Measuring Gravity at the Micron Scale with Laser-Cooled Trapped Microspheres: a Continuation Proposal

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Gravity is the least understood fundamental force of nature: there is a 16 order of magnitude disparity between the energy scale of quantum gravity and that of the electromagnetic and nuclear forces. The mystery can be cast in another way: why is gravity so much weaker? As a number of recent theories have suggested, important clues related to this "hierarchy problem" can be obtained by measuring how gravity behaves at sub-millimeter distances. Such measurements could lead to exciting new discoveries of physics beyond our current knowledge. However, the gravitational force between massive objects becomes weak very rapidly as their size and separation distance changes, thus making ultra-sensitive measurements a necessity at sub-millimeter length scales. This group is developing an experiment based on new technology which could advance our understanding of gravity by several orders of magnitude at the micrometer length scale. In this approach, a test mass is suspended in a "container" made of light, leading to greatly reduced friction and enhanced sensitivity. This research program is at the forefront of current knowledge and will enhance the scientific competency of the state of Nevada, which is currently under-represented in terms of scientific endeavor. The collaboration with researchers at Stanford and the NSF-sponsored Stanford Nanofabrication facility will improve the research infrastructure for the state, opening new pathways for future collaborations involving nanotechnology. Students and postdoctoral researchers will be broadly trained in experimental physics and nanofabrication and will be well positioned for entry into the scientific workforce. The fundamental nature of this project appeals to our sense of wonder about the natural world. The nation will benefit from an improved understanding of high-energy physics related to gravitational physics at the micron-length scale, at a fraction of the cost of particle-collider experiments.This award supports work on an experiment with the goal of using laser-cooled trapped microspheres to test for Yukawa-type deviations from Newtonian gravity at the micron length scale. By optically levitating the force sensor, an exquisite decoupling from the environment is possible, potentially yielding sub-attonewton force sensitivity. This new technique could ultimately advance our understanding of gravity at the micron length scale by a factor of 100,000 or more, probing deep into the parameter space for theoretically predicted deviations from Newtonian gravity. In addition to studies of short-range gravitational forces, the experimental technique we propose could also enable novel investigations of Casimir forces in unexplored regimes. The project is conceptually divided into three tasks: (1) calibration and optimization of the force sensitivity of the trapped cooled microspheres, (2) final assembly of the source mass and its driving mechanism, (3) investigation of systematic errors in preliminary gravity measurements. We will also investigate novel methods for cooling the levitated nanospheres, involving sympathetic cooling with cold atoms. A graduate physics course "Hybrid Quantum Systems" will be developed to complement and augment the proposed research, leading to new ideas and opportunities for advancing science, while developing students' breadth and expertise in STEM topics. This award is supported by the Gravitational Physics and the Atomic, Molecular and Optical Physics programs.