Keeping BEC Cold

I know I said there weren't going to be physics posts for a while, but yesterday our Communications office passed along a media request about this paper on feedback cooling of BEC, from some sort of communications-person mailing list. I'd seen it talked up elsewhere-- here, for example, so I banged out an email to the reporter in question. Who didn't use any of my stuff in the story that ran late last night.

Having put in the work, though, I may as well get something out of it, so here's the email I sent. Questions in bold are from the original request. The paper is in the New Journal of Physics, which is an open access journal, if you want to read it. I'd add figures and links to old posts and that sort of thing, but I've already spent longer on this than I should've, so you just get the email straight.

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Questions for the sources include: 1) What is the importance of this finding regarding the "coldest" material in the universe?

One of the main applications people talk about for ultracold atoms is in the area of precision measurement, using the quantum nature of the atoms to make extremely sensitive detectors of acceleration, rotation, gravitational forces, and magnetic fields. These can be useful both for fundamental physics studies--looking at tiny interactions in fine detail-- and for more practical projects. When I was a post-doc, the lab I worked in got funding from the Navy to develop atom-based detectors of rotation and gravity gradients because they might be useful in submarine navigation. A sensitive rotation detector can tell you how you've changed the orientation of the submarine, and a gravity sensor could help detect underwater mountains and other obstacles, in both cases without needing to send out sonar pulses that other people could detect.

These sorts of detectors mostly work by using the wave nature of matter, taking a bunch of atoms and splitting them apart onto two different paths, then bringing them back together recombining them. Differences in the interactions experienced by the part of the atoms that went along one path versus the other show up as changes of the final state when you recombine them. People do the same thing with light waves all the time-- the best current sensors for a lot of these things involve light-- but atoms offer a couple of advantages having to do with the fact that they're massive particles, and thus interact much more strongly with gravity.

For this to work really well, you need the best possible source of atom waves-- a collection of atoms that behave basically like a laser, with all of them having the same wavelength and "phase," meaning that they're all in step, with the peaks and valleys in the same places. Ultra-cold atoms from a Bose-Einstein Condensate are, in principle, a great way to do this-- a BEC is, loosely speaking, a large collection of atoms all occupying a single quantum state, so all of the atom waves are perfectly in step. The practical problem with this is that a BEC is a fragile thing, so cold that a single atom absorbing a single photon can significantly heat the sample, which disturbs the waves, and thus reduces the sensitivity. But you need to shine light on the atoms to know what they're doing, which means you always have a bit of heating going on. This limits the length of time you can keep a BEC around as a useful source of cold atom waves.

The new study is a theoretical investigation of a way to fix this problem. Having light shining on the atoms to monitor them will give you some heating, but if you're clever about it, you can use the information you get about what they're doing to correct for that heating-- making small tweaks to the light and magnetic fields used to confine the atoms that takes that heat out. This kind of feedback system allows you to adjust the parameters in a way that can extend the useful lifetime of a BEC dramatically.

Some previous simulations had suggested that this might be possible, but this is a very tricky problem to solve, and the methods the previous studies used relied on some approximations to simplify the problem to something they could actually work with. That always leaves open the possibility that you've approximated away the killer problem that will prevent the system from working the way you'd like. This new paper uses a different approximation, that captures more of the details of the interaction, and while it shows the limits of the previous work, it also suggests that the feedback scheme could work in a practical system.

2) How cold is cold in this instance?

That's a harder question to answer than you might think. There are two different ways to talk about temperature in physics: one is in terms of the speed at which the atoms in your sample are moving, and the other is in terms of the number of quantum states that they occupy.

In speed terms, you can never totally get rid of the motion of the atoms. If you let the sample go, turning off the trap that confines them, you'll see them fly away at some speed, that probably corresponds to a billionth of a degree (Celsius) above absolute zero.

In occupied-state terms, the sample is essentially all in a single one of the possible energy states an atom can have inside the trap that holds them. There's a tiny residue of atoms in higher-energy states, which corresponds to a really tiny temperature. Again, this is probably something in the nanokelvin (billionth of a degree) sort of range.

Those values are purely practical limits-- you can never remove all of the energy from the system, because it would require an infinitely long time to get all the energy out. You can get as close to zero energy as you have the patience to wait for, though. The numbers they use in their simulation (and that people reach in experiment) are a sort of compromise--they could get colder, but it's not worth the extra time.

Probably the best thing to say about the temperature is that it's as close to absolute zero as you'd like it to be.

(Though, again, this is a theory paper, so their atoms are simulated in a computer. The numbers they put in are based on real experiments, though.)

3) What is the next step in the research?

This is a theoretical study, using a computer to simulate the behavior of atoms in a BEC exposed to their particular feedback scheme. The next step is to actually do it with real atoms in a real trapped BEC.

This shouldn't be too hard to arrange, as they propose two different ways of realizing the feedback, closely based on two different systems people use for studying BEC. There are several different experimental research groups that could potentially implement this sort of scheme, scattered all around the world, including in Australia where some of these people are based. It's a good bet that they've been talking to experimenters about how to do this, and there are almost certainly groups tooling up to implement this right now (if they're not already well into the experiments).

Once they show it can work to keep the BEC around and coherent for a long time, then it's a matter of integrating this with the various precision-measurement systems. Again, there are a lot of people who do this sort of thing, and I'm sure they're watching this very closely.