Although there are many candidates for quantum computers, the potential of diamond is second to none in other physical systems (Fig. 1). In this project, we aim to develop a superconducting optical quantum interface, in which superconducting microwave photons and optical photons are quantum-connected by diamond (Fig. 2), to realize a large-scale distributed superconducting quantum computer system. In this R&D theme, we have developed optomechanical cavities that will be the building blocks of the interface. We have achieved the following outcome.

Subject 1: Photonic Crystal Optical Cavity

We further advanced the fabrication process and successfully realized a diamond optical nanocavity structure with an optical quality factor of 12,000 (Fig. 3(a),(b)), which is more than ten times higher than the initially targeted value. In addition, by leveraging the fabrication techniques established so far, we fabricated a number of optical nanocavities for integration with photonic circuits as well as optomechanical resonator structures, making a significant contribution to the realization of integrated device prototypes.

Subject 2: Photonic Crystal Optical Cavity Mounting

We have developed an interface that connects optical fiber to a diamond optomechanical cavity. The cavity is transfer-printed on a silicon nitride (SiN) waveguide chip fabricated by silicon photonics process, and optically coupled to the optical fiber. For the first prototype chip fabricated in 2023, light propagation from the fiber to the cavity was confirmed. Based on this result, a second prototype was fabricated in 2024, equipped with super-conducting wiring and microwave resonator. The fiber connection is expected to be stabilized at cryogenic temperatures, and optical photons are expected to be converted to microwave photons (Fig. 4).

Subject 3: Phononic Crystal Sound Cavity

To confine photons and phonons within a resonator and enable strong optomechanical coupling, we designed and fabricated diamond and silicon resonators (Fig. 5). The design takes fabrication constraints into account, ensuring that both the designed optical and mechanical Q-factors exceed 10⁵. By applying a voltage to the piezoelectric pads located on the resonator using proximity electrodes, the inverse piezoelectric effect is triggered, thereby driving the acoustic modes.