Monday, 4 April 2011

Optical Lattices for Dummies

From the dawn of time, physicists have always been looking for new ways to manipulate atoms. At extremely cold temperatures, it's not quite so straightfoward; one does not simply walk into Mordor contain them in a bowl. When I say "extremely cold", I mean close to absolute zero. Liquid helium is a tropical paradise in comparison. Moreover, usually when atoms are so cold, there are not so many of them. Usually we talk about trapping billions of atoms, and if Bose-Einstein condensates are in question, then a few orders of magnitude less! So solid containers won't work.

Instead, people use magnetic or optical traps, or a combination of both. I'm going to talk about optical traps here. A fun fact about atoms: they can be trapped in laser beams! If you've heard of optical tweezers, this is basically the same thing. The electric field of the light induces a dipole force on the atoms, and they get stuck in the the most intense part of the beam (actually, they can be repelled by the beam as well - it depends on the wavelength of the light).

Now we can get creative! You can do all kinds of stuff with laser light. The nice thing about laser light is that you can make it interfere - peaks and troughs add up or cancel out, and you get something out like water ripples. If you shine two laser beams against each other, they will interfere and produce a periodic structure which we call an optical lattice. If you do this in three dimensions (6 counter-propagating beams, like axes in a 3D plot), you get something that looks like a stack of egg cartons, and in each well you can trap some atoms! If you tune your lattice parameters just right, you can trap a single atom in each well.

There are several awesome things about 3D lattices. One is that they are very much like a crystal structure, so you can simulate a quite different area of physics. Another neat thing about them is that they can be used for quantum computing (at least proof of principle, though perhaps not really for practical purposes - if you've ever seen a cold atoms setup, you'll know why). If you manage to get a single atom in each lattice site, and can somehow manipulate individual atoms, you can make a quantum register with a few hundred qubits!

This brings me to what I actually wanted to talk about: a paper that talks about just this! A group in Germany managed to trap a whole lot of single atoms in a lattice and are able to see the individual atoms (more or less) AND can manipulate single atoms surrounded by other atoms in the lattice! The paper was recently published in Nature (you have to pay to see it, unfortunately, but if you are on a university network, you should be subscribed through your university library). The figures seem to be available, so here is one:

The different patterns demonstrate the level of control they have over the atoms. They can change the spin state of individual atoms and get rid of them if they want to. The result? Pixel art. If it was me, I'd probably draw something inappropriate and see how much I could get away with and still get it published.

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