into resonance and absorbs a photon, taking a momentum kick that opposes its motion; an atom moving away sees the light shifted further out of resonance and absorbs nothing. Every atom is therefore preferentially slowed whichever way it moves, producing a viscous 'optical molasses' made entirely of light. Absorption is directional but re-emission is random, so the recoils from emission average to zero and only the slowing survives.

Doppler cooling alone has a floor set by that random re-emission — the Doppler limit, a few hundred microkelvin for typical alkalis. Sub-Doppler mechanisms, notably the Sisyphus cooling identified by Cohen-Tannoudji's group when experiments inexplicably beat the limit, reach a few microkelvin. Going lower means abandoning light: atoms are transferred to a magnetic or optical trap and cooled evaporatively, letting the most energetic atoms escape exactly as a cup of tea cools, which reaches nanokelvin and below and produces Bose–Einstein condensates.

Laser cooling appears to violate the second law — an ordered light field is imposing order on a gas — but the books balance. The incoming beams are near-monochromatic and directional, a very low-entropy resource; the photons scattered out leave in random directions across a broader spectrum, carrying the entropy away with them. What the laser supplies is free energy, and the cooling spends it. Beyond cold itself, the technique underpins atomic fountain clocks, atom interferometers, and neutral-atom quantum computing.

Hänsch and Schawlow proposed Doppler cooling of neutral atoms in 1975, and Wineland and Dehmelt proposed the ion analogue the same year. William Phillips slowed a sodium beam with the Zeeman slower in 1982 and later found that optical molasses was reaching temperatures below the Doppler limit, which theory said was impossible; Claude Cohen-Tannoudji explained it with the Sisyphus mechanism, and Steven Chu's group at Bell Labs demonstrated molasses and the optical trap. The three shared the 1997 Nobel Prize in Physics. Their cold atoms became the raw material for the first Bose–Einstein condensate in 1995.