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Michael Cavagnero
Understanding Ultracold Atomic Collisions

by Jeff Worley & Robin Roenker

Studying the collision of two atoms is one thing. Studying atomic collisions near the absolute zero of temperature is another. Since the early 1990s, scientists have been able to cool a gas to one one-billionth of a degree Kelvin, also known as a nanokelvin, the coldest temperature existing anywhere in the known universe. Since then, it has become clear that atomic collisions in such an ultracold gas are extremely delicate processes giving rise to largely uncharted phenomena.

Photo of Michael Cavagnero"The coldness means that the atoms are colliding at close to zero speed," says Michael Cavagnero, professor of physics. "And that means that they interact with each other for an extremely long time and over extremely large distances."

During collisions at nanokelvin temperatures, atoms interact for several milliseconds—millions of times longer than what's seen in room-temperature atomic collisions. And they interact over a distance of tens or hundreds of microns—a vast length in atomic terms, about a million times the size of an atom.

Because of the enormous scales of distance and time involved in ultracold processes, collision outcomes—and, by implication, macroscopic properties of the gas—depend critically on tiny interatomic forces, such as relativistic corrections, nuclear spin, and hyperfine interactions, which typically are negligible in room-temperature collisions.

"These ultracold collision processes are extremely sensitive to everything. You have to have everything right. If you make just one part per billion error in the force, that will basically destroy the calculation," says Cavagnero.

To date, theoretical predictions about how these ultracold collision processes work—predictions based on fundamental theories of quantum mechanics—do not align with lab-based experimental results. Scientists have not yet been able even to accurately theoretically describe the workings of the simplest ultracold benchmark system: two hydrogen atoms colliding at very low temperatures.

Solving that hydrogen-hydrogen system theoretically is one of Cavagnero's intermediate-term goals. To do that, he will utilize supercomputing capacity together with his own newly developed theoretical techniques, designed to reduce the scale of the computation to a manageable size.

"I'm training my sights on developing a set of codes and algorithms that will allow me to calculate these ultracold hydrogen-hydrogen collisions. From the point of view of pure science, you'd like to say you could handle that," Cavagnero says. "That's the simplest place to start. And I think it's reasonable to think we can do it within the next three to five years."

While Cavagnero's interest primarily lies in understanding the fundamental processes associated with ultracold atomic interactions, other scientists have begun to investigate the many potential long-term applications for super-cooled gasses—including creating atom lasers and quantum computers—and for conducting zero-temperature chemistry. Another important application is ultra-high precision spectroscopy and improving standards of measurement, and for these, comparison and correspondence to the fundamental theory of atomic interactions will be essential.

About Michael Cavagnero

Michael Cavagnero, chair of the physics department, came to UK in 1990. He was a visiting faculty member at the Institute for Theoretical, Atomic, Molecular, and Optical Physics at Harvard University in 1998 (when this project was initially conceived), and he was named a Fellow of the American Physical Society in 2000 for creative analyses of atomic collision processes.

Cavagnero Research Team