In a recent breakthrough spearheaded by researchers from the University of California, Santa Barbara under the leadership of Professor Chris Van de Walle, a novel silicon-based qubit platform has been theoretically revealed. This new defect center, known as the carbon-nitrogen (CN) complex, offers compelling advantages over previously studied centers. The findings are detailed in a forthcoming publication in the prestigious journal Physical Review B, marking a significant advancement in the quest for silicon-compatible quantum emitters. The research leverages high-fidelity first-principles computational methods to elucidate the atomic-scale structure and quantum properties of the CN center, providing a roadmap for experimental realization.

Defect centers in crystals have a celebrated history in quantum science, notably the nitrogen-vacancy (NV) center in diamond which serves as an archetypal solid-state qubit. NV centers enable coherent electron spin manipulation alongside the emission of single photons, facilitating applications in quantum sensing and communication. The paradigm extended to silicon has incorporated the so-called T center, a defect formed by carbon and hydrogen atoms. The T center stands out for its ability to emit light in the telecom wavelength band, crucial for fiber-optic quantum communication, and for demonstrating long spin coherence times competitive with NV centers. However, the inclusion of hydrogen atoms inherently injects instability, given hydrogen’s mobility and sensitivity during semiconductor fabrication processes, thereby complicating reproducibility and device scalability.

Addressing this crucial limitation, the CN center replaces hydrogen with nitrogen, thereby forming a more structurally stable and chemically robust defect complex within the silicon lattice. According to lead postdoctoral researcher Kevin Nangoi, the absence of hydrogen means that the CN center is less susceptible to the migration and diffusion phenomena that plague hydrogen-containing defects. This stability enhances its viability as a reliable quantum emitter for device integration, overcoming a major hurdle in silicon-based quantum photonics. The team’s computational exploration confirms that the CN center preserves essential electronic and optical characteristics akin to the T center, producing photon emission precisely within the technologically vital telecom window.

The research utilized state-of-the-art ab initio simulations based on density functional theory (DFT) combined with many-body perturbation techniques to capture the defect’s electronic structure and excited-state properties. Such computational tools empower scientists to predict material behavior from first principles—without recourse to prior experimental data—thus accelerating discovery by guiding synthetic strategies. Mark Turiansky, a former member of the group now affiliated with the U.S. Naval Research Laboratory, emphasized the significance of the CN center’s structural resilience and telecom emission profile, underscoring its suitability for quantum information processing and photonic network devices.

The implications of identifying a hydrogen-free quantum-light emitter embedded in silicon stretch far beyond academic curiosity. By leveraging silicon’s well-established fabrication ecosystems, the CN center could catalyze the development of scalable quantum communication infrastructure, quantum repeaters, and integrated quantum photonic circuits. Unlike diamond or other exotic materials, silicon’s compatibility with existing CMOS processes holds the promise of mass production and functional quantum devices seamlessly integrated with classical electronics. This synergy is critical for achieving practical quantum advantage and transitioning quantum systems from laboratory curiosities to commercial technologies.

Moreover, the telecom wavelength emission characteristic of the CN center is particularly advantageous. Telecom bands experience minimal attenuation in optical fibers, enabling photons to travel long distances with negligible loss—a prerequisite for building quantum networks spanning metropolitan, continental, or even global scales. The CN center’s ability to generate such photons on-demand within a stable and controllable silicon matrix addresses a long-standing bottleneck in realizing fiber-based quantum communication systems, potentially reshaping secure communication paradigms.

Beyond communication, the CN center’s coherent spin states coupled with its optical addressability position it as a versatile qubit candidate for quantum sensing. Precision measurements of magnetic and electric fields, temperature, and strain at the nanoscale utilize the quantum coherence properties of defect centers to surpass classical sensor limits. The enhanced stability imparted by nitrogen substitution could ensure consistent performance across diverse environmental conditions and device cycles.

Although the CN center’s theoretical promise is compelling, experimental verification remains a crucial next step. Fabricating and characterizing this defect at atomic precision will require refined doping and annealing protocols, supported by advanced spectroscopy and microscopy techniques. The theoretical predictions serve as a vital compass directing these experimental efforts, optimizing conditions to realize the CN center reproducibly and harness its quantum functionalities effectively in silicon photonic architectures.

