A significant breakthrough in levitated optomechanics has been achieved as a team of scientists led by Prof. Tongcang Li discovered the Berry phase of electron spins in tiny diamonds that are floated in a vacuum.
A significant breakthrough in levitated optomechanics has been achieved as Prof. Tongcang Li’s team observed the Berry phase of electron spins in tiny diamonds that are floated in a vacuum.
Researchers at Purdue University are hosting the smallest disco in the world. At the center of this event is a fluorescent nanodiamond that they have levitated and spun at astonishing speeds. As the diamond spins, it scatters colorful lights in various directions. This “party” allows them to investigate how rapid rotation impacts the spin qubits within the diamond, leading to the observation of the Berry phase. The findings were published in Nature Communications and acclaimed by reviewers as a “potentially groundbreaking moment for the exploration of rotating quantum systems and levitodynamics,” marking a new achievement for the levitated optomechanics field.
“Think of tiny diamonds hovering in vacuum. Encased within these diamonds are spin qubits that can enable scientists to take precise measurements and delve into the intriguing link between quantum mechanics and gravity,” explains Li, who is also part of the Purdue Quantum Science and Engineering Institute. “Previously, research with these floating diamonds struggled to retain them in a vacuum and read the spin qubits. In our study, we successfully floated a diamond in a high vacuum using a unique ion trap. This is the first time we have been able to observe and manage the behavior of spin qubits in a levitated diamond within high vacuum conditions.”
The team managed to make the diamonds spin at astonishing speeds of up to 1.2 billion rotations per minute! This rapid movement allowed them to uniquely observe how the rotation impacted the spin qubits, specifically in an effect known as the Berry phase.
“This discovery enhances our understanding and exploration of the captivating realm of quantum physics,” he states.
These fluorescent nanodiamonds, which are on average about 750 nanometers in diameter, were created through a high-pressure, high-temperature process. They were treated with high-energy electrons to produce nitrogen-vacancy centers, which are essential for electron spin qubits. When a green laser illuminated the diamonds, they emitted red light used to read their electron spin states. An additional infrared laser was directed at the levitated nanodiamond to track its rotation. As the nanodiamond spun like a disco ball, the direction of the scattered infrared light shifted, conveying information about its rotation.
The authors of this study predominantly hail from Purdue University and are part of Li’s research group: Yuanbin Jin (postdoc), Kunhong Shen (PhD student), Xingyu Gao (PhD student), and Peng Ju (recent PhD graduate). Li, Jin, Shen, and Ju conceptualized and designed the project, with Jin and Shen constructing the experimental setup. Jin performed measurements and calculations while the entire team engaged in discussions about the findings. Two external contributors are Alejandro Grine, a principal member of the technical staff at Sandia National Laboratories, and Chong Zu, an assistant professor at Washington University in St. Louis. Li’s team collaborated with Grine and Zu to refine the experiment and the manuscript.
“For designing our integrated surface ion trap,” Jin clarifies, “we utilized commercial software, COMSOL Multiphysics, to conduct 3D simulations. We evaluated the trapping position and microwave transmittance with various parameters to enhance the design. Additional electrodes were integrated for better control of the levitated diamond’s motion. For the fabrication, the surface ion trap was made on a sapphire wafer through photolithography. A 300-nm-thick layer of gold was then added to form the trap’s electrodes on the sapphire surface.”
Are the diamonds spinning in specific directions and is it possible to manipulate their speed or orientation? Shen confirms that adjustments can indeed be made to the spin direction and levitation.
“We can modify the driving voltage to change the direction of rotation,” he elaborates. “The levitated diamond can rotate around the z-axis (which is perpendicular to the ion trap’s surface), as shown in the schematic, either clockwise or counterclockwise depending on the signal we provide. Without applying a driving signal, the diamond rotates in all directions, similar to a ball of yarn.”
Levitated nanodiamonds containing spin qubits have potential applications for precision measurements and creating large quantum superpositions to explore the boundaries of quantum mechanics and the quantum nature of gravity.
“General relativity and quantum mechanics are two pivotal scientific advancements of the 20th century. Yet, the quantization of gravity remains an enigma,” Li remarks. “Being able to study quantum gravity experimentally would represent a significant leap forward. Furthermore, rotating diamonds with embedded spin qubits present an opportunity to investigate the interplay between mechanical motion and quantum spins.”
This discovery could lead to wide-ranging industrial applications. Li mentions that levitated micro and nano particles in vacuum have excellent potential as accelerometers and electric field sensors. For instance, the US Air Force Research Laboratory (AFRL) is utilizing optically levitated nanoparticles to address critical navigation and communication challenges.
“Here at Purdue University, we have cutting-edge facilities dedicated to our research in levitated optomechanics,” Li points out. “We operate two specialized, custom-built systems for this research area, and we also benefit from shared resources at the Birck Nanotechnology Center that allow us to fabricate and analyze the integrated surface ion trap right on campus. We’re fortunate to have skilled students and postdocs engaged in pioneering research. Additionally, my group has been exploring this field for a decade, and our vast experience has propelled our progress.”
This research was funded by the National Science Foundation (grant number PHY-2110591), the Office of Naval Research (grant number N00014-18-1-2371), and the Gordon and Betty Moore Foundation (grant DOI 10.37807/gbmf12259). The project also received partial support from the Laboratory Directed Research and Development program at Sandia National Laboratories.
