An 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