Bose, Fermi, Hanbury Brown, and Twiss

Via Doug Natelson, a very nice paper from the arxiv on Hanbury Brown and Twiss experiments with atoms. The Hanbury Brown and Twiss experiment (that's two guys, one with a double unhyphenated last name) is a classic experiment from the field of quantum optics, which can be interpreted as showing the bosonic nature of photons. I posted some lecture notes about it during my Quantum Optics class.

(The Hanbury Brown and Twiss experiment can also be understood classically, but that's not as much fun...)

The key idea here goes back to the symmetry business I talked about a little while back. Particles that are bosons want to collect in the same state, while fermions are forbidden to be in the same state. As photons are bosons, they tend to clump together, and arrive at a detector in groups. If there were a fermionic version of a photon, they'd "anti-bunch," and you'd never see two arrive at the detector at the same time. Of course, there aren't any fermionic photons running around (not that anybody has seen, anyway), so you can't test the experiment with light.

The linked paper desribes an experiment that does the same thing with atoms, which come in both varieties. In fact, they use two different isotopes of the same atom, and demonstrate both the "bunching" of bosons and the "anti-bunching" of fermions.

This is a really nice experiment, but of course, I would say that, since one of the key experiments of my thesis involved looking at the difference between bosons and fermions in collisions at very low temperatures. I also know the members of the French group who did this experiment, and the first experiment showing bunching of bosonic atoms was done by the guy I worked with when I was in Japan, so I have a bunch of connections to this paper.

The measurement is extremely difficult to do-- they basically have to look at arrival times of single atoms, and determine the correlations between them-- but as always with the french groups, the data are beautifully clean. The effect isn't large-- they see at most a few percent increase or decrease in the probability of finding two atoms right on top of each other-- but it's unmistakable:

i-7704b92f13320578298b9bbf4c5d2cfc-HBT.jpg

OK, maybe you need to be a geek like me to appreciate that, but trust me, it's terrific work.

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Really a cool experiment! Of course photons can be antibunched too, if made in the right way. But I reckon fermions can't be bunched, unless there are other quantum numbers involved.

Really a cool experiment! Of course photons can be antibunched too, if made in the right way. But I reckon fermions can't be bunched, unless there are other quantum numbers involved. The original explanation of the "Hanbury-Brown Twiss" experiment was "photons is bosons", but it was more subtle. Course Glauber figured it out and later got the Nobel!

I heard a probably apocryphal story about Feynman hearing a lecture by Hanbury-Brown or Twiss, leaving after 10 minutes as he thought it was crazy wrong, came back with 5 or so minutes left. The guy asked Feynman why he did that, and he said he came back after he figured it out. Course it doesn't say if Feynman just had the boson idea or if he figured it out more correctly!