This experimental research program will make precision measurements of quantum scattering phase shifts of ultracold cesium (Cs) atoms in an atomic clock. The group has demonstrated these measurements with a new type of scattering experiment that scatters a cesium atom in a coherent superposition of its two clock states off atoms in a pure state at ultracold temperatures. Each clock state experiences an s-wave scattering phase shift and, by detecting only the scattered part of each atom's wavefunction, the difference of the scattering phase shifts is directly observed as a phase shift of Ramsey fringes. A unique feature is that the observed difference of the scattering phase shifts is independent of the atomic density, providing atomic clock accuracy to scattering measurements. With high precision, the group has observed a number of Feshbach resonances with two clouds colliding at energies between 10 and 50 microKelvin, and recently at collision energies of order 1 microKelvin, within a single cloud. They will perform a first accuracy demonstration, which is expected to significantly improve the knowledge of the Cs-Cs interactions. The scattering lengths are presently insufficiently known that they impact the accuracy and operational parameters of cesium clocks, particularly the Atomic Clock Ensemble in Space (ACES) clock scheduled to fly on the International Space Station. The group will evaluate systematic errors, principally due to backgrounds, and improve the precision of the phase measurements. An improved theoretical understanding can guide future measurements, potentially studying different angular momenta, versus energy and magnetic field, and for various internal states. The group will also collaborate with national labs around the world to improve the accuracy of the best atomic clocks. The group provides a unique expertise on the design of microwave cavities for primary atomic clocks and participates in the accuracy evaluation of primary atomic clocks. Ideas developed as part of the juggling experimental work provided the explanation of the interactions of ultracold fermions for optical lattice clocks, recently experimentally observed in a chip-scale atomic clock and with a radio frequency transition in Li-6 fermions.
Accurate time keeping is crucial for navigation, communication, global positioning, and national security. Recent research of this group has significantly advanced the accuracy of the atomic clocks that contribute to International Atomic Time (TAI). Their contributions were essential to realize the currently most accurate atomic clock. Novel advancements by the research have reduced many large systematic errors that previously limited the performance of atomic clocks. Earlier work had led to rubidium being adopted as the first amendment to the definition of the International System of Units (SI) second, novel space clock designs, and an understanding of the frequency shifts due to ultracold collisions in precision measurements. The research will further advance the most accurate precision measurements and the understanding of ultracold atom-atom interactions. The training of graduate and undergraudate students in many areas of modern technology from lasers, electro-optics, radio-frequency and microwave techniques, and atomic clocks and frequency control, is important for the development of the nation's scientific workforce.
