Engineers have developed an innovative version of nuclear quadrupolar resonance (NQR) spectroscopy using quantum sensors. This advanced technique, historically employed for identifying drugs and explosives or examining pharmaceuticals, now boasts the ability to detect NQR signals from individual atoms—something previously deemed impossible. This exceptional sensitivity paves the way for advances in areas like drug development, where understanding atomic-level molecular interactions is essential.
Since the 1950s, researchers have harnessed radio waves to uncover the unique “fingerprints” of materials, aiding various applications from MRI scans in medicine to security checks for explosives at airports.
Previously, these techniques relied on signals averaged from massive numbers of atoms, which prevented the detection of subtle differences among individual molecules. This limitation has posed challenges in areas such as protein research, where minute variations in structure can influence functionality and impact health outcomes.
Sub-Atomic Insights
Engineers from the University of Pennsylvania’s School of Engineering and Applied Science (Penn Engineering) have now engineered a revolutionary variation of NQR spectroscopy that employs quantum sensors.
According to a report in Nano Letters, this new technique offers such incredible precision that it is capable of discriminating NQR signals from single atoms, an accomplishment once believed to be beyond reach. This groundbreaking sensitivity opens new avenues for advancements in drug development, where insights into molecular interactions at the atomic scale are crucial.
“This technique enables us to distinguish individual nuclei and discover minute differences between what were previously considered identical molecules,” explains Lee Bassett, an Associate Professor in Electrical and Systems Engineering (ESE) and the senior author of the paper. “By targeting a single nucleus, we can unveil details about molecular structure and behavior that were previously obscured. This ability allows us to explore the fundamental building blocks of nature from a completely new perspective.”
An Unexpected Discovery
The groundbreaking discovery arose from an unanticipated finding during routine experiments. Alex Breitweiser, a recent Ph.D. graduate in Physics at Penn’s School of Arts & Sciences and co-first author of the paper, was investigating nitrogen-vacancy (NV) centers in diamonds—atomic-scale defects generally used in quantum sensing—when he observed some unusual data patterns.
Though the periodic signals initially appeared to be mere experimental noise, they persisted through extensive troubleshooting. Delving into nuclear magnetic resonance literature from the 1950s and 1960s, Breitweiser eventually identified a physical mechanism that could explain their observations, even though it had been deemed insignificant in previous experiments.
Thanks to technological advances, the team could measure effects once thought to be beyond the capabilities of scientific instruments. “We soon realized we were not merely observing an anomaly,” Breitweiser states. “We were tapping into a new realm of physics that this technology allows us to access.”
Unprecedented Precision
The team’s understanding of this effect was further refined through collaboration with researchers at Delft University of Technology in the Netherlands, where Breitweiser previously conducted research as part of an international fellowship. By blending expertise in experimental physics, quantum sensing, and theoretical modeling, the team devised a method to capture single atomic signals with remarkable accuracy.
“It’s akin to isolating a single row from a vast spreadsheet,” says Mathieu Ouellet, another recent ESE doctoral graduate and co-first author of the paper. “Traditional NQR methods yield an average, providing a general sense of the data but revealing nothing about individual data points. This new method allows us to extract all the data behind the averages, isolating the signal from one nucleus and uncovering its distinct properties.”
Deciphering the Signals
Unraveling the theoretical basis behind these surprising experimental results demanded substantial effort. Ouellet meticulously tested various hypotheses, running simulations and performing calculations to align the data with possible explanations. “Diagnosing this was somewhat akin to treating a patient based on their symptoms,” he elaborates. “The data indicated something unusual, but multiple interpretations often exist. It took considerable time to arrive at the right conclusion.”
Looking forward, the researchers envision their method addressing significant scientific challenges. By revealing previously hidden phenomena, this new approach could enhance our understanding of the molecular mechanisms that govern our world.
This research was conducted at the University of Pennsylvania School of Engineering and Applied Science, supported by the National Science Foundation (ECCS-1842655, DMR-2019444). Additional backing was provided by the Natural Sciences and Engineering Research Council of Canada through a Ph.D. Fellowship awarded to Ouellet, and from IBM through a Ph.D. Fellowship for Breitweiser.
Additional co-authors include Tzu-Yung Huang, a former doctoral student in ESE at Penn Engineering now with Nokia Bell Labs, and Tim H. Taminiau from Delft University of Technology.
