Real-world materials contain elements of disorder, such as atomic impurities or imperfect alignments in the interfaces of their crystals. These imperfections cause electrons in the material to become confined to a particular location—an effect known as Anderson localization—which can give rise to unexpected macroscopic behaviors, such as turning a metal into an insulator. It can also cause the so-called quantum-boomerang effect, in which a localized particle launched in any direction returns to its original location. Now, Roshan Sajjad of the University of California, Santa Barbara, and colleagues report the first experimental observation—in the momentum space of a Bose-Einstein condensate (BEC)—of this effect [1].
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