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arXiv:2605.04277

Seeing the forbidden: overcoming optical selection rules through nanophotonic integration

Alex H. Rubin, Vytautas Žalandauskas, Pranta Saha, Rubek Poudel, Aurora Teien, Liam Hofmann, Nathan R. Gonzalez, Scott Dhuey, Marianne Etzelmüller Bathen, Marina Radulaski

physics.opticsquant-phSubmitted May 5, 2026• Announced May 7, 2026
arXivHTMLPDF

Abstract

Optically addressable spin defects in silicon carbide, including the neutral divacancy (VV$^0$) and the negative nitrogen-vacancy (NV$^-$), are among leading building blocks of solid-state quantum technologies. Integrating these defects into photonic structures such as nanopillars improves photon collection efficiency, but the consequences extend further. We show that the sub-wavelength geometry of nanopillars drastically modifies the local electromagnetic environment, providing optical access to defect transitions that are otherwise suppressed by selection rules in bulk material. Using low-temperature photoluminescence spectroscopy, we observe that emission from the PL3 divacancy, which is nearly absent in planar devices, becomes pronounced in nanopillars owing to a polarization transformation of the excitation field within the pillar. We further leverage the orientation-dependent collection of nanopillars to resolve the origin of previously ambiguous spectral lines. In particular, the NV4$'$ feature displays the signal enhancement expected for axially oriented NV$^-$ centres, consistent with assignment to a higher excited state of the $kh$ defect configuration. Our results establish nanophotonic integration as a symmetry-sensitive probe that can both activate nominally dark transitions and identify the dipole character of poorly understood defect states.

