laser cooling
Temperature is just the jiggling of atoms, so to cool a gas you must slow its atoms down. It sounds paradoxical to do that with light — light carries energy, which usually heats things — but laser cooling turns the tiny momentum kick each photon delivers into a brake, and it routinely brings atoms from room temperature to a few millionths of a degree above absolute zero. It is the technology that opened the door to the coldest matter in the universe.
The basic mechanism is Doppler cooling. Tune a laser slightly below (red-detuned from) an atomic transition and shine it on an atom from opposite directions. An atom moving toward one beam sees that beam Doppler-shifted up into resonance, so it preferentially absorbs photons from the beam opposing its motion; each absorption delivers a momentum kick hbar k directed against its velocity. The subsequent spontaneous re-emission goes in a random direction, so over many cycles its recoil averages to zero. The net effect is a velocity-dependent force that always opposes motion — a viscous drag, which with six beams along three axes creates 'optical molasses'. Doppler cooling has a floor, the Doppler limit, set by the random recoil of spontaneous emission: k_B T_min ~ hbar Gamma/2, where Gamma is the transition linewidth.
Laser cooling underlies atomic clocks, atom interferometers, quantum simulation, and the production of Bose-Einstein condensates, and earned the 1997 Nobel Prize. Honest caveats worth stating: optical molasses cools but does not confine — trapping needs a magneto-optical trap, which adds a magnetic-field gradient to make the force position-dependent as well. Cleverer sub-Doppler schemes (Sisyphus cooling) beat the Doppler limit by exploiting polarization gradients, but even they stop at the single-photon recoil limit; to reach the nanokelvin regime of a condensate one hands off to evaporative cooling, which is not laser cooling at all.
For sodium, with linewidth Gamma corresponding to a ~10 MHz natural width, the Doppler limit works out to about 240 microkelvin — atoms moving at only tens of centimetres per second. Reaching that from a 300 K oven means each atom has absorbed and re-emitted tens of thousands of photons, each nudging it a little slower.
Tens of thousands of photon kicks brake an atom from thermal speed to a crawl.
Optical molasses provides friction but no restoring force, so it cools without trapping; confining the atoms requires a magneto-optical trap, and reaching Bose-Einstein condensation needs a further, non-laser stage of evaporative cooling.