This experimental research program will precisely study quantum scattering of ultracold cesium atoms in an atomic clock that juggles clouds of atoms. The group has recently demonstrated a fundamentally new type of scattering experiment in which a cesium atom, prepared in a superposition two states, scatters off of atoms in another cloud launched in the atomic clock. Each state of the superposition experiences a scattering phase shift, a measure of the strength of the interaction between the atoms. By detecting only the scattered part of each atom's wave function, the difference of the scattering phase shifts is directly observed as a frequency shift of the clock. A unique feature of this technique is that the frequency shift is independent of the atomic density. The technique allows scattering measurements with atomic-clock-like accuracy. Measurements of atomic scattering lengths with unprecedented accuracy are expected. They will precisely study the scattering of the different ground-substates of cesium atoms as a function of collision energy and magnetic field to unambiguously constrain ultracold cesium-cesium interactions. They will measure threshold effects and try to identify Feshbach and shape resonances. They can also probe the frequency shifts due to juggling collisions, important for improving the stability of future clocks. Highly precise measurements of scattering phase shifts near a narrow Feshbach resonance may stringently constrain the time variation of fundamental constants, such as the electron-proton mass ratio.
Broader impacts of this program include the training of graduate students and postdoctoral researchers in many areas of modern technology from lasers, electro-optics, radio-frequency and microwave techniques, ultra-high vacuum, and atomic clocks and frequency control. This group has significantly contributed to the development of atomic clocks, including laser-cooled rubidium clocks, space clock design, juggling atomic fountains, ultra-stable lasers for optical frequency clocks, and studies of microwave cavities and cold collisions. The proposed research will lead to higher accuracy and stability of the cesium clocks that realize the definition of the SI second. The work will impact the understanding of ultracold atom-atom interactions with a breakthrough in clarity and precision.
The award supported experimental research to study atom-atom scattering at ultralow energies, as low as a millionth of a degree above absolute zero. Ultracold scattering plays an important role in many forefront areas of atomic physics research, including Bose-Einstein condensed and degenerate Fermi gases and atomic clocks using laser-cooled atoms, for which collisions are the largest problem to overcome to achieve high accuracy. In low-energy scattering, there can be resonances, where the scattering probability changes dramatically, by a factor of 10 or more, over a narrow range of collision energies. One type of resonance is a Feshbach resonance. Using a novel technique in the group’s laser-cooled cesium clock, they probed a series of Feshbach resonances and studied them by scanning the collision energy. Such resonances give very precise knowledge of the interatomic interactions, which are needed to improve the accuracy of the atomic clocks that define the second. The experimental program was a major part of the graduate education of four Ph.D. students and the career development of a post-doctoral researcher. The students and post-doc gained experience with a wide variety of experimental techniques, from lasers and optics, radio-frequency and microwave electronics, to modern atomic physics methods. The proposed experiments directly lead to several spin-offs. One was the theoretical prediction and experimental observations of a collisional frequency shift for fermions at ultracold temperatures. Previously, ultracold fermions were thought to be immune to collision shifts. However, a frequency shift was observed in optical lattice frequency standards using fermions, candidates for a next generation of atomic clocks. While it was later found that those experiments were not in the ultracold regime, these ultracold shifts for fermions will be important for future optical lattice clocks. In collaboration with groups in Paris and at Penn State, the group experimentally observed the novel collisional frequency shifts for ultracold fermions that they theoretically predicted. Another activity was analysis to support the accuracy of the world’s most accurate atomic clocks that form international atomic time. The leading systematic error was a (first order) Doppler shift. They developed the first comprehensive treatment of this long-known shift and experimentally verified it in collaboration with the clock group in Paris, establishing a clock in Paris as the world’s most accurate. Their model was subsequently used by the German and UK clock groups. With the UK group, they advanced their model of the second largest shift, due to the quantum mechanical focusing of atomic waves in the clock by the microwave field, resetting the bar for the highest clock accuracy. Applying that model to the Paris clocks reset the accuracy bar once more, improving the accuracy of the best clocks by almost a factor of two. The award also supported a theoretical collaboration that allowed the evaluation of the frequency shifts due to blackbody radiation at room temperature, a leading error source of optical-lattice frequency standards.
