CAREER: Equivalence Principle Test with Ytterbium Atom Interferometry

The equivalence principle (EP) states that all massive objects fall at the same rate in a given gravitational field. This idea can be tested by measuring the difference in acceleration of two freely falling objects. Since the EP is the foundation of general relativity, our most precise theory of gravity, EP tests probe the relationship between gravity and the other fundamental forces. In addition, EP tests are sensitive to the existence of new particles that could comprise “dark matter,” the matter of unknown composition that has been observed in astronomical data but has not yet been detected in the laboratory. In this project, the PI and team will perform an EP test by measuring the difference in acceleration between two atomic isotopes in an ultracold atom cloud. The positions of the atoms as they fall will be read out with high precision. The experiment will use recent advances in quantum science to reach a higher accuracy than previous EP tests, providing the most stringent searches for new gravitational physics, new long-range forces, and leading dark-matter candidates. In addition, this program will provide opportunities for students to learn the experimental techniques of modern quantum science and will support a summer school on quantum sensing and precision measurement. To carry out this measurement, the research team will produce an ultracold atom cloud containing two isotopes of ytterbium, an alkaline-earth-like atom. The ytterbium atoms will be optically launched into an atomic fountain, where they will interact with a sequence of laser pulses to measure their accelerations as they freely fall. The laser system will be designed to enable hundreds of atom-light interactions, greatly increasing acceleration sensitivity, while avoiding atom loss and unwanted forces from the light pulses themselves. The team will characterize and control systematic effects from many sources, including gravity gradients, magnetic fields, black-body radiation, and the rotation of the Earth. After characterizing systematic effects, the results of the differential acceleration measurement will be analyzed to discover or set constraints on new gravitational physics and dark matter candidates. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.