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.

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.

Quantum computer chip based on spin qubits in diamond uses modules that are entangled with on-chip optical links. This enables an increased connectivity and a negligible crosstalk and error-rate when the number of qubits increases onchip. Here, 3D integration is the key enabling technology for a large-scale integration of the diamond spin qubits with photonic and electronic circuits for routing, control and readout of qubits. There are several engineering challenges to integrate the large number of spins in diamond with the on-chip circuits operating at a cryogenic temperature. In this paper we will address challenges, present recent results and discuss future outlook of the integration technology for realization of a scalable quantum computer based on diamond spin qubits.

Quantum computer chip based on spin qubits in diamond uses modules that are entangled with on-chip optical links. This enables an increased connectivity and a negligible crosstalk and error-rate when the number of qubits increases on-chip. Here, 3D integration is the key enabling technology for a large-scale integration of the diamond spin qubits with photonic circuits and CMOS electronics for routing, control and readout of qubits. Several engineering challenges exist in order to integrate the large number of spins in diamond with the on-chip circuits operating at a cryogenic temperature. We will review trends, address challenges and discuss future outlook of the integration technology for realization of a scalable quantum computer based on diamond spin qubits.