Thursday, April 24, 2008

AMO physics coolness

I saw two things in Science this week that I found quite interesting. First was a mention in Editor's Choice of this paper from my old stomping grounds at Stanford. The arxiv version is here. The idea is another great example of using essentially table-top physics (if you have a large, stainless steel vacuum chamber and lasers on your table) to test the limits of the Standard Model of particle physics, usually the domain of the high energy folks. Here's the story: there are many weird alternatives to the standard model where things like charge quantization (the idea that charge comes in chunks of exactly -e for electrons, and +e for protons, for example) and charge neutrality are approximate rather than exact, due to the breaking of some far out symmetries at very high energy scales. This paper points out that this idea can be tested very precisely (to 1 part in 1028) using interferometry of Bose-condensed atoms. In an optical interferometer, light (consider only one particular color) is split into two beams that take different paths, and then recombined. As light travels on each path, you can figure out how much phase the light waves accumulate by dividing the pathlength by the wavelength (and multiplying by 2 pi if you want your phase to be in radians). The intensity when the beams are recombined is proportional to the cos of the phase difference between the paths. This can be an incredibly precise way of measuring relative path lengths, and is essential to lots of modern technology. In the proposed experiment, the Bose-condensed atoms act like matter waves, and the idea is to do the same thing. However, in quantum mechanics the phase difference that builds up is related not just to the path length, but also picks up a contribution due to the (integrated) difference in (potential) energy (times time, divided by hbar) between the two paths. This is the way AMO and neutron interferometry measurements of gravity work: send waves along paths at different heights and recombine them, and the phase difference will include a contribution proportional to (m g h) where m is the mass of the particles, g is the gravitational acceleration, and h is the height difference. In the proposed experiment the atom waves are sent through regions of different electrostatic potential (voltage). If the atoms aren't exactly neutral, the voltage will couple to their charge and lead to a phase difference that would otherwise be absent. It's very elegant, and may be a way to test advanced high energy ideas without TeV particle accelerators.

The second bit that I read was this article about the race to use cold fermionic atoms trapped in optical lattices as a means of implementing condensed matter models of interesting systems (e.g., the Hubbard model of high-Tc superconductors). The theoretical models are computationally nightmarish to solve exactly, in large part because of the Fermi-Dirac statistics problem that the correct many-body wavefunctions must pick up a minus sign if the positions of any two electrons are swapped. The plan is to implement what are basically analog computers - cold atom systems that can be poked, prodded, and tuned - to map out the solutions. Using tunable model systems to explore strong correlations in quantum matter also happens to be the focus of Rice's Keck Program in Quantum Materials. (One note for regular commenter Sylow: now do you believe me that there is a DARPA program on this?)


Anonymous said...

Doug, I am not religious. I do not believe in things or people unless I see concrete evidence or statistical data. If you look at the last two papers coming from Hulet's group in arxiv, there is no acknowledgment to DARPA. (Note that ONR and DARPA are different government agencies)

Douglas Natelson said...

So, your baseline assumption is that everyone that speaks to you is lying unless there is empirical evidence that you have directly experienced or observed proving their statements to be accurate? That must be an interesting worldview.

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