So, what is the last thing you picture happening to something when you hit it with a laser? I’d imagine cooling is pretty close to the bottom of the “stuff I think lasers do” list. The whole idea is so counterintuitive. We know that lasers deliver energy in the form of coherent optical radiation, and that this energy is absorbed by atoms if it happens to be in the frequency of an absorption band. We also know that, in general, adding energy to a system should increase its temperature. So how does hitting a cloud of atoms with lasers decrease the temperature? Let me tell you how!
The Doppler Effect and Quantum Mechanics
In order to explain the strange phenomenon of laser cooling, I must explain first the two
physical effects that allow its mechanism to work. The first is rather simple to explain. The Doppler effect is the shrinking or stretching of the wavelengths of a wave when there is relative motion between the emitter of the wave and the observer. When the emitter approaches in the observer’s frame of reference, the waves appear to be “compressed”, shrinking the wavelength and increasing the frequency (as the emitter is “catching up” to the waves it is emitting). The opposite occurs when the emitter is receding from the observer. This is the reason that the sound of a car approaching you changes as it passes by. The second essential component of laser cooling is the quantum mechanical properties of light. The first property is the quantization of energy absorption in atoms. Because the electrons orbiting an atom are quantum objects, they can only absorb photons at a certain frequency. The second is the fact that photons carry momentum. This means that an atom that absorbs or reflects a photon will rebound in the direction the photon was traveling. This is the principle by which solar sails work, bouncing solar radiation back and traveling forward on the rebound.
So How Does Laser Cooling Actually Work?
Very deviously. If we recall the statistical mechanical definition of temperature, it depends on the
entropy of the object. If you could selectively slow down the atoms random walk through a gas, this would drop their entropy, and cool the gas. And this is exactly what laser cooling does. By tuning two lasers, pointing in opposite directions, slightly off of the absorption frequency of the atoms being cooled, so that the atoms can only absorb photons traveling in the opposite direction they are, they will absorb the momentum of these photons, and slow down. As you have done this in both directions on the axis of these two lasers, the total speed of all the atoms in that direction will decrease. Do this in all three dimensions (using a total of six lasers), and you can reduce the motion of the atoms altogether, and bring them very close to absolute zero (as their entropy is reduced by reducing their motion through the container). How is that for brilliant physics trickery (brilliant enough to win the three co-inventors the Nobel Prize in Physics).