Intrinsically Quantum

Garnet Chan appreciates a good challenge. A Caltech theorist who works at the interface of theoretical chemistry, condensed matter physics, and quantum information theory, Chan says he is drawn to scientific problems that have long resisted theoretical study.

One knotty question he investigates is why some materials are able to conduct electricity with zero resistance even when they are not close to absolute zero in temperature, a phenomenon known as high-temperature superconductivity. “We’ve had these materials for more than 30 years, but we don’t know why a particular collection of atoms arranged in a particular way gives rise to high-temperature superconductivity,” says Chan, the Bren Professor of Chemistry at Caltech.

Another question relates to the biological process by which some bacteria use catalyzing proteins, or enzymes, to capture nitrogen from the air and turn it into soil fertilizer that plants can use. Commercial production of fertilizer can mimic this natural process only under very high temperatures and pressures, and at very high energy costs.

“These are two types of problems that previously were not resolvable by theory, but for which we have developed techniques that bring them into the scope of computational quantum chemistry,” Chan says.

READ MORE FROM THE CALTECH EFFECT: A Quantum Life

“I made a careful decision to come here and not to go anywhere else, and that’s because of the uniquely collaborative culture. Here, we can regularly participate across different divisions, and we actually do it in practice.”
- Garnet Chan

Predicting the Nonintuitive

The field of quantum chemistry, pioneered at Caltech by Linus Pauling, aims to understand and predict the behaviors of atoms, molecules, and materials by modeling their constituent electrons using quantum mechanics.

“Chemical reactions and chemical bonding are intrinsically quantum because they involve electrons, which are very small and light particles,” Chan says, “and quantum mechanics is the theory of the very small.”

To simulate these quantum mechanical phenomena, Chan enlists the aid of computers. The laws of quantum mechanics, which describe the vast number of ways that electrons can interact and arrange themselves within a material, are well known. But solving the resulting equations to make testable predictions about a given material’s properties appears to require so much computational power that it was long considered to be beyond the capabilities of classical computers.

“If you naively consider the equations of quantum mechanics, they look like they’re too hard to solve using even the most powerful computers today,” Chan says. “But under most circumstances, most of the potential solutions are not actually found in nature. By limiting the simulations to the restricted set of quantum mechanical possibilities in the natural world, it becomes practical to solve the equations.”

Chan is betting that this computational approach will be especially useful for modeling the behaviors of electrons in molecules and materials that contain transition metals. These metals include iron, titanium, the precious metals, and others located in columns three to 12 of the periodic table. Transition metals form chemical bonds with other atomic elements using electrons (called d electrons) that are distributed around the nucleus in a complex shape like a clover leaf (called a d orbital). Whereas the electrons in most materials travel sufficiently quickly that they interact only briefly with each other, the d electrons are comparatively sluggish. This means that they spend a lot of time interacting, which creates complicated correlated motions.

Transition metals are highlighted in gray in the periodic table above. Chan uses computation to study their properties.

“In other materials, you can assume that the electrons just do their own thing,” Chan says. “But the electrons of transition metals are not like this. They kind of talk to each other, and the behavior of one electron depends strongly on the behaviors of the others.”

Chan and other researchers think this correlated movement could help explain the role transition metals play in everything from superconductivity to biological nitrogen fixation. A successful simulation of the complex interactions of d electrons could lead to an improved understanding of these phenomena and eventually enable scientists like Chan to create new materials with bespoke properties.

“Transition metals are at the core of all the interesting enzymes,” Chan says. “They are also at the heart of all the unusual quantum materials. The complexity generated by these electron interactions is a rich seam running through chemistry and physics.”

Fearless Collaboration

Chan says the interdisciplinary nature of his work is one reason he chose to move to Caltech. “I made a careful decision to come here and not to go anywhere else, and that’s because of the uniquely collaborative culture,” he says. “Here, we can regularly participate across different divisions, and we actually do it in practice.”

The promise of working in a close-knit research community was appealing to Chan. “There’s a degree of personal connection between the faculty and the administration that does not exist at any other institution I’ve worked at,” he says. “At Caltech, you can be sitting next to the president for lunch and actually talk about quantum physics!”

Chan (right) with graduate student Erika Ye and postdoctoral scholar Joshua Kretchmer

Chan is also thankful for the philanthropically endowed chair he holds at Caltech, which provides flexible funds he can use to supplement federal research grants.

“That’s extremely important because it means you have a way to set money aside when times are good so that you are prepared when times are lean,” Chan says. “It means you’re not just at the mercy of the winds. There’s a fearlessness to pursue very difficult questions at Caltech, and that is due in part to the financial stability philanthropy helps provide.”

To learn more about how you can help researchers push the boundaries of quantum science and technology at Caltech, contact Janny Manasse, senior director of development for the Division of Chemistry and Chemical Engineering, at (626) 395-1530 or jmanasse@caltech.edu.

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