Another response copied/adapted from the Physics Stack Exchange. The question was:
What are the main practical applications that a Bose-Einstein condensate can have?
Bose Einstein Condensation, for those who aren't familiar with it, is a phenomenon where a gas of particles with the right spin properties cooled to a very low temeprature will suddenly "condense" into a state where all of the atoms in the sample occupy the same quantum wavefunction. This is not the same as cooling everything to absolute zero, where you would also have everything in the lowest energy state-- at the temperatures where BEC occurs, if you ignored their quantum character, you would expect the particles in the BEC to be distributed over a bunch of different states, moving with different amounts of energy. BEC is a purely quantum effect, having to do with the way particles relate to one another.
For the purpose of answering, I assume this refers to the relatively recent phenomenon of Bose-Einstein Condensation in dilute atomic vapors (first produced in 1995 in Colorado). The overall phenomenon of Bose-Einstein Condensation is closely related to superconductivity (in a very loose sense, you can think of the superconducting transition in a metal as the formation of a BEC of pairs of electrons), and that application would trump everything else. We don't have flying cars or levitating trains yet, but superconductors are very common in research settings, and their use in something like the LHC approaches industrial scale.
The primary application of atomic BEC systems is in basic research areas at the moment, and will probably remain so for the foreseeable future. You sometimes hear people talk about BEC as a tool for lithography, or things like that, but that's not likely to be a real commercial application any time soon, because the throughput is just too low. Nobody has a method for generating BEC at the sort of rate you would need to make interesting devices in a reasonable amount of time. As a result, most BEC applications will be confined to the laboratory.
One of the hottest areas in BEC at the moment is the use of Bose condensates (and the related phenomenon of degenerate Fermi gases) to simulate condensed matter systems. You can easily make an "optical lattice" from an interference pattern of multiple laser beams that looks to the atoms rather like a crystal lattic in a solid looks to electrons: a regular array of sites where the particles could be trapped, with all the sites interconnected by tunneling. The big advantage BEC/ optical lattice systems have over real condensed matter systems is that they are more easily tunable. You can easily vary the lattice spacing, the strength of the interaction between atoms, and the number density of atoms in the lattice, which allows you to explore a range of different parameters with essentially the same sample, which is very difficult to do with condensed matter systems where you need to grow all new samples for every new set of values you want to explore. As a result, there is a great deal of work in using BEC systems to explore condensed matter physics, essentially making cold atoms look like electrons. There's a good review article, a couple of years old now, by Immanuel Bloch, Jean Dalibard, and Wilhelm Zwerger (RMP paper, arxiv version) that covers a lot of this work. And people continue to expand the range of experiments-- there's a lot of work ongoing looking at the effect of adding disorder to these systems, for example, and people have begun to explore lattice structures beyond the really easy to make square lattices of the earliest work.
There is also a good deal of interest in BEC for possible applications in precision measurement. At the moment, some of the most sensitive detectors ever made for things like rotation, acceleration, and gravity gradients come from atom interferometry, using the wavelike properties of atoms to do interference experiments that measure small shifts induced by these effects. BEC systems may provide an improvement beyond what you can do with thermal beams of atoms in these sorts of systems. There are a number of issues to be worked out in this relating to interatomic interactions, but it's a promising area. Full Disclosure: My post-doc research was in this general area, though what I did was more a proof-of-principle demonstration than a real precision measurement. My old boss, Mark Kasevich, now at Stanford, does a lot of work in this area, and that's his principal interest in the BEC stuff we were doing at Yale.
The other really hot area of BEC research is in looking for ways to use BEC systems for quantum information processing. If you want to build a quantum computer, you need a way to start with a bunch of qubits that are all in the same state, and a BEC could be a good way to get there, because it consists of a macroscopic number of atoms occupying the same quantum state. There are a bunch of groups working on ways to start with a BEC, and separate the atoms in some way, then manipulate them to do simple quantum computing operations.
There's a lot of overlap between these sub-sub-fields-- one of the best ways to separate the qubits for quantum information processing is to use an optical lattice, for example. But those are what I would call the biggest current applications of BEC research. None of these are likely to provide a commercial product in the immediate future, but they're all providing useful information about the behavior of matter on very small scales, which helps feed into other, more applied, lines of research.
This is not by any stretch a comprehensive list of things people are doing with BEC, just some of the more popular areas over the last couple of years. Other areas that people are working on actively include ultracold chemistry-- sticking atoms from a BEC together to make molecules, or trying to directly condense a gas of molecules-- and the study of strongly-interacting systems-- there was a paper not long ago about some sort of black hole analogue in a BEC system, and if you crank the interaction strength up, you can apparently get into some interesting many-body physics. My knowledge of these isn't as good as the other areas, so I'm not sure how popular they are at the moment.
(Feel free to mentally insert a loud "Huh!" right after the colon in the post title, a la Edwin Starr doing "War.")
Chad, I have a bit of a problem understanding what "the same quantum state" means. If you made a BEC of iron atoms, would it suddenly become a magnet because the spins are aligned? If you made a BEC of radioactive atoms, would they all decay at the same time? Just how "the same" are these atoms? Thanks for any elucidation!
Chad, I have a bit of a problem understanding what "the same quantum state" means. If you made a BEC of iron atoms, would it suddenly become a magnet because the spins are aligned?
They need to already be in the same state in order to Bose condense. If you have a sample in a mixture of spin states that all have the same energy, they won't all magically align their spins at the transition temperature. The best you'll get in that case is a bunch of little condensates, one for each spin state.
If you've got a bunch of atoms with spin, and they're all in the same spin state, though, the resulting condensate will be magnetic. You can then do things to perturb the spin state, and watch what happens. It's kind of complicated, because the different spin states usually have different collisional properties, which changes the behavior of the condensate, but there are a lot of people studying this sort of system. The buzzword to look for is "spinor condensate."
If you made a BEC of radioactive atoms, would they all decay at the same time?
Just because you have made a BEC of a bunch of atoms doesn't mean that the atoms will stay a BEC forever. If you scatter light off them, or they spontaneously decay, they leave the condensate, and don't take any other atoms with them.
You can make a BEC of atoms and have enhanced superradiant decay, but you can do this with regular atoms too. Many of the applications like electromagnetically induced transparency and slow light don't make use of the real quantum weirdness, but you have a great atomic sample with high density, and low temperature (so no Doppler broadening). Ben Lev's group has a nice paper on how to experimentally make light/atom crystals, really fun stuff
I'm doing my master on the quantum Zeno effect in BEC, and the hardest part is to explain to people what am I doing. Everyone keep asking for applications, disregarding the fact that in a relatively new field the transition from pure research to applications may take many years. I had to admit that I myself was totally ignorant of possible applications, and had to search in google to answer that question...