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A Quantum Universe

As a kid, Ruixing Zhang loved to look up at the night sky and stargaze, inspired by the vast wonders overhead and the beauty of nature. He followed that curiosity to become a physicist and now explores the smallest of physical systems, each a universe of its own.

Zhang, a condensed matter theorist, joined the faculty in August as an assistant professor. As an undergrad at the University of Science and Technology in China, the Dongying native was drawn to particle physics, but by the time he finished his bachelor’s degree in 2012 his motivation to understand the universe took a different direction.

"That’s when I started to appreciate the beauty of quantum materials," he said.

Each quantum material, he explained, can be thought of as a quantum universe with its own rules. (The electrons and quasiparticles inside it are more like stars and galaxies.)

While classical physics describes a system such as a ball falling from a balcony, that model doesn’t work in a microscopic system. Quantum mechanics defies the predictable when electrons and particles interact or behave in unexpected ways. Though atomic scale materials may seem mysterious, they’re actually part of our everyday experience.

"Quantum materials are basically everywhere," Zhang explained. "Without understanding them, we would not have our hard drives, our central processing units … they’re all based on the state-of-the-art understanding of these quantum materials."

Zhang is particularly interested in topological systems, whose properties remain unaltered even if a material is moderately disordered or deformed.

"For me there are two major motivations for understanding topological quantum materials," he said. "We want to understand them and see how they can actually better serve us and also give a boost to modern technology."

He explained that energy dissipation in electronic devices is one important example. If you’re watching a movie or working on your laptop, you’ll notice the device gets hotter. That heat comes from energy dissipation. Topological quantum materials can curtail those losses by providing electrons with a more orderly traffic pattern.

"The reason an electric current can generate heat is because these moving electrons are bouncing back and forth, getting scattered because of impurities in the materials," he said. "In some sense it’s like if you’re driving on a very crowded street, you can’t drive very fast."

He continued: "In some topological materials we can have one-dimensional electron channels where electrons can only move to the left, and in some part you can have some other channels where the electrons can only move to the right, like a divided highway. Everyone is moving in one direction. This is how you can promote energy efficiency if you’re making electronic devices with these materials."

This is one motivation for Zhang’s work with quantum materials. Another is their potential to help scientists realize anyons.

"We call them fractionalized quasi-particles," he explained. "An anyon can be made of a thousand electrons: it’s like a composite object. But the surprising thing is, it can only behave like a fraction of one single electron."

Anyons are interesting to physicists like Zhang in part because they’re exotic and also because they could be the foundation for topological qubits, which in turn can be used as building blocks for quantum computers.

"I think the most exciting aspect in quantum science is how we can make use of quantum materials to revolutionize our way of processing information," he said.

Zhang earned a PhD in physics at the Pennsylvania State University in 2018 and then joined the University of Maryland Joint Quantum Institute as a postdoctoral fellow. Next he came to UT as part of the university’s cluster hire program in Quantum Materials for Future Technologies and holds a joint appointment with the Department of Materials Science and Engineering.

"When I first saw the opening, I was super excited, because you were asking for exactly the person that I am," he said. The cluster combines expertise from multiple disciplines to build a dynamic research and development presence in quantum science, artificial intelligence, and their technological applications. Faculty from physics, computer science, and materials science and engineering all play a role.

"We all have partial overlaps with each other’s research interests, but we all have some different expertise and directions," Zhang said. "We’re creating links."

He’s already working with experimentalist Joon Sue Lee, assistant professor of physics, on higher-order topological materials and has been actively interacting with other faculty members in both departments to establish collaboration opportunities. He’s also excited about collaborating with the new cluster hires who’ll join the university in the coming months, including physicist Alan Tennant, who arrives in January. In the meantime, two students have joined his group and one postdoc is joining in mid-January 2022. Zhang enjoys mentoring young scientists, both in research and in the classroom.

Though he had a teaching release for his first semester, Zhang chose to postpone it so he could get a jump on classroom experience. He’ll teach Structure of Matter in the spring (a joint physics/engineering course) and started off last fall by teaching Electricity and Magnetism.

"I love teaching," he said. "You design the course and you see the students become excited about learning something new. (You) see that they make progress and improve their understanding—it’s really rewarding for me."

For Zhang, the most important elements of teaching are communication and direction.

"I learned a lot about teaching this course from my students," he said, laughing.

Once the term started he surveyed the class and let them vote on whether they were happy with his approach or would like to see some changes. They indicated they’d like to see more examples worked out, so he updated his format to include more handwritten examples in the classroom.

"That helped me understand better what the students need," he said.

Zhang’s, teaching, research, and even his hobbies have an underlying theme—appreciating the beauty of something and pursuing it joyfully. His outside-of-physics interests range from photography to badminton to singing. He’s also an avid cook—a skill he started developing when he moved to the US and missed dishes from home.

"If you start appreciating the beauty of cooking, then you can start to enjoy it," he said. "I’m good at appreciating the beauty of something."

From the wonder of the stars he watched growing up to the right combination of ingredients in the kitchen or a syllabus, seeing that beauty is key.

"Enjoying the research subject that I’m working on, or enjoying the life of being a physicist, that’s actually a very important thing," he said. "In research you will encounter a lot of difficulties (and) different problems. You need to have some motivation to keep you moving forward. For me, that motivation is to enjoy the beauty of the research subject I’m working on and also to enjoy the life of a physicist."