Traditional nuclear magnetic resonance spectroscopy, familiar to many through its medical imaging counterpart—magnetic resonance imaging (MRI)—has revolutionized our ability to visualize internal structures non-invasively. Yet, the inherently limited resolution of conventional NMR has imposed a boundary on examining molecular structures at the atomic scale. Diagnostic MRI and standard NMR methods require large ensembles of atoms to produce signals, preventing scientists from observing single molecules or even individual atoms. This limitation has been an enduring obstacle in both fundamental research and applications demanding unparalleled precision, such as quantum computing components and highly sensitive molecular sensors.

Li’s team has capitalized on the unique qualities of 2D materials, which form crystalline sheets just atoms thick. Hexagonal boron nitride, specifically, exhibits a lattice structure of alternating boron and nitrogen atoms arranged in hexagonal rings and hosts naturally occurring vacancies—missing atoms that create localized sites capable of trapping electrons or nuclear spins. By introducing carbon-13 isotopes into these vacancies, the researchers transformed these sites into controllable spin defects. Unlike its most abundant isotope, carbon-12, carbon-13 has a nuclear spin that interacts with magnetic fields, enabling it to be directly probed by magnetic resonance techniques.

The process of embedding carbon-13 isotopes into hBN involved a sophisticated technique where carbon-13-enriched carbon dioxide gas was accelerated toward the hBN crystal using an electric field, causing some atoms to replace boron or nitrogen atoms in the lattice. These substitutions created a new class of spin defects that serve as sensitive probes of their atomic surroundings. By exploiting optically detected NMR, a method that couples nuclear magnetic resonance with optical readout through emitted photons, Li’s group succeeded in achieving single-spin detection. This approach allows the direct observation of the quantum state of a single nuclear spin in a material only a few atoms thick—a feat never accomplished before.

One of the hallmark achievements of this study is the ability to classify the newly discovered spin defects into three distinct groups based on their characteristic spectroscopic signals. Collaborating with theorist Yuan Ping from the University of Wisconsin-Madison, the team combined experimental observations with advanced computational modeling to identify the specific atomic structures corresponding to two of these groups. These insights are crucial because understanding the precise defect geometry is fundamental for reproducible quantum device engineering and for tuning the coherence properties of spin qubits.

Coherence time, or how long a quantum state remains unperturbed, is a critical parameter for quantum technologies. Remarkably, the carbon-13 spin defects in hBN demonstrated long coherence times even at room temperature, an attribute that positions these defects as promising quantum memories. Quantum memories are the backbone of many quantum computing and communication schemes, storing quantum information reliably during processing and transmission. The discovery that these nuclear spins maintain coherence without requiring cryogenic cooling represents a major leap toward practical quantum devices operating under ambient conditions.

The implications of this advancement extend beyond the realm of quantum computing. Magnetic resonance microscopy enhanced with atom-scale resolution can revolutionize molecular analysis by enabling the direct detection and structural characterization of individual biological molecules. This capability opens up possibilities for unprecedented insight into protein folding, enzyme mechanisms, and pharmacological interactions at the ultimate level of detail, potentially transforming drug discovery and molecular diagnostics.

Historically, Li’s research group has pursued using the electron spins in boron vacancies within hBN as quantum sensors. While these electron spins emitted light to signal local magnetic environments, their optical emission was too weak for single-defect resolution. The pivot toward carbon-13 nuclear spins represents a strategic evolution, overcoming the sensitivity barrier by directly targeting nuclear rather than electronic spins, which are less prone to environmental noise and thus capable of longer coherence times.

The sophisticated interplay between nuclear and electron spins in these 2D materials allows for precise manipulation and readout of quantum states using combinations of magnetic resonance and optical techniques. Optically detected nuclear magnetic resonance uniquely enables this control by using laser excitation to polarize and detect nuclear spins indirectly via changes in emitted light, thus merging the strengths of optical measurement with the intrinsic information contained in nuclear spins.

