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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 can withstand extreme forces 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 in Arts & Sciences and a member of Washington University in St. Louis' 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 their in the 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. Guanghui He also spent six months at Harvard University working with physicist Norman Yao, another 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 previously had 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 also could 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.

"Now that we have these sensors, the high-pressure chamber and the diamond anvils, we'll have more opportunities for exploration," Zu said.