Thought Experiments Made Real

We’re back from Boskone, which included lots of fun stuff, and not enough sleep. I also cleverly forgot to bring my lecture notes home from work, which means I need to go in early to figure out what the hell I’m talking about in class today, so there’s not much time for blogging at the moment.

I would be remiss in my physics-blogging duties, though, if I failed to point people to this Physics Web story about a new single-photon interference experiment (you’ll need a subscription to read the Science article). A French group including Alain Aspect (who else?) has done a beautifually clean realization of Wheeler’s delayed-choice experiment:


The experiment in this: a beam froma single-photon source is split between two paths, which travel some 48 meters before coming back together. A beamsplittler can then be inserted to recombine the two beams, or the beamsplitter can be removed to allow the paths to fall on two separate detectors. If the beamsplitter is in, you see intereference between the two paths, and if it’s out, you see which path the photon took.

The trick to the experiment is that you don’t decide whether the beamsplitter will be there or not until after the photon is in flight. They have their single-photon source connected to a random number generator, which spits out a random number some time after the photon is emitted, and that number determines whether they put the “beamsplitter” (which is an electronically controlled polarization modulator) in the path or not. They still see exactly the results you expect: with the beamsplitter, they see beautiful interference fringes, and without it, they see nothing.

There’s nothing really all that stunningly new, here. Similar experiments have been done in the past, and gotten the same results. The thing about this experiment that makes it interesting enough to make Science is what a clean realization of the thought experiment it is. They’ve done almost exactly what John Wheeler proposed, lo these many years ago, and with the random number generator and extremely long optical paths (they must’ve used optical fibers, though the paper doesn’t specify), they’ve really separated the photon emission and the decision about which measurtement to make in a way that makes it almost impossible to find a loophole.

It’s always really nice to see simple thought experiments brought to reality. It’s not a paper that’s going to change the world, but it’s excellent work, and nicely readable.