Dropping Bose Condensates for Fun and Science

ResearchBlogging.orgAn experiment in Germany has generated a good deal of publicity by dropping their Bose-Einstein Cendensate (BEC) apparatus from a 146 meter tower. This wasn't an act of frustration by an enraged graduate student (anybody who has worked with BEC has probably fantasized about throwing at least part of their apparatus down a deep hole), but a deliberate act of science: They built a BEC apparatus that is entirely contained within a two-meter long capsule inside the evacuated drop tower at the Center of Applied Space Technology and Microgravity (which in German leads to the acronym ZARM, which just demands an exclamation point. ZARM!) in Bremen. The whole thing looks like this:


(Figure lifted from the Science paper. I can't find this on the arxiv, so you can't read it for free.)

This is a really cool achievement from the point of view of an experimental AMO physicist, because it's kind of amazing that BEC technology has advanced to this point in under 15 years. The publicity about the experiment sadly is not based on the AMO awesomeness, but rather on the promise of future applications to tests of the equivalence principle or BEC interferometry for gravity sensing. As such, it's getting a little ahead of things. The experiment as it currently stands is nothing more than a cool technical accomplishment, a step on the road to interesting science.

The dropping of BEC's is nothing new-- in fact, it's a standard technique in the field. The way you measure the properties of a BEC is generally to release it from the trap that holds it, and let it fall and expand for some time before taking a picture of the distribution of atoms.

The limit to this is, of course, that the atoms fall under the influence of gravity, and that means you need either a really big apparatus or a short drop time-- tens of milliseconds at most, for typical experiments. This limits what you can do with a falling BEC, particularly in cases where you would like to investigate the effects of gravity. The German group's approach is a little more radical-- their entire apparatus can be compact because they drop the whole thing at once. They release the BEC from the trap in which it's made just the way everybody else does, but because the apparatus is in free fall at the same rate as the atoms, they can track them for a full second of drop time.

The simplest experiment you can do with such a system would be just to watch the expanding cloud, and see if it moves relative to the rest of the apparatus. If everything else is under control, a shift relative to the trap and imaging system would indicate something funny with gravity-- the atoms were falling at a different rate than the capsule, contrary to everything we think we know about gravity.

They do see some deviation-- a few mm of shift-- but when they look closely, the effect can be explained by small inhomogeneities in the magnetic fields. The give-away for this is that in addition to an upward shift, they also see a suppression of the expansion in the horizontal direction-- that is, the cloud stays smaller than you would expect, given the temperature. A little bit of stray magnetic field is enough to account for both effects, meaning that they don't see any obvious problems.

The paper identifies at least one obvious fix that they can implement to reduce the sensitivity to these stray fields: their condensate is created in a magnetic trap, which means that the atoms are in a quantum state whose energy depends on the magnetic field (that energy shift is what makes the trap work). They can relatively easily move their atoms into a different state that would not be affected by the local magnetic field, and plan to do so in the near future.

At the moment, that's all in the future, though. The current paper is very impressive from a narrow experimental technique perspective, but they have a long way to go before they can use it to test any really interesting science.

van Zoest, T., Gaaloul, N., Singh, Y., Ahlers, H., Herr, W., Seidel, S., Ertmer, W., Rasel, E., Eckart, M., Kajari, E., Arnold, S., Nandi, G., Schleich, W., Walser, R., Vogel, A., Sengstock, K., Bongs, K., Lewoczko-Adamczyk, W., Schiemangk, M., Schuldt, T., Peters, A., Konemann, T., Muntinga, H., Lammerzahl, C., Dittus, H., Steinmetz, T., Hansch, T., & Reichel, J. (2010). Bose-Einstein Condensation in Microgravity Science, 328 (5985), 1540-1543 DOI: 10.1126/science.1189164


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I know the equipment is massive, but this so calls out for repetition in near earth orbit.

By D. C. Sessions (not verified) on 21 Jun 2010 #permalink

The first experimental observation on BECs postdates my college time, so I never got a good grip on the theory. But one question I'm missing is why we would expect a change in gravity when we cool stuff down so far that it's in the lowest quantum state and stops interacting?

Interesting. It's cool to see a paper from Bremen. I have fond recollections of being shown the tower by my host parents, and wondering what kind of experiments they were doing in there precisely. It's pretty easy to spot standing above the landscape as you're driving from Lilienthal in to Bremen via some routes.

By DrScienceDaddy (not verified) on 21 Jun 2010 #permalink

This is so weird. I remember distinctively only a few months back I talked to a friend about some (nutty) idea and we ended up wondering if anybody had measured the gravitational force on a BEC. I'll spare you the details but neither of us had any clue there was an experiment planned on this. Isn't it funny how sometimes completely unrelated people think about the same things at the same time? It seems to indicate to me that we're much more influenced by what we hear and read than we think.

Aren't there easier ways to measure g? What about putting the optical lattice vertical, and measuring the Bloch oscillations?

But one question I'm missing is why we would expect a change in gravity when we cool stuff down so far that it's in the lowest quantum state and stops interacting?

I don't think there's any reason to expect the force to be different because it's a BEC. What they're looking for is a sign that really light things (single atoms) are affected by gravity in a different way than heavy things (the rest of the apparatus). The BEC is just a really convenient source of atoms for this, as it's a dilute vapor of single atoms at extremely low temperature, so there's hardly any thermal motion to deal with.

Aren't there easier ways to measure g? What about putting the optical lattice vertical, and measuring the Bloch oscillations?

Measuring anything involving gravity is a royal pain in the ass. You can measure the Bloch oscillations, but you have all the same problems with magnetic field gradients and the like that you do with dropping things, plus you don't get to look at the atoms for all that long. You can do interferometry experiments where you send half the wavefunction up and the other half down, then recombine them and look for a phase shift, but you need a long interaction time and field stability and all that stuff. And so on.

Pretty much every trick you might use to measure g works a lot better if you can let something fall for a really long time. Which makes this sort of falling-apparatus experiment a logical approach to the problem.

Ultimately, you would want to do this in orbit, where everything is in freefall all the time, not just for five seconds at a time, but that's really expensive. That's one of the goals they have in mind, though.