The Coldest Thing You've Ever Seen

Where is the coldest place in the universe? Siberia? Deep space? Not even close. In vacuum chambers around the world, atomic physicists use clever arrangements of magnetic fields and laser beams to trap dilute clouds of atoms and cool them to ridiculously cold temperatures only a few billionths of a degree above absolute zero. To give you an idea how cold this is, consider that deep space is a balmy 2.7 Kelvin or -454.8 degrees Farenheit.One of the interesting properties of these gases is that you can see them with the naked eye. The inset picture shows a glass vacuum chamber with an ultra-cold blob of rubidium atoms in its center. The laser beams used to confine and cool the gas interact strongly with the hotter atoms while ignoring the colder ones to produce what is aptly named optical molasses. The lasers used in this process lie in the red part of the spectrum, so the atom cloud naturally appears reddish in color.

The rubidium cloud pictured has a temperature around ten millionths of a degree above absolute zero and is almost certainly the coldest thing you have ever seen. It seems kind of strange that something this cold would not instantly solidify, but the density is so low that this is not possible. The rubidium cloud contains about a billion atoms whereas the same volume of air would contain about a billion times more atoms and a normal solid a million times more atoms still.

You get the idea--the gas is crazy cold and there are not many atoms. So, what are they good for? These ultra cold gases were originally created to study a novel form of matter called Bose-Einstein condensate in which all of the atoms are so cold that they move in lockstep with each other. It was an incredible feat when this new state of matter was first created in 1995, but now these gases are used mainly as tools to study a number of phenomena.The group I work for takes these condensates and loads them into an optical lattice, this is just an array of interfering laser beams used to create a periodic array of little wells in the gas. When these wells are prepared properly, just a few of the atoms will sit in each well, and the system as a whole will behave like a normal crystal. This opens an incredible number of possibilities for understanding materials. In traditional material science, powerful microscopes are used to look at a surface of atoms, but as the saying goes, that is just scratching the surface. Our crystal of ultra cold gases are much more dilute than normal materials, and we can actually look inside them to study materials in the bulk.

It turns out that the crystals made with an optical lattice are actually more perfect than those that exist in nature. You would think this would be great, but everything in the real world has imperfections and a lot of interesting phenomena arise from have these imperfections. The novel thing my lab does is introduce a small level of disorder into the optical lattice to simulate the disorder in traditional systems. With this you can emulate a number of interesting material properties such as superfluidity and high-temperature superconductivity and learn how disorder affects these systems.