In the first direct observation of 3-D Anderson localization of matter, laser light has shown the ability to completely localize atoms. The experimental demonstration will show how 3-D conduction is affected by the defects that plague materials, which could have implications for many applications, including ultrasonic waves in medical imaging, lasers for imaging and sensing, and electron waves for electronics and superconductors. “The physics behind disorder is fundamental to understanding the impact of unavoidable material imperfections on these kinds of applications,” said Brian DeMarco, physicist at the University of Illinois. Scientists have long theorized, but never observed, Anderson localization — that strong disorder causing interference on all sides can trap a matter wave in one place. Physics professor Brian DeMarco, center, and graduate students Stanimir Kondov, left, and William McGehee were the first to trap waves of quantum matter in three dimensions. (Images: L. Brian Stauffer) DeMarco said that Anderson localization is analogous to a trumpeter playing in a concert hall filled with randomly placed barriers that reflect sound waves. Instead of traveling in all directions, the sound stays at its source, never propagating outward because of destructive interference. “The result? Perfect silence everywhere in the concert hall. The trumpeter blows into his instrument, but the sound never leaves the trumpet,” DeMarco said. “That’s exactly the case in our experiment, although we use quantum matter waves instead of sound, and the barriers are created using a speckled green laser beam. An illustration of Anderson localization. The green balloons represent disordered barriers that localize the sound of the trumpet at its source. To simulate electrons moving in waves through a metal, the group uses ultracold atoms moving as matter waves in a disordered laser beam. Using laser light as an analogy for a material allows the researchers to completely characterize and control the disorder — a feat impossible in solids, which has made understanding and testing theories of Anderson localization difficult. “This means that we can study Anderson localization in a way that is relevant to materials,” he said. “Now, theories of Anderson localization in 3-D can be compared to our ‘material’ and tested for the first time.” The impact of disorder on waves depends strongly on their energy in three dimensions. The high-energy red wave can freely propagate outward through the disordered green laser light, but the low-energy blue wave is trapped, or localized, by reflections from the disorder. The team also measured the energy a particle needs to escape localization, known as the mobility edge. By tuning the power of the speckled green laser beam, the researchers measured the relationship between the mobility edge and disorder strength. They found that as disorder increased, so did the mobility edge, meaning that materials with high concentrations of defects induce more localization. DeMarco plans to use his measurements of Anderson localization and the mobility edge, along with future work exploring other parameters, to engineer materials to better perform specific applications — in particular, high-temperature superconducting. The group published its findings in the journal Science. For more information, visit: www.illinois.edu