Disorder Effects in Ultracold Atomic Physics

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The scientific research in this project is directed towards the interface between atomic and condensed matter physics. A particular focus is quantum optics in high-density and ultra cold atomic gases, in which disorder-driven electromagnetic interactions can develop strongly correlated character, typified by Anderson localization of light and the onset of random lasing. The overriding theme of the research program is investigation of the physics of strongly correlated atomic-radiative systems, including phase transitions in ultracold atomic vapor. A closely related secondary theme appears when such systems are electromagnetically dressed by an external field. Then new phenomena, including formation of a diffusive dark state quasiparticle, are expected to emerge. This fundamental research project focuses on manifestations of localization of electromagnetic waves in ultracold gases. As individual photons are noninteracting bosons, a nearly idealized system can be realized with a spatially disordered gas of atomic scatterers. Measurement of the time evolution of light emerging from a sufficiently dense sample provides strong measures of the development of localized states. These include a departure from diffusive light transport at a characteristic localization time, the temporal evolution of the light intensity for times larger than the localization time, and the appearance of critical fluctuations in the vicinity of the localization transition. These quantities are analyzed by time-dependent models of localization, and by atomic models of light localization, to obtain critical exponents and parameters characterizing the localization transition. The role of interactions among the scatterers, as induced by the localized field itself, forms an essential focus area of the research. A second general area, coherent control of weak light localization, is also being pursued. In these studies, a spatially disordered atomic gas is dressed with a strong control field, modifying the localization of a multiply scattered, weak probe beam. Among other applications, this serves as a measure of the fidelity of single photon storage and retrieval, and can have an important impact on an array of problems in quantum linear and nonlinear optics and quantum information. Overall, this research project has significant potential scientific and technical impact in a wide range of basic and practical areas, these associated with the inevitable presence of disorder in realistic systems. These include quantum phase transitions, deeper understanding of the role of gain and the development of random lasing in disordered systems, and new mechanisms for quantum light storage in quantum information science. Beyond these more technical aspects, the research program also has significant impact on science education, infrastructure, and diversity. This is partly accomplished by substantially involving undergraduate and graduate students in all parts of the research projects, including attendance at undergraduate-focused symposia at professional conferences. Students participate in undergraduate (independent study and Senior Thesis research) and M.S. Thesis study associated with the experiments. In the past decade, 14 Ph.D. and many M.S. students, nearly half of whom are women, have been involved in our research projects. Through active recruitment of talented undergraduates, paid internships, and our previous track record, we expect that pattern to continue. Most graduated students have gone on to technical industrial positions, while the remainder are involved in science education at all levels. Finally, the research impacts international science education and infrastructure through involvement of Ph.D. students and faculty in international components of this research with academic colleagues and students in Russia and France.