This research was enabled by meticulous experimental craftsmanship combined with theoretical expertise, supported by funding from the Gordon and Betty Moore Foundation, the U.S. National Science Foundation, and the Department of Energy. By revealing the pathways to harness individual nuclear spins in scalable and accessible materials like hexagonal boron nitride, the study marks a transformative moment in quantum science, where the manipulation of matter at the smallest scales can translate into revolutionary technologies.

Looking forward, the ability to deterministically place and control carbon-13 spin defects promises the creation of quantum sensors of unparalleled sensitivity and spatial resolution. These detectors could transform a wide array of scientific disciplines, ranging from nanoscale magnetic resonance imaging to quantum-enhanced biological sensing. Moreover, the research enriches the toolbox for engineering novel qubits in 2D materials, essential for developing scalable quantum networks that integrate with existing semiconductor technology.

In summary, Purdue University’s recent demonstration of single nuclear spin detection and precise control in hexagonal boron nitride heralds a new era of quantum sensing and molecular microscopy. By weaving together the subtle intricacies of materials science, quantum physics, and cutting-edge spectroscopy, this work not only solves a longstanding challenge in NMR spectroscopy but also lays the groundwork for breakthroughs in quantum computing, communications, and biomedical research—ushering us ever closer to the long-envisioned realm of atomic-scale exploration and manipulation.

Subject of Research: Not applicable

Article Title: Single nuclear spin detection and control in a van der Waals material

News Publication Date: 9-Jul-2025

Web References:

• Article in Nature: https://www.nature.com/articles/s41586-025-09258-7
• Purdue Physics Faculty: https://www.physics.purdue.edu/people/faculty/tcli.php
• Purdue Quantum Science and Engineering Institute: https://quantum.research.purdue.edu/
• Purdue National Science Foundation’s Quantum Technologies Center: https://www.purdue.edu/cqt/
• Purdue Strategic Initiatives: https://www.purdue.edu/president/strategic-initiatives

References:
Li, T., et al. “Single nuclear spin detection and control in a van der Waals material.” Nature (2025). DOI: 10.1038/s41586-025-09258-7

Image Credits: Purdue University photo/Charles Jischke

Keywords:

• NMR spectroscopy
• Quantum information
• Qubits

At the heart of this advance is the concept of exceptional points—singularities in non-Hermitian systems where two or more eigenvalues and their corresponding eigenvectors coalesce. Unlike ordinary degeneracies, exceptional points arise due to the presence of gain, loss, or non-reciprocity, giving rise to unique physical phenomena, including unidirectional invisibility, enhanced sensitivity, and anomalous dispersion. While exceptional points have been explored extensively in optics, implementing them within scalable, CMOS-compatible platforms has remained a challenge due to the necessity of precisely balancing system parameters.

The researchers tackled these hurdles by leveraging all-dielectric metasurfaces fabricated directly on-chip. Metasurfaces, ultrathin arrays of subwavelength resonators, have revolutionized photonics by allowing deterministic control over phase, amplitude, and polarization of light. However, embedding topological features within such metasurfaces elevates their functionality beyond mere wavefront shaping. All-dielectric designs circumvent the losses inherent in plasmonic or metallic counterparts, enabling high Q-factors and strong light confinement indispensable for maintaining coherent interactions necessary for topological phenomena.

In their experimental setup, the team engineered the metasurface to exhibit carefully tailored anisotropic resonant modes, resulting in non-Hermitian coupling conditions conducive to forming exceptional points. By manipulating geometrical parameters and refractive indices, the metasurface’s band structure was tuned to achieve a precise degeneracy, leading to the emergence of a topological exceptional point. This intricate interplay between geometry and material dispersion highlights the nuanced control achievable through state-of-the-art nanofabrication techniques.

One of the defining features of this work is the demonstration that such exceptional points possess robust topological characteristics, protected against certain perturbations and disorder. This robustness is crucial for practical device applications, where environmental fluctuations and fabrication imperfections typically degrade system performance. The topological protection ensures that the unique optical properties associated with the exceptional point remain stable, opening pathways for reliable on-chip devices harnessing non-Hermitian physics.

