I remember doing laser cooling of rubidium atoms in physics class. It's really cool how it works:<p>* Rubidium will absorb photons of a certain wavelength and re-emit it in a random direction. Since photons have momentum, it will get some net momentum change from absorbing photons all coming from a particular direction, but the re-emissions are in random directions so they have net zero momentum.<p>* Shine laser light at it from all directions but at a wavelength slightly longer than the wavelength at which it will absorb. Now, if the rubidium atom is moving, the light hitting it head on will be Doppler shifted into the wavelength that it absorbs, slowing it down, whereas the other light wouldn't affect it. So, no matter how it is moving, it will slow down.<p>Rubidium is a good material since it has an S shell on the outside as though it were a big hydrogen atom, but it is so massive that it is a lot nicer to work with for this application.