A major goal in physics is the deep understanding of different forms of matter, and the prediction and design of new materials.  In our project, we use ultracold atoms for this purpose, and employ laser beam, radio frequency and microwave radiation to assemble atoms into interesting and novel forms of matter.

One focus area has been spin physics, or the physics of magnetic properties.  In our work, we have studied spin dynamics for paradigmatic models of interacting spins.  Atoms with spins are like microscopic compass needles, and with the help of laser beams, we can align them like beads on a string and study their behavior.  In several of our experiments, we twisted the spin chain into a spiral or helical pattern and then looked at the time evolution.  We found how spin transport depends on the forces between the spins, and discovered helical patterns which decayed much more slowly.  More technically, we have observed ballistic, diffusive and superdiffusive transport, depending on the anisotropy of the spin-spin interactions.

Another focal area of our research has been ultracold molecules.  Such molecules can be assembled from ultracold atoms.  At low temperatures, only a single or a few quantum states are populated, which greatly simplifies the description of collisions and chemical reactions.  One major goal of the field of ultracold molecules is to obtain a microscopic understanding of how molecules collide and react.  We have studied collisions between NaLi molecules with Na atoms, and between NaLi molecules, and made several discoveries. A long-standing goal in chemistry is to control reactions and collisions by external fields.  We have realized magnetic control of reactive scattering in an ultracold mixture of Na atoms and NaLi molecules via so-called Feshbach resonances. In most molecular systems, particles form lossy collision complexes at short range with unity probability for chemical reaction or inelastic scattering leading to the so-called universal loss rate. In contrast, spin-polarized Na+NaLi was shown to have only <4% loss probability at short range.  By controlling the phase of the wavefunction via a Feshbach resonance, we have modified the loss rate by more than a factor of hundred, from far below the universal limit to far above, demonstrated here for the first time. The results are explained in analogy with an optical Fabry-Perot interferometer by constructive and destructive interference of reflections at short and long range.  Our work demonstrates quantum control of chemistry by magnetic fields with the full dynamic range predicted by our models.

Whether such resonances exist for collisions between ultracold ground state molecules has been debated due to the possibly high density of states and/or rapid decay of the resonant complex.  We have discovered a very pronounced Feshbach resonance in collisions between two NaLi molecules in the triplet ground state.  Our observations prove the existence of long-lived coherent intermediate complexes even in systems without reaction barriers.

The realization of new forms of ultracold matter will advance our understanding of materials and provide guiding principles for materials research.  Our research is fundamental in its immediate impact, but in the long run, it should lead to devices and advanced materials with yet unknown properties, and open new possibilities and applications.  Besides promoting the progress of science; our program educates students and postdocs and prepares them for a career in areas of advanced technology.

Last Modified: 01/29/2023
Modified by: Wolfgang Ketterle