Comments
23 pages, 8 figures

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Optically addressable spin defects in silicon carbide, including the neutral divacancy (VV0^00) and the negative nitrogen-vacancy (NV−^-−), are among leading building blocks of solid-state quantum technologies. Integrating these defects into photonic structures such as nanopillars improves photon collection efficiency, but the consequences extend further. We show that the sub-wavelength geometry of nanopillars drastically modifies the local electromagnetic environment, providing optical access to defect transitions that are otherwise suppressed by selection rules in bulk material. Using low-temperature photoluminescence spectroscopy, we observe that emission from the PL3 divacancy, which is nearly absent in planar devices, becomes pronounced in nanopillars owing to a polarization transformation of the excitation field within the pillar. We further leverage the orientation-dependent collection of nanopillars to resolve the origin of previously ambiguous spectral lines. In particular, the NV4′'′ feature displays the signal enhancement expected for axially oriented NV−^-− centres, consistent with assignment to a higher excited state of the khkhkh defect configuration. Our results establish nanophotonic integration as a symmetry-sensitive probe that can both activate nominally dark transitions and identify the dipole character of poorly understood defect states.
Color centers are optically active point defects in semiconductor materials, many of them carrying a spin degree of freedom that couples coherently to emitted photons. This makes them a leading candidate for building quantum computers, networks, and sensors, but realizing that promise depends on the interface between the emitter and light. Color centers must be integrated into nanophotonic structures, such as nanopillars that collect their emission efficiently and optical cavities that couple them to a single mode. This dissertation studies the physics of quantum emitters inside such structures along two complementary threads: the direct cryogenic spectroscopy of color centers integrated into silicon carbide nanostructures, and the study of the physics of many-body cavity quantum electrodynamics (CQED) through simulation on superconducting and trapped-ion quantum hardware. The first thread concerns the nitrogen-vacancy (NV) center in 4H silicon carbide integrated into nanopillars. We show that the sub-wavelength geometry of a nanopillar reshapes the local electromagnetic environment, granting optical access to transitions that selection rules forbid in bulk material: emission from the PL3 divacancy, nearly absent in planar devices, becomes pronounced in nanopillars through a polarization transformation of the excitation field, and the orientation-dependent collection of the pillar resolves the previously ambiguous NV4′ line, establishing nanophotonic integration as a symmetry-sensitive probe of defect structure. We then take up the complementary problem of quantifying how many emitters a device contains, adapting the number-and-brightness technique from fluorescence microscopy to solid-state emitters. This calibration-free method counts dense NV ensembles in silicon carbide nanopillars, a regime beyond the reach of existing optical techniques, and extracts their areal density and formation yield. The second thread turns to many-body cavity QED, in which placing N emitters in a common cavity enhances the light-matter coupling by √N and gives rise to collective effects that are central to strong-coupling applications yet difficult to achieve in solid-state devices based on defects. We realize one such effect on a superconducting circuit, demonstrating photon blockade, the conversion of a coherent drive into a stream of single photons, in a Tavis–Cummings system of N = 1, 2, 3 transmon qubits coupled to a common resonator. To study the open-system dynamics that govern these systems, we digitally simulate the dissipative Tavis–Cummings model on both a superconducting and a trapped-ion processor. Platform-specific compilation and error mitigation yield population dynamics in close agreement with exact solutions, providing a recipe for simulating cavity-emitter systems on contemporary quantum computers. Together, the two threads connect the fabrication and characterization of color-center devices to the many-body cavity-QED physics that will ultimately govern their performance.
Defect based qubit systems like the nitrogen vacancy center in diamond have recently emerged as promising candidates for quantum technologies due to their combination of long coherence times, room temperature operation, and robust optical interface. In order to realize many of their proposed applications, defect qubits must be incorporated into scalable devices architectures consisting of photonic, mechanical, or electrical degrees of freedom. Despite much recent progress, many challenges remain for diamond growth and device fabrication. As an alternate approach, we engaged in a search for nitrogen vacancy center analogues in alternative materials with the hope of obtaining a greater degree of control over defect and material properties. Ultimately, we discovered that divacancy-related point defects in all three of the most common forms of silicon carbide- termed 4H, 6H, and 3C- act as analogues to the nitrogen vacancy center in diamond. We chose to focus our research primarily on defects in 3C silicon carbide (termed 'Ky5' defects) because of its availability as a single crystal heteroepitaxial thin film grown on silicon, an advantage that greatly facilitates the fabrication of functional devices. We characterized the spin and optical properties of Ky5 defects in thin film geometries and observed many similarities to the nitrogen vacancy center. We performed the first measurements of spin dynamics in 3C silicon carbide and demonstrate coherent control of defect spins up to room temperature and observe coherence times of up to 22 microseconds. To demonstrate their use in real devices, we designed, fabricated, and characterized photonic crystal cavities in 3C silicon carbide thin films with mode volumes of less than (lambda/n)^3 and Q's as high as 1,500 with integrated Ky5 defects. Additionally, we performed simulations and analysis of the fabricated structures using observed structural imperfections to determine that the Q's are likely limited primarily by the non-vertical structure sidewall angle. Despite the modest Q's of these structures, they can be utilized to generate large local field intensities to enhance optical interactions with Ky5 qubit states within the cavities. We accomplish this by performing cavity enhanced photoluminescence excitation spectroscopy on cavity modes tuned to the zero phonon line of the defects and observe large (as high as 30 times) increases in the luminescence and optically detected magnetic resonance signals originating from the defect states and approximately 2x faster rates of ground state spin initialization. In addition, we use these techniques to probe the photoluminescence dynamics of the Ky5 defects' optical pumping cycle, perform excitation wavelength dependent studies of spin and spectral inhomogeneity, and use the small mode volume and narrowband photoluminescence enhancements of the cavities to observe spectrally distinct subensembles of defects with linewidths as narrow as 25 GHz within the inhomogeneously broadened zero phonon line. Although much is still unknown regarding the properties of these defects, they show great promise as a candidate system for defect qubit based quantum devices and technologies.
Silicon carbide (SiC) is a promising platform for scalable quantum technologies owing to its well-established, wafer-scale industrial processing. SiC also hosts a variety of optically active color centres including the nitrogen vacancy defect that has a spin-triplet ground state. However, strong phonon coupling in the infrared range limits photon extraction from these defects. Here, we use nanophotonic structures, specifically micro-pillar and micro-disk resonators, to enhance optical collection and spin-readout. The micro-pillar geometry yields a 4-fold increase in photon collection, accompanied by a 2.4-fold reduction in spectral noise in optically detected magnetic resonance measurements. Consequently, the magnetic field sensitivity is improved by 24%. The large mode volume of the micro-disk supports resonances spanning 1150-1250 nm, enabling broadband coupling to nitrogen vacancy emission lines. Our results demonstrate that fabrication of scalable photonic structures efficiently improves performance of silicon carbide color centers for integrated quantum light generation and sensing.
Divacancy defects in silicon carbide have long-lived electronic spin states and sharp optical transitions. Because of the various polytypes of SiC, hundreds of unique divacancies exist, many with spin properties comparable to the nitrogen-vacancy center in diamond. If ensembles of such spins can be all-optically manipulated, they make compelling candidate systems for quantum-enhanced memory, communication, and sensing applications. We report here direct all-optical addressing of basal plane-oriented divacancy spins in 4H-SiC. By means of magneto-spectroscopy, we fully identify the spin triplet structure of both the ground and the excited state, and use this for tuning of transition dipole moments between particular spin levels. We also identify a role for relaxation via intersystem crossing. Building on these results, we demonstrate coherent population trapping -a key effect for quantum state transfer between spins and photons- for divacancy sub-ensembles along particular crystal axes. These results, combined with the flexibility of SiC polytypes and device processing, put SiC at the forefront of quantum information science in the solid state.
Spin defects in silicon carbide (SiC) are promising candidates for integrated quantum photonics, offering long-lived spin states and near-infrared emission suitable for low-loss photonic integration and fibre-based quantum communication. However, light extraction from these defects remains challenging due the relatively high refractive index of SiC. Metalenses offer a compact approach to enhance light collection by engineering the wavefront directly at the material interface. Here, we design and fabricate monolithic metalenses from SiC bulk material that simultaneously operate at 860 and 1240 nm, matching with emission from the nitrogen vacancy and silicon vacancy colour centers. By independently engineering the phase response at both wavelengths, the metalens enables collection and polarization manipulation of the emitted light. We further employ the metalenses to demonstrate optically detected magnetic resonance of both defects simultaneously. These multifunctional metalenses provide a compact optical interface for scalable integrated SiC photonic devices.
Negatively charged nitrogen vacancy centers (NV−) in diamond serve as highly sensitive, optically readable sensors for magnetic fields. Improved sensing approaches rely on NV− centers embedded in diamond nanopillar waveguides, which enable scanning probe imaging and use multi-color laser schemes for efficient spin readout. In this work, we investigate the free-beam coupling of the most relevant laser wavelengths to diamond nanopillars with different geometries. We focus on cylindrical pillars, conical pillars, and conical pillars with an added parabolic dome. We study the effects of the pillar geometry, NV− position, laser wavelength, position of laser focus, and excitation geometry (excitation from the top facet or from the substrate side). We find a pronounced impact of the laser wavelength that should be considered in multi-color excitation of NV−. Within the pillars, exciting laser fields can be enhanced up to a factor of 11.12 compared to bulk. When focusing the laser to the interface between the substrate and the nanopillar, even up to 29.78-fold enhancement is possible. Our results are in accordance with the experimental findings for green laser excitation of NV− in different pillar geometries.