Scientists Detect Single Proton Spin with Diamond Sensor

Researchers have achieved a breakthrough in quantum sensing by detecting and controlling the spin of a single proton—the nucleus of a hydrogen atom—using a diamond-based sensor. This marks the first time scientists have observed the free precession of an individual proton spin, a fundamental advance that opens the door to atomic-scale imaging with applications ranging from materials science to quantum memory. The work, published in a recent study, demonstrates that quantum sensors can now resolve nuclear magnetic resonance (NMR) at the level of single spins, pushing the boundaries of what's possible in nanoscale measurement.

The key finding is the successful detection, characterization, and polarization of a single proton nuclear spin. The researchers observed its free precession—a wobbling motion similar to a spinning top slowing down—induced by a radio-frequency pulse. This observation is critical because free induction decay, the signal produced during precession, is the foundation of conventional high-resolution NMR, but until now, it hadn't been achieved for individual protons. The team also determined spatial coordinates of the proton, specifically the distance (r) and one angular parameter (θ), and noted that their setup is compatible with protocols to find the remaining azimuthal angle (φ), as shown in prior work by some of the authors.

Ology relied on a nitrogen-vacancy (NV) center in diamond, a defect in the crystal structure that acts as a highly sensitive quantum sensor. Think of the NV center as a tiny magnetic compass that can detect nearby nuclear spins. The researchers used radio-frequency pulses to manipulate the proton spin and observed its response through the NV center. This approach builds on established quantum sensing protocols previously tested mainly on carbon-13 nuclei in diamond, but here it was adapted for protons, which have a higher Larmor frequency—meaning they can be manipulated faster, like a faster-spinning top.

From the study, detailed in the paper, show that the proton spectrum revealed a fine structure, indicating detailed information about the spin's environment. The data support the detection of free induction decays, which are essential for NMR's power in chemical analysis. The researchers interpreted the detected protons as residing within the diamond, likely incorporated during chemical vapor deposition (CVD) growth. This interpretation suggests that protons in diamond could serve as built-in quantum memories, coupled with the NV center for faster operations compared to carbon-13 nuclei.

This advancement matters because it enables atomic-scale imaging, akin to taking a super-high-resolution picture of individual atoms. In practical terms, it could improve materials characterization—for instance, in developing better semiconductors or quantum computing components—by allowing scientists to map atomic positions with unprecedented precision. The compatibility with existing quantum protocols means this technique could be integrated into devices for quantum information processing, where fast, reliable spin control is crucial.

Limitations noted in the paper include that the full set of spatial parameters, such as the azimuthal angle, wasn't determined in this particular experiment, though the setup is designed to do so. Additionally, the protons were inferred to be in the diamond, but their exact origin and distribution require further investigation. The study focuses on proof-of-concept detection, leaving broader applications like imaging complex molecules or external samples for future work.