This breakthrough research exemplifies the power of integrating computational materials science with quantum technology development, illustrating how predictive modeling can pioneer solutions to long-standing material challenges. Supported by the U.S. Department of Energy’s Office of Basic Energy Sciences through the Co-design Center for Quantum Advantage, and leveraging computational resources at the National Energy Research Scientific Computing Center, this work exemplifies cooperative interdisciplinary innovation.

Looking forward, the realization of the CN defect center in silicon could unlock a host of quantum devices that benefit from both the extraordinary physics of quantum information science and the practical advantages of silicon technology. The potential to accelerate the deployment of quantum communication networks, quantum processors, and quantum sensors harnessing a stable and manufacturable silicon qubit is an inspiring milestone on the path toward the quantum age.

In summary, the identification of the carbon-nitrogen complex as an alternative to the hydrogen-dependent T center in silicon marks a pivotal advance in quantum material research. It blends atomic-scale precision, advanced computational modeling, and strategic materials engineering to push the boundaries of what silicon quantum technology can achieve. This innovation holds promise not only for fundamental quantum science but also for scalable quantum technology ecosystems compatible with today’s semiconductor manufacturing infrastructure, potentially transforming the quantum landscape for decades to come.

Subject of Research: Quantum defect centers in silicon for quantum information technologies

Article Title: Carbon-nitrogen complex as an alternative to the 𝑇 center in Si

News Publication Date: 10-Feb-2026

Web References: [Physical Review B publication DOI: 10.1103/zy5b-fskh]

Keywords: Quantum information, Materials engineering, Quantum computing, Qubits

At the heart of this scientific advancement is the concept of Fano interference, a quantum mechanical phenomenon first described in the context of atomic physics, which emerges from the interaction between a discrete quantum state and a continuum of states. Applied to photon pairs, this interference pattern offers a rich terrain of quantum correlations and coherence effects that can be actively modulated in practical photonic systems. The innovative aspect of this research lies in harnessing these quantum interferences within a planar metamaterial, thereby transcending conventional limitations imposed by bulk optics and conventional nonlinear crystals.

The metasurface employed in this study is a sophisticated two-dimensional array of subwavelength nanoantennas, engineered with extreme precision to tailor light-matter interaction at the quantum scale. These nanoantennas function as resonant scatterers, supporting localized plasmonic modes that couple strongly with incident electromagnetic fields. When photon pairs, generated through spontaneous parametric down-conversion (SPDC) or other nonlinear optical processes, interact with this structured environment, their quantum states experience modulation leading to distinctive Fano interference patterns. This marks a paradigm shift in the ability to design quantum light sources with enhanced functional attributes.

Fundamentally, the phenomenon relies on balancing the discrete resonances of the metasurface’s nanoantennas with the broad continuum of photonic modes, resulting in asymmetric interference line shapes that are highly sensitive to environmental parameters and device geometry. The researchers meticulously characterized the spectral and quantum properties of the emitted photon pairs, employing advanced coincidence counting techniques and Hong-Ou-Mandel interferometry to verify the presence and tunability of the Fano resonances. These experimental validations underscore the robustness and reproducibility of the interference effects in realistic device architectures.

The implications of achieving controlled Fano interference in photon pairs are multifold. Quantum coherence and entanglement properties intrinsic to the photon pairs can be fine-tuned, allowing for enhanced control over quantum state preparation and measurement. This tunability is crucial for the development of scalable quantum networks, where the efficient routing and manipulation of quantum information carriers dictate the overall system performance. By integrating metasurfaces into chip-scale quantum photonic circuits, the study effectively paves the way for ultra-compact, versatile platforms capable of quantum state engineering on demand.

Moreover, the research highlights the adaptability of metasurfaces in tailoring energy transfer processes at the quantum level. Their planar nature enables seamless integration with existing semiconductor and photonic technologies, enhancing compatibility and fostering cross-disciplinary innovations. The ability to engineer Fano interference patterns in situ offers unprecedented control over the photonic density of states, which can be exploited to interact with a variety of quantum emitters beyond photon pairs, such as quantum dots and color centers, potentially revolutionizing quantum light-matter interfaces.

