Sunday, 3 July 2011

Laser Cooling

One of the key techniques used in ultracold atoms experiments is laser cooling. Normally people think of lasers being used to heat things up since they are intense beams of light, so how would you cool something with one? I will answer this question, to spare you tossing and turning all night, unable to sleep because of the inner turmoil of not knowing how lasers can possibly used for cooling.

I'll start with atoms. Atoms in a gas at room temperature move very fast and therefore have a lot of energy, since energy is proportional to velocity squared. Light is made up of photons, which carry momentum that is proportional to the frequency (or colour) of the light. When an atom and a photon collide, they exchange energy. If you get the laser frequency right, then when the photon and atom bounce off each other after the collision, the photon will come out more energetic, and the atom less energetic. The atom is now slower, and therefore has less energy, and is therefore cooler (high temperature = high energy).

There are a few applets at Physics 2000 that let you play around with a few parameters and help you see how laser light can be used to slow atoms down. Check out the rest of the site too! I found it great for learning about Bose-Einstein condensation for the first time, a long time ago...

So far, this is a very simple picture. In real life, things are more complicated. The atoms and photons don't literally bounce off each other. Instead, the atom absorbs the photon and then spits it out again! One might naively think that atoms could be fully slowed to a stop by doing this, but alas, it is not so. There are a couple of limits related to the fact that the atom is always absorbing and emitting photons. However, when physicists hear about limits, they consider them to be a challenge. If that atom won't get any colder, can we trick it into getting colder? The answer is yes.

Have you heard of the legend of Sisyphus? He was a king in ancient Greece who angered the gods, and as punishment, he had to push a huge boulder up a hill, but when he got to the top, the boulder rolled back down and he had to start again. For eternity.

It turns out that by playing some tricks with polarisation, one can replicate this experience for an atom. The atom will always see an uphill potential and will climb the hill. When it gets to the top, it gets transfered into another state and suddenly finds itself at the bottom of a new hill. It looks something like this:

Nicked from here.

This is actually called Sisyphus cooling. You can't stop an atom completely like this (or really at all), but you can make it damn cold! I should note that this is exactly how I feel on Sunday mornings when I voluntarily run up and down a hill for "fun". Try it! You'll love it!

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