Furthermore, the researchers meticulously characterized the device’s response through a combination of near-field imaging and far-field spectroscopy, revealing hallmark signatures of the exceptional point. Observable phenomena included asymmetric mode switching and enhanced sensors’ responsivity, directly attributable to the non-trivial topology of the system’s eigenmodes. Such experimental validation underpins the theoretical predictions and confirms the feasibility of integrating these metasurfaces into complex photonic circuits.

The implications of creating topological exceptional points on-chip extend across multiple disciplines. For instance, in optical sensing, the enhanced sensitivity near exceptional points can lead to devices capable of detecting minute changes in environmental parameters such as refractive index or temperature with unprecedented precision. Additionally, the capability to engineer unidirectional light propagation and modal selectivity is a boon for optical isolators and circulators vital in photonic networks and quantum communication.

Moreover, this advancement dovetails with burgeoning interest in non-Hermitian topological photonics, where gain and loss are harnessed as resources rather than detriments. The all-dielectric metasurface platform offers an experimentally accessible and scalable means to probe complex physical phenomena such as parity-time symmetry breaking, topological lasers, and exceptional rings. By embedding these functionalities on-chip, the technology promises compact, integrable solutions for next-generation photonic systems.

Importantly, the design principles elucidated in this study set a precedent for future explorations into active metasurfaces. By incorporating tunable elements or nonlinear materials, it would be possible to dynamically modulate exceptional points, enabling reconfigurable topological devices responsive to external stimuli. Such adaptability would revolutionize optical computing architectures, allowing for real-time control of light propagation and enhanced information processing capabilities.

From a fabrication standpoint, the demonstrated approach capitalizes on mature silicon photonics processes, ensuring compatibility with existing semiconductor manufacturing infrastructure. This compatibility greatly facilitates the transition of topological exceptional point-based devices from laboratory curiosity to deployable technology. The all-dielectric metasurface’s low-loss and high-damage threshold characteristics further cement its suitability for practical applications requiring long-term stability and high power handling.

In the broader context of physics, this work bridges the gap between abstract mathematical concepts of non-Hermitian topology and tangible physical implementations. The realization of exceptional points in an all-dielectric metasurface platform not only adds a new dimension to photonics but also enriches the understanding of wave dynamics in complex media. It establishes a concrete example of how topology and non-Hermitian physics converge to produce novel functionalities inaccessible to conventional systems.

Critically, the team’s results also stimulate discussion on potential new device paradigms. The unique mode coalescence at exceptional points could inspire novel laser designs with tailored emission properties or sensors with tunable detection thresholds. Additionally, integrating these metasurfaces with other photonic elements, such as waveguides or resonators, could yield hybrid systems capitalizing on the synergy between topology and traditional photonic components.

As research in this field progresses, the principles demonstrated here might unlock pathways toward topological quantum photonics, where quantum states of light are manipulated through non-trivial topological structures. Exceptional points may serve as critical nodes for enhanced light–matter interaction or robust entanglement generation, advancing quantum technologies’ scalability and resilience.

In conclusion, the creation of a topological exceptional point via an on-chip all-dielectric metasurface represents a landmark achievement, merging the frontiers of nanofabrication, photonics, and topological physics. This innovation not only deepens fundamental understanding but also drives technological development toward integrated photonic devices with unprecedented control over light behavior. As these findings disseminate across the scientific community, a new wave of topological photonic devices is anticipated to reshape our interaction with light in the foreseeable future.

Subject of Research: Creating topological exceptional points using all-dielectric metasurfaces integrated on-chip.

Article Title: Creating topological exceptional point by on-chip all-dielectric metasurface.

Article References:
Yi, C., Wang, Z., Shi, Y. et al. Creating topological exceptional point by on-chip all-dielectric metasurface. Light Sci Appl 14, 262 (2025). https://doi.org/10.1038/s41377-025-01955-2

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41377-025-01955-2