Quantum information technology promises unprecedented computational power and secure communication through entangled quantum computers. Diamond, hosting optically accessible spin qubits, is a leading platform for quantum memory nodes in quantum networks. Efficient spin-photon interfaces are crucial for quantum communication, requiring fast and low-loss transfer of information between stationary qubits (spins) and flying qubits (photons). Photonic crystal (PhC) cavities are highly efficient spin-photon interfaces due to their high-quality factors (Q) and small mode volumes (V), which enhance light-matter interaction. Previous work has explored various structures like microcavities, micro rings/disks, waveguides, and nanophotonic cavities for this purpose. However, achieving high-Q cavities in diamond, particularly in the visible spectrum, has been challenging. Existing methods, like focused-ion-beam milling, angled etching, and quasi-isotropic etching, often result in lower Q factors than theoretically predicted, limited by fabrication imperfections. While diamond thin films offer potential advantages, previous attempts have resulted in lower Q values due to thickness variations and imperfect crystal quality. This research aims to address these limitations by developing a novel thin-film diamond platform and fabrication process to create high-Q PhC cavities for efficient spin-photon interfaces.

Diamond's suitability for quantum applications stems from its atom-like defects, such as nitrogen vacancies (NV), silicon vacancies (SiV), and tin vacancies (SnV), which possess long-lived, optically accessible spin qubits. These color centers have enabled advances in quantum communication. Previous work has explored various methods for creating spin-photon interfaces in diamond, including microcavities, micro rings/disks, waveguides, and nanophotonic cavities. Photonic crystal (PhC) cavities, however, stand out for their potential to achieve high Q factors and small mode volumes, leading to strong light-matter interactions. While PhC cavities have been successfully implemented in other qubit platforms (quantum dots, defects in Si or SiC, rare-earth ions), achieving high Q values in visible-wavelength diamond PhC cavities has remained a challenge. Fabrication techniques based on bulk diamond, such as angled etching and quasi-isotropic etching, have resulted in Q factors up to the low 10⁴ range, significantly lower than theoretical predictions. Approaches utilizing diamond thin films, though promising for simplifying fabrication, have yielded Q factors only in the 10³ range due to thickness variations and crystal imperfections. This work aims to overcome these limitations by employing a novel thin-film fabrication strategy.

The researchers employed a novel approach to fabricate high-Q PhC cavities in thin-film diamond. The process began with creating high-quality, homogenous thin-film diamonds (approximately 200 × 200 µm and 160 nm thick) with surface roughness < 0.3 nm and thickness variation ~1 nm. These films were bonded to a SiO₂/Si substrate using ion implantation, regrowth, electrochemical etching, and transfer printing. Silicon vacancies (SiVs) were generated via implantation across the membrane before transfer. One-dimensional (1D) and two-dimensional (2D) PhC cavities were then fabricated using electron beam lithography and reactive ion etching in O₂ chemistry. The 1D cavities (lattice constant *a*₁ᴅ = 184–226 nm, hole radius *r*₁ᴅ = 65 nm) were created by introducing a quadratic hole shift near the waveguide center, while the 2D cavities (*a*₂ᴅ = 236–269 nm, *r*₂ᴅ = 65 nm) were formed by shifting center holes outward in the PhC line-defect waveguide. For practical applications, 1D PhC cavities coupled to a feeding waveguide were also fabricated to facilitate efficient transfer of quantum information. This involved reducing the number of holes in the photonic crystal mirror on one side of the waveguide, enabling preferential coupling to the waveguide. The fabricated devices were characterized using several techniques: Photoluminescence (PL) measurements at room temperature to identify cavity resonances; cross-polarized measurements using a tunable CW laser to accurately determine the Q factors; and fiber-coupling measurements to characterize waveguide-coupled 1D PhC cavities. Finally, confocal PL measurements at 4 K were performed to characterize the SiVs and their optical coupling to the high-Q cavities, including second-order correlation (g²(τ)) measurements and time-resolved PL measurements to determine the Purcell factor.

The study yielded several significant findings. First, the fabricated 1D and 2D PhC cavities in thin-film diamond achieved record-high Q factors of 1.8 × 10⁵ and 1.6 × 10⁵, respectively, for visible-wavelength PhC cavities in any material. These values represent a significant improvement over previously reported results. The high uniformity and yield of the fabrication process were also demonstrated: 93% (53 out of 57) of the 1D cavities showed high-Q modes with resonances closely matching the design. The 1D PhC cavities coupled to waveguides exhibited a loaded Q of 8.4 × 10⁴ (intrinsic Q ~1.8 × 10⁵) and a waveguide-cavity coupling efficiency of ~65%. Crucially, the researchers demonstrated optical coupling between a single SiV center and a high-Q 1D PhC cavity at 4 K, observing a threefold reduction in the SiV's radiative lifetime, resulting in a Purcell factor of 13. The measured linewidth of the SiV's C transition was 605 MHz, and g²(0) measurements confirmed its single-photon nature. Moreover, the observed 20-fold intensity enhancement of the SiV D line emission under resonant conditions (compared to far-detuned conditions) strongly suggests Purcell effect enhancement of spontaneous emission rates. The study showed that the Q factors were one order of magnitude lower than theoretical predictions, possibly due to surface absorption and scattering from fabrication imperfections. However, the demonstrated results highlight the significant potential of this approach for creating high-performance spin-photon interfaces.

The results demonstrate a significant advancement in the fabrication of high-Q photonic crystal cavities in diamond for quantum information science applications. The record-high Q factors achieved in both 1D and 2D cavities, along with the high yield and uniformity of the fabrication process, address major limitations of previous approaches. The successful demonstration of optical coupling between a single SiV center and a high-Q cavity, resulting in a measurable Purcell factor, showcases the platform's immediate applicability to cavity quantum electrodynamics (QED) experiments. The observed enhancement in spontaneous emission rates and the single-photon nature of the SiV confirm the platform's potential for efficient spin-photon interfacing. While the Q factors are still lower than theoretical predictions, the observed performance represents a substantial step forward. The high Q-factors, coupled with the ability to efficiently couple the cavities to waveguides and fibers, pave the way for constructing scalable and integrated quantum networks.

This research successfully demonstrated a high-yield fabrication process for creating high-Q photonic crystal cavities in thin-film diamond, achieving record-high quality factors. The integration of these cavities with single silicon-vacancy (SiV) centers demonstrated efficient spin-photon interfacing, confirmed by a measurable Purcell factor. This platform significantly advances the field of quantum photonics by enabling efficient, scalable, and integrated quantum nodes for quantum communication. Future research could focus on improving the Q-factor further by optimizing the fabrication process to minimize scattering losses, and exploring the integration of this platform with other materials and technologies for advanced quantum networks.

The achieved Q factors, while record-high, are still lower than theoretical predictions. This discrepancy might be attributed to surface absorption and scattering due to fabrication imperfections such as lithography errors, surface roughness, and sidewall roughness of the air holes. Further research could focus on optimizing the fabrication process to minimize these imperfections and further improve the Q factor. The Purcell factor achieved in the SiV-cavity coupling experiment, while significant, could be enhanced by using more precise methods for SiV implantation to improve the spatial overlap between the SiV and the cavity mode.