Low-loss visible-light photonic circuits are crucial for high-performance photonic quantum processors. By using aluminum oxide (Al₂O₃) for its low visible-light absorption, we achieved waveguides exhibiting an exceptionally low propagation loss (1.39 dB/cm for the transverse electric mode) at red-light wavelengths. Directional coupler beam splitters fabricated using this platform exhibited good controllability of the optical splitting ratios. Furthermore, we fabricated a half beam splitter, which is an essential component of entangled photon generation in quantum optics. These results represent a significant advance toward developing low-loss photonic circuits, paving the way for improved performance in photonic quantum processors.

Surface-activated direct bonding of diamond (100) and c-plane sapphire substrates is investigated using Ar atom beam irradiation and high-pressure contact at RT. The success probability of bonding strongly depends on the surface properties, i.e, atomic smoothness for the micron-order area and global flatness for the entire substrate. Structural analysis reveals that transformation from sapphire to Al-rich amorphous layer is key to obtaining stable bonding. The beam irradiation time has optimal conditions for sufficiently strong bonding, and strong bonding with a shear strength of more than 14 MPa is successfully realized. Moreover, by evaluating the photoluminescence of nitrogen-vacancy centers in the diamond substrate, the bonding interface is confirmed to have high transparency in the visible wavelength region. These results indicate that the method used in this work is a promising fabrication platform for quantum modules using diamonds.