What's the application? Using lasers to reduce the speed of a sample of atoms, thereby reducing their temperature to a tiny fraction of a degree above absolute zero.
What problem(s) is it the solution to? 1) "How can I make this sample of atoms move slowly enough to measure their properties very accurately?" 2) "How can I make this sample of atoms move slowly enough for their quantum wave-like character to become apparent?"
How does it work? I've written about laser cooling before, but the nickel version of the explanation is this: You can think of a beam of light as being made up of photons, little particles of light each carrying a discrete amount of energy. In addition to having energy, those photons also carry a little bit of momentum (despite having zero mass). when an atom absorbs a photon, it picks up that momentum, so a stationary atom absorbing one photon gets a "kick" that starts it moving in the same direction the photon was headed. And atom that is moving either speeds up a little (if it was headed in the same direction as the photon), or slows down a little (if it was headed in the opposite direction).
So, you can use light to exert forces on atoms, and change their motion. Laser cooling is based on using these forces to make atoms move very slowly. The temperature of a gas of atoms is a measure of the kinetic energy due to the motion of the atoms, so slow atoms are cold atoms. Using laser cooling, you can take a gas of atoms from their room-temperature speed of something close to the speed of sound down to a pokey few centimeters per second-- from the speed of a jet airplane, to the speed of a crawling insect.
"Don't lasers usually heat things?" you ask. The key to cooling rather than heating is the Doppler effect. Atoms will only absorb very specific frequencies of light, and if you tune your laser to a frequency slightly below that which the atoms want to absorb, stationary atoms will not absorb any of the photons, and won't start moving. Atoms that happen to be moving toward the laser, though, will see the laser frequency Doppler shifted closer to their resonant frequency. This makes them more likely to absorb photons from the laser, which slows them down, because the alser is headed in the opposite direction from their motion. This gives you a way to selectively slow atoms down, cooling the sample.
Why are lasers essential? For laser cooling to work, you need a very narrow spread of frequencies in your light source, and you need to control the frequency of the light very precisely in order to take advantage of the Doppler cooling mechanism. Typical Doppler shifts are around one millionth of the overall frequency of the light used (MHz, rather than THz). The best way to get this kind of control over light is through using lasers.
Why is it cool? Dude, you can get a gas of atoms within a few one-millionths of a degree of absolute zero! If that doesn't count as "cool," I don't know what does.
At those temperatures, atoms are moving so slowly that you can do exquisitely precise spectroscopic studies of their properties, because the Doppler shifts due to their motion are so small. Laser-cooled atoms are now the basis for the best atomic clocks in the world.
You can also use laser-cooled atoms to look at the quantum-mechanical behavior of atoms as matter waves, for example, in collisions. Using laser cooling as a starting point, you can use other techniques to cool the atoms still further, eventually reaching Bose-Einstein Condensation, where all the atoms in the sample "condense" into a single quantum state, occupying a single wavefunction. This allows you to study a huge range of phenomena in quantum physics, atom optics, and condensed matter physics.
Why isn't it cool enough? Laser cooling only works well for a limited number of atoms and molecules, due to limitations of laser technology. There are other ways to make cold atoms an molecules that are more generally applicable, and some of them can be employed to reach BEC.
(This post is the second of twelve highlighting amazing laser applications, in honor of the 50th anniversary of the first laser. These posts serve as a lead-up to an audience poll asking what the coolest laser application is, so if you like lasers and radio buttons, watch this space over the next week or so.)
Wow cool. That's the best layman explanation of laser cooling I've read yet. Thanks!
I heard a guy give a talk on using the AC Stark effect to cool electrons, excitons and trions in quantum dots last week. I think he's crazy.