Pushing the limits of quantum technology with spin qubits and levitated particles

Shattering world records and proving the seemly impossible to be possible isn’t an occasional happenstance with Tongcang Li, professor at Purdue University; it’s part of his daily routine. Over a century ago, Albert Einstein said it would never be possible to observe the instantaneous velocity of particles undergoing Brownian motion due to collisions with surrounding molecules. Over a decade ago, Li helped prove Einstein wrong about that. In 2018, Li’s research team shattered the records for the fastest spinning object when his team created the fastest nanorotor in the world with an optically levitated nano-dumbbell. Science that could easily be confused with magic is daily practice in Li’s lab where levitation, qubits, and spintronics enable him to delve into fundamental physics and to advance quantum technology for a wide range of applications.

Li, a professor in both the Department of Physics and Astronomy and Elmore Family School of Electrical and Computer Engineering, has his roots in many aspects of physics and engineering. This ability to wade into multiple scientific areas allows his research to canvas a growing scientific territory.

Li has been recognized by multiple awards, including the Experimental Physics Investigator Award from the Gordon and Betty Moore Foundation in 2023 and the CAREER award from the U.S. National Science Foundation in 2016. In addition, his work has been featured in Highlights of the Year of 2018 by the American Physical Society and Optics in 2022 by the Optics & Photonics News.

“My group studies the quantum interaction between light and matter at atomic and nanoscales for quantum sensing and quantum information processing,” says Li. “For example, we are exploring spin qubits in 2D materials, levitated optomechanics, quantum biological and chemical sensing, and quantum vacuum fluctuations.”

Quantum technology relies on the qubit which is the quantum counterpart of a classical computer bit. In electron or nuclear spin qubits, the classical binary states "0" and "1" are represented by spin, a characteristic akin to magnetic polarity, making it sensitive to electromagnetic fields. This allows spin qubits to act as sensors, capable of probing structures like proteins or measuring temperature with nanoscale precision.

“Recently, we have established an interface between photons and nuclear spins in ultrathin hexagonal boron nitrides. We are also using spin qubits in 2D materials to study biological molecules,” he explains. “In another study, we utilized light to levitate nanoparticles in a vacuum. In 2018, we successfully levitated and driven a nano-dumbbell to rotate at over 60 billion rpm, setting a world record for the fastest human-made rotor at that time. By employing a levitated nanoparticle, we also developed the world's most sensitive torque detector. More recently, we levitated a nano-dumbbell near a surface to investigate particle-surface interactions. This ultrasensitive device holds the potential to explore quantum vacuum fluctuations and gravity at the nanoscale.”

Li’s research has several offshoots heading into seemingly unrelated branches of physics and engineering. Because he has explored many of these branches in his educational path, he is able to bring them together seamlessly.

“As a high school student, I was inspired by my science teachers and renowned scientists like Isaac Newton and Albert Einstein to pursue a career in physics. When I became an undergraduate student, I realized that there are now many different branches in physics,” he says. “I specialized in condensed matter physics during my undergraduate studies before transitioning to atomic, molecular, and optical physics (AMO) for my graduate research. After earning my PhD, I joined a research group in the Mechanical Engineering department at UC Berkeley. Now at Purdue University, I am dedicated to exploring new frontiers in physics and pushing the boundaries of knowledge. It's especially thrilling when we set world records or discover new phenomena.”

Setting records is a skill that Li is especially adept at mastering. He shows zero signs of stopping when it comes to setting records and has his eye on smashing even more of them with his students at Purdue.

“My proudest work so far is measuring the instantaneous velocity of the Brownian motion of a suspended particle for the first time, a task deemed impossible by Albert Einstein over one hundred years ago,” he says. “This work was published in Science magazine in 2010. At Purdue University, we created the fastest rotor in the world in 2018 with an optically levitated nano-dumbbell. This work was featured as one of the ten Highlights of the Year in 2018 by the American Physical Society. We are hopeful that our students at Purdue will generate more important results in the future.”

Li’s lab, the Quantum Sensing and Quantum Optomechanics Laboratory, is located in the Physics Building at Purdue’s West Lafayette campus. Because he works in both physics and engineering, he has collaborated with many fellow faculty members, including Yong Chen (Physics), Francis Robicheaux (Physics), Yulia Pushkar (Physics), Zubin Jacob (ECE), Sunil A. Bhave (ECE), Pramey Upadhyaya (ECE), and Vladimir M. Shalaev (ECE).

“Purdue University is famous for world-class research. I am thankful to our colleagues who are so supportive. Our research involves both fundamental physics and engineering applications,” says Li. “During our research, we aim to understand the physical principles underlying the phenomena we observed and apply them to practical uses. For example, when studying spin qubits in 2D materials, we want to understand the atomic structures of spin defects, which involves physics, and use spin defects to build quantum sensors for broader applications, which involves engineering. Our research group includes students from both Physics and ECE. They work closely and have produced many high-impact results through collaborations.”

As important as research is to Li, teaching is also something he holds as equal in value. Because Purdue is a Land Grant university, research and teaching are fundamentally tied together.

“One of the most rewarding aspects of being a professor is witnessing the growth of my students. I take great pride in their achievements, as evidenced by the numerous awards they have received,” he says. “Notably, one of my PhD students, Zhujing Xu, was awarded the Harvard Quantum Initiative Postdoctoral Fellowship from Harvard University. Additionally, she was selected as one of only seven trailblazing young researchers to present at the Trailblazers Symposium at the California Institute of Technology in 2023, highlighting her exceptional contributions to the field.”

Li has advised about ten graduate students and ten undergraduate students. He encourages students who might be interested in his research to contact him by email or visit his lab’s website. He hopes that students who are passionate about research will reach out.

Li’s path to record-setting research started in a small village in China where he grew up. He received his bachelor's degree from the University of Science and Technology of China and subsequently his PhD degree from the University of Texas at Austin. He then pursued his post-doctoral research at the University of California, Berkeley. Then in 2014, he joined the faculty at Purdue University.

“I have experienced several dramatic changes in my life. When I was a small kid over 30 years ago, our house and the neighborhood did not have electricity for most of time. So, we had to use oil lamps and candles for illumination during the nights,” says Li. “Now my group is using advanced lasers and single photon emitters to perform state-of-the-art research. I am grateful to so many people who have helped me and would like to help my students to be successful.”