This work also addresses fundamental questions pertaining to decoherence mechanisms in quantum systems. By manipulating interference through precise design of the metasurface, the researchers demonstrated pathways to mitigate environmental noise and loss channels, thereby preserving the fragile quantum correlations essential for high-fidelity quantum operations. These insights contribute valuable knowledge to the broader endeavor of achieving fault-tolerant quantum information processing.

On the theoretical front, the integration of Fano interference principles with metasurface physics enriches the conceptual framework for describing open quantum optical systems. The interplay between discrete resonant states and continuum modes now finds a tangible experimental embodiment in engineered nanostructures, bridging gaps between abstract quantum theories and applied photonics. This synergy of theory and experiment forms the cornerstone for future explorations into nonlinear and quantum optical phenomena within artificially structured media.

The article further explores the spectral response and emission dynamics of the photon pairs, elucidating how geometrical parameters of the metasurface elements influence the resonance positions, linewidths, and interference contrasts. This deep understanding invites customizable designs, where metasurfaces can be tuned to specific operational wavelengths and quantum protocols, enhancing their functionality in practical applications ranging from secure quantum key distribution to quantum metrology.

Importantly, the work transcends pure scientific inquiry to hint at real-world technological impact. Controllable quantum interference effects realized through metasurfaces could lead to breakthroughs in creating on-chip quantum light sources with tailored emission profiles, critical for quantum computing architectures reliant on indistinguishable photons. Additionally, this technology may facilitate the development of quantum sensors with superior accuracy by exploiting interference-based sensitivity enhancements inherent to Fano resonances.

In terms of fabrication, the study leverages state-of-the-art nanolithography and material deposition techniques to achieve the requisite precision in metasurface construction. The reproducibility and scalability of these fabrication methods underscore the feasibility of industrial-scale applications and open a viable route toward commercialization of metasurface-enabled quantum photonic devices. This practical perspective ensures the research is not confined to laboratory curiosity but advances the frontier of next-generation quantum technologies.

The team’s interdisciplinary approach, combining expertise in quantum optics, plasmonics, and materials science, exemplifies the collaborative efforts needed to accelerate progress in quantum photonics. Their experimental strategies, alongside comprehensive theoretical modeling, form a comprehensive toolkit for exploring complex quantum phenomena in nanostructured environments. This holistic methodology further establishes metasurfaces as versatile platforms for exploring new physics and engineering challenges at the quantum scale.

Looking forward, this discovery is poised to inspire extensive research into hybrid metasurface-based quantum systems that incorporate active control mechanisms such as electrical tuning or optical modulation. Such advancements would propel adaptive quantum devices capable of dynamic response and reconfiguration, essential for complex quantum networks and quantum machine learning applications. The foundational work presented here is thus anticipated to catalyze a vibrant area of quantum photonics research over the coming decade.

In conclusion, the demonstration of Fano interference of photon pairs from metasurfaces epitomizes a landmark achievement in controlling quantum light-matter interactions with nanoscale precision. This convergence of quantum optics and nanophotonics not only enhances our capability to engineer quantum states but also unveils new avenues for practical quantum technologies. As metasurfaces continue to evolve, their integration with quantum systems will likely redefine the landscape of quantum information science, ultimately leading to transformative advances in secure communication, computation, and sensing.

Subject of Research: Quantum photonics; Fano interference of photon pairs; metasurfaces; quantum light manipulation

Article Title: Fano interference of photon pairs from a metasurface

Article References:
Noh, J., Santiago-Cruz, T., Doiron, C.F. et al. Fano interference of photon pairs from a metasurface. Light Sci Appl 14, 371 (2025). https://doi.org/10.1038/s41377-025-01998-5

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41377-025-01998-5

Quantum entanglement, long regarded as one of the most perplexing phenomena in physics, reveals a remarkable connection between pairs of particles. When entangled, the measurement of one particle instantaneously influences the state of its partner, irrespective of the distance separating them. This correlation has spurred numerous advancements in technologies such as quantum computing, where the ability to manipulate entangled states can enhance processing power and data security exponentially.

