A marginally less cranky physics post than the previous: the big story in my area of physics this week is probably the Harvard experiment involving the storage and transport of light pulses. Like the ILC announcement, this has been written up in the Times, and you can also read the Harvard press release or the much more informative PhysicsWeb report.
The basic idea here is an extension of the “stored light” trick that Lene Hau’s group at Harvard did a few years ago. They illuminate a sample of atoms with two different laser beams, which leads to the absorption of one of the two. They then switch the other beam off, wait a little while, and turn it back on, and the absorbed light is regenerated.
The new wrinkle here is that the atom sample they used is a Bose-Einstein Condensate split into two parts, and the initial absorption causes some atoms from one of the condensates to start moving. They switch the second laser off for long enough for the moving atoms to make it from one condensate to the other, and then turn it back on, and recover the original pulse from the neighborhood of the second condensate.
It’s a clever experiment, but I think I’ll need to read the actual paper to figure out what the fuss is about. I mean, it’s cool and all, but I’m not quite sure what the point is. But then, I’ve never really understood the point of the “slow” and “stopped” light stuff in the first place…
The “stopped” light thing is particularly puzzling to me. The light has basically been absorbed. Granted, it’s been used to put the atoms into a long-lived superposition state, rather than a typical short-lived excited state, but it’s still essentially absorption, and not a big shock to anyone who’s heard of the dressed state picture. The fact that the recovered pulse is still coherent with the original absorbed pulse is kind of neat, but that’s stimulated emission for you.
I’m not really clear on what this new experiment adds to the picture. Specifically, I don’t understand the role of the second condensate– I get that the absorption causes some of the atoms in the first BEC to start moving, and separate themselves from the others, but I don’t see what’s gained by having those atoms enter a second condensate before turning the laser back on. The pulse recovery ought to work exactly the same way with spatially separated atomic wavefunctions as with a single excited BEC, so it can’t be that the second condensate is needed for the recovery to work. It would be really cool if the second condensate amplified the signal in some way, so you got more light out, but there are some tiny little issues with conservation of energy there, so that can’t be it.
I dunno. I’ll have to get the paper from Nature, and see if I can figure out why this is so exciting. In my copious free time…