Exploring Ultracold Matter Along the Complexity Axis

It is a fundamental notion that simple things can combine into more complex things, and this complexity underlies the rich variety of phenomena that we see in nature. It is therefore important to the progress of science to understand this richness and complexity in as many environments as possible; from many examples will come general ideas about the intricacies of how the world works.

This project relies on the recognition that things as seemingly simple as atoms can present surprisingly complex behavior. Specifically, atoms held in the gaseous state at temperatures one millionth of a degree above absolute zero can be measured with high precision that reveal surprising complexity. For instance, the atoms can be pointed in a certain direction, like little bar magnets, and this direction can influence the thermodynamics of the gas as well as the interactions of the atoms with each other, altering their trajectories in sometimes unpredictable ways.

On the other hand, the comparative simplicity of the atoms lends hope that these intricacies can be understood in detail, sorting out the parts that might have an underlying simplicity from those parts that are truly chaotic. This work is therefore also an exercise in understanding the capabilities and limits of engineering atoms to particular purposes.

The work proposed will probe in detail new aspects of ultracold collisions that have emerged in recent years. One is the occurrence of an enormous density of resonant states in collisions of lanthanide atoms such as dysprosium and erbium. This circumstance may lead to novel phenomena in many-body gases on the one hand and to new insights into chaotic behavior and dynamics of rethermalization of isolated systems on the other, hence contribute to small-scale nonequilibrium dynamics.

Part of this development seeks to bring the techniques of information theory to bear on questions of the structure of complex molecules. A second aspect is the possibility of super-resonant scattering of ultracold molecules, with far higher densities of states than lanthanide atoms. The occurrence of long-lived collision complexes has been empirically verified, and their lifetimes measured.

The proposed work will seek to understand the properties of these unusual, four-atom microdroplets, using statistical techniques.

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.