The recent research conducted by a collaborative team from the University of Exeter and institutions in Hong Kong introduces a novel approach to producing quantum holograms using metasurfaces. Traditionally viewed as mere flat surfaces, metasurfaces are engineered from arrays of nanostructures that can manipulate light in unprecedented ways. This unique capability allows scientists to encode vast quantities of information, laying the groundwork for high-resolution holography that transcends the limitations of conventional optics.

Central to this advancement is a process called spontaneous parametric down-conversion (SPDC), which generates pairs of entangled photons through the interaction of a laser beam with a nonlinear crystal. By carefully controlling the polarization states of the emitted photons, researchers can establish the entangled relationship vital for the effective functioning of quantum holograms. Notably, when one photon’s polarization is determined, the other instantly adopts its complementary state, creating a reliable mechanism for entanglement.

In their study, the researchers demonstrated that by strategically designing the orientations of the nanostructures embedded within the metasurfaces, they could foster a quantum hologram where the polarization of entangled photons and the holographic information are intricately bound. This illuminating discovery represents a pivotal leap in seamlessly merging the concepts of holography with quantum phenomena, paving the way for novel experimental frameworks and potential technologies.

The practical applications stemming from this research are as diverse as they are promising. For instance, the encoding of information in both holographic letters and their corresponding polarization states holds significant implications for quantum communication. This method could create more efficient systems for quantum key distribution, a secure communication protocol that safeguards sensitive information against eavesdropping.

To visualize their innovation, the researchers successfully generated four distinct holographic letters—“H,” “V,” “D,” and “A”—that were entangled with the polarization of the pairs of photons. This meticulous control over holographic representation not only exemplifies the versatility of metasurfaces as a medium for quantum applications, but also emphasizes the precision achievable in manipulating entangled states. By altering the polarizer orientations for one of the photons, researchers could effectively erase specific letters from the holographic display, showcasing a profound level of control over quantum information.

Moreover, the implications of this research extend beyond the realm of quantum communication. Metasurfaces demonstrate potential use in anti-counterfeiting technologies, where their intricate designs and the dynamic interplay between the holograms and their polarization states create complex patterns that are exceedingly challenging to replicate. This unique feature could provide added layers of security against forgery, highlighting a functional aspect of quantum technology in everyday life.

Another intriguing aspect of the study is the research team’s note regarding the relationship between their quantum holograms and the concept of a quantum eraser. This mechanism, which has long captivated the imagination of physicists, enables the selective erasure of “which-path” information associated with quantum particles. By substituting holograms for traditional double-slit setups, the researchers illustrated how the quantum eraser effect manifests at a holographic level, offering an enlightening perspective on the nature of information retrieval within quantum systems.

As the boundaries of quantum mechanics continue to be explored, this research underscores the promise of nanofabrication technologies in harnessing quantum effects for practical applications. The ultrathin nature of metasurfaces, combined with their ability to perform complex operations, presents a shift away from bulky optical setups that have previously dictated the field.

In conclusion, this groundbreaking work represents a convergence of fundamental physics and applied technology, offering invaluable insights into the behavior of entangled states while paving the way for future innovations. The coupling of metasurfaces with quantum entanglement not only enhances our understanding of quantum mechanics but also emphasizes the potential societal impacts of such advancements.

This revolutionary approach encapsulates the essence of modern scientific inquiry—blurring the lines between theoretical physics and real-world applications. By leveraging the power of quantum mechanics, researchers are taking significant strides toward developing technologies that could transform the landscape of communication, security, and information processing.

Subject of Research: Quantum holography and entangled states
Article Title: Metasurface-enabled quantum holograms with hybrid entanglement
News Publication Date: 11-Mar-2025
Web References: Advanced Photonics
References: DOI: 10.1117/1.AP.7.2.026006
Image Credits: Figure courtesy of J. Li (University of Exeter).

Keywords: Quantum entanglement, holography, metasurfaces, quantum computing, quantum communication, nanotechnology, optical engineering, information security.