This week in cond-mat

Where did summer go? Several interesting things on the arxiv recently. Here are two from this past week that caught my eye.

arxiv:0806.3547 - Katz et al., Uncollapsing of a quantum state in a superconducting phase qubit
This paper first appeared on the arxiv last year, and it made it onto this week's mailings because the authors uploaded the final, published version (PRL 101, 200401 (2008)). This experiment is important as a technical development in the quantum computing community, since the ability to restore some measure of purity to a quantum state after that state gets entangled with some environmental degrees of freedom could be very useful. It is also a great example of why simplistic thought experiments about wavefunction collapse are misleading. A better way to think about this experiment is in (an imperfect) analogy to spin echo in nuclear magnetic or electron spin resonance. In a spin echo experiment, an ensemble of spins is set precessing, and the evidence of their coherent precession gets smeared out as a function of time as the spins "dephase" (get out of sync because of perturbing interactions with other degrees of freedom). However, in these echo experiments, a properly defined external perturbation (a pulse of microwaves) can flip all of these spins around, so that the ones originally going ahead of the pack are put in the back, and the slow ones are put in the front. The spins rephase, or become coherent in their motion again. The authors do something rather analogous here using superconducting devices. Nice!

arxiv:0908.1126 - N. P. Armitage, Electrodynamics of correlated electron systems
I'm not promoting this just because Peter sometimes comments on this blog. This is a great set of lecture notes from a 2008 summer school at Boulder. These notes provide a very good, pedagogical overview of how electromagnetic radiation interacts with the electronic systems of real materials, and how one can use measurements ranging from the THz (mm-wave) to the ultraviolet to infer details of the electronic properties. These sorts of reviews are a wonderful feature of the arxiv.