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.

For quantum computing modules using diamond color centers, we propose an integrated structure of a quantum chip with photonic circuits and an interposer with electric circuits. The chip and interposer are connected via gold stud bumps using flip-chip bonding technology. For evaluating the proposed integrated structure, we bonded a test chip of 15 × 15 mm2, corresponding to the area that allows the allocation of color center qubits in the order of 102, with an interposer of 20 × 20 mm2, including test measurement lines. We confirm all connections of 16 lines with two bumps for each line at 10 K. The resistance of the lines with two bumps at 10 K is ~ 3.5O, These resistances are mainly attributed to the gold lines on the interposer, which is confirmed by simulations. The shear strength of the flip-chip bonded structure is 67 g/bump. It is larger than that of previous reports where the chips passed the standard temperature cycle test. Moreover, we integrate the flip-chip bonded structure with a printed circuit board (PCB). We confirm a connection between the connector terminal of the PCB and the test chip at 80 K. It is shown that the integrated structure using gold stud bumps has a potentially highly reliable connection at cryogenic temperature. These results will lead to realizing large-scale diamond spin quantum processors.