Harvard researcher unlocks potential of quantum technologies – Harvard Gazette

Xu’s physics background and experimental chemistry experience enable him to test quantum theories in the…

Xu’s physics background and experimental chemistry experience enable him to test quantum theories in the real world. “Even though physicists and chemists both study materials, physicists tend to look at them more as abstract equations, while chemists engage with their emergent properties,” Xu said. “Since I have a pure physics background and speak the language of chemistry, I can translate difficult theories into real space.”

With a few well-reasoned assumptions and some innovative techniques, Xu and his team bridge the gap the between quantum physics and chemistry, testing theories with materials. First, they predict which materials may realize topological properties. The chemical formulas for the elements in such materials do not provide adequate insight; Xu is also interested in their macroscopic properties.

“If I were to study water, steam, and ice only by looking at their H20 equation, I would learn nothing about their different properties.” Xu said. “As a chemist, I am trying to find certain elements and organize them microscopically, so that they can produce a topological property.”

Xu’s lab then tests current theories about chemical reactions against experimental data to expand the map of topological materials. Using specialized refrigerators in which atoms and molecules are cooled to temperatures just above absolute zero, at which they become highly controllable and more visible, Xu and his team test the flow of electrons through materials with currents.

They are also interested in the optical properties of materials, testing to see their interaction with light. The team fires photons at the materials and gathers quantum mechanical topological data based on how light scatters, reflects, and transmits. Xu has already yielded strong evidence for theoretical particles that answers one of the most vexing problems in quantum science.

In a study reported last year in Nature, Xu and his team set out to study the properties of axions, a theoretical elementary particle proposed by physicist Frank Wilczek. The Nobel Prize winner named it after a brand of laundry detergent because it “cleaned up” the complex, highly technical Strong Charge Parity problem in quantum chromodynamics by filling in a gap between theory and observation.

In addition one of the most enticing predictions about axion states is that we may be able to use them to control magnetization, which could revolutionize all kinds of technology as magnetism and magnetic materials are at the heart of many, many applications.

In a class of topological materials called axion insulators, Xu’s team sought to simulate the behavior of the axion. They fabricated a dual-gated MnBi2Te4 device in an argon environment, and measured its electrical and optical properties, uncovering new pathways to detect and manipulate the rich internal structure of topological materials.

“We discovered a real material that can support the axion insulator state,” Xu said. “We confirmed that it had the predicted properties, a strong coupling between electricity and magnetism.”

Having provided evidence for a theorized particle, Xu plans to explore the spin properties of Weyl semimetals, a new state of matter that has an unusual electronic structure that has deep analogies with particle physics and leads to unique topological properties.

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