Wednesday, 24 March 2010

Even colder still

In a previous post I was talking about how you can use a laser to cool atoms. By tuning the laser to just below the energy of an atomic transition you can selectively kick atoms that are moving towards the laser. If you fire six lasers in (one for each side of the cube) you can selectively kick any atom that is trying to leave the centre. So we've made a trap!

There is a hitch unfortunately. There is a minimum to which one can cool the atoms, once the atoms have an energy that is comparable to the photons coming from the laser then that's about as low as they can go. After all, there's only so much you can cool something by kicking it. We're already pretty cold - around 100 micro Kelvin - we'd like to go a bit colder if we can. Now we're into magnetic traps.

Magnetic Traps

Up to now we've been acting quite aggressively towards the atoms - kicking anything that's moving too quickly. To do better we're going try and round them up where we can control things better. Fortunately there's a neat way to do this. We can make use of an inhomogeneous magnetic field and the Zeeman effect.

If you apply a magnetic field to our gas of atoms then the magnetic dipoles of the atoms tend to line up with the field. Being quantum physics they can only do so in a discrete number of ways. What happens is that the transition that used to be a line splits and shifts into a number of different lines.

If we use a stronger field then the shift is larger. We can finely tune the energy at which our laser will interact with the atoms. So now we do this; if we put a magnetic field that is zero in the middle of the trap and gets bigger as you move away from the centre (you can do this) then we can control how hard we kick the atoms depending where they are. If we do it right then inside the trap we hardly kick them at all and outside trap we kick them back in.


We've managed to confine the atoms in our trap, the final step is to switch off the lasers (to stop all that noisy kicking and recoiling) and to try and use evaporation to get rid of as much energy as possible. It is understandably quite complicated to stop them all flying out once you've switched off the lasers and unfortunately it's at this point I start getting lost! The actual cooling mechanism is nothing more complicated than why your cup of tea goes cold.

After all this we're down the micro Kelvin level - a millionth of a degree above absolute zero! At these sort of temperatures the atoms can undergo a quantum phase transition and become a Bose-Einstein Condensate (BEC). This is a new state of matter, predicted by theory and finally observed in the nineties. As far as I know this is as cold as it gets anywhere in the universe.

Well I think I'm done with cooling things now. It starts off beautifully simple and then gets a bit harder! Needless to say I salute anyone that can actually do this - it's back to simulations for me.

EDIT: I over-link to wikipedia but this is a good page on Magneto-optical traps

Wednesday, 17 March 2010

Ghost Jams

via Lester, a nice video showing ghost jams in action

See New Scientist for more.

The drivers were asked to drive around at a constant speed. For a while this works OK, eventually a ghost jam develops and propagates at the same speed that they're observed in real traffic. I don't know if they tried to apply any external stimulus to see if they could guide it better.