With NSF support, WashU physicists create quantum sensors that track stress and magnetism at pressures exceeding 30,000 times Earth’s atmosphere.
The world of quantum physics is already mysterious, but what happens when that strange realm of subatomic particles is put under immense pressure? Observing quantum effects under pressure has proven difficult for a simple reason: Designing sensors that withstand extreme force is challenging.
In a significant advance, a team led by physicists at WashU has created quantum sensors in an unbreakable sheet of crystallized boron nitride. The sensors can measure stress and magnetism in materials under pressure that exceeds 30,000 times the pressure of the atmosphere. “We’re the first ones to develop this sort of high-pressure sensor,” said Chong Zu, an assistant professor of physics and a member of WashU’s Center for Quantum Leaps. “It could have a wide range of applications in fields ranging from quantum technology, material science, to astronomy and geology.”
The team described the breakthrough in the prestigious journal Nature Communications. The paper’s co-authors are graduate students in Zu’s lab, including Guanghui He, Ruotian “Reginald” Gong, Zhongyuan Liu, and Changyu Yao; graduate student Zack Rehfuss; postdoctoral researcher Mingfeng Chen; and Xi Wang and Sheng Ran, both assistant professors of physics.
The research was supported in part by a National Science Foundation Research Traineeship (NRT) grant, which allowed He to spend six months at Harvard working with physicist Norman Yao, also a co-author.
To create the sensors, the team used neutron radiation beams to knock boron atoms out of the thin sheets of boron nitride. The vacancies can immediately trap electrons. Because of quantum-level interactions, the electrons change their spin energies depending on magnetism, stress, temperature, and other qualities of nearby materials. Tracking the spin of each electron provides deep quantum-level insights into whatever material is being studied.
Zu and colleagues had previously created quantum sensors by making vacancies in diamonds, which power WashU’s two quantum diamond microscopes. While effective, diamond sensors have a drawback: Because diamonds are three-dimensional, it’s hard to place the sensors close to the material being studied.
In contrast, sheets of boron nitride can be less than 100 nanometers thick — about 1,000 times thinner than a human hair. “Because the sensors are in a material that’s essentially two-dimensional, there’s less than a nanometer (a billionth of a meter) between the sensor and the material that it’s measuring,” Zu said.
Diamonds still play an important role. “To measure materials under high pressure, we need to put the material on a platform that won’t break,” He explained. Diamonds, the hardest substance in nature, serve this purpose. He and other members of the Zu lab created “diamond anvils” — two flat diamond surfaces, each about 400 micrometers wide, roughly the width of four dust particles — that squeeze together in a high-pressure chamber. “The easiest way to create high pressure is to apply great force over a small surface,” He explained.
Tests showed that the new sensors could detect subtle shifts in the magnetic field of a two-dimensional magnet. Next, the researchers plan to test other materials, including specimens of rocks like those found in the high-pressure environment of the Earth’s core. “Measuring how these rocks respond to pressure could help us better understand earthquakes and other large-scale events,” Zu said.
The sensors could also advance research on superconductivity, the ability to conduct electricity without resistance. Currently, known superconductors require extremely high pressure and low temperatures. Previous claims that some materials can act as superconductors at room temperature have proven to be highly controversial. “With this sort of sensor, we can collect the necessary data to end the debate,” said Gong, who, along with He, was co-first author of the paper.
The new sensors underscore the value of the NSF NRT training grant, Zu said. “The program encourages collaboration between universities,” he said. “Now that we have these sensors, the high-pressure chamber, and the diamond anvils, we’ll have more opportunities for exploration.”
Header image: Schematic image of a 2D sensor squeezed between two diamond anvils. (Image courtesy of Chong Zu)
