This project aims to develop a quantum interface that connects superconducting qubits with photons for communication to realize a large-scale distributed superconducting quantum computer. In this R&D theme, we have developed a piezoelectric microwave resonator that will be a component of the system (Fig. 1). In this R&D theme, we have achieved the following outcome.

Subject 1: Piezoelectric Microwave Cavity

Microwave photons emitted from superconducting qubits are converted into phonons via the piezoelectric effect. To enable highly efficient interaction between the resulting phonons and diamond color centers, a microcavity with an extremely small mode volume is required, confining the phonons within a region on the order of one wavelength or smaller. In this study, we successfully excited vibrations in a diamond nanobeam structure fabricated by nanomachining diamond, with a size comparable to the wavelength (approximately 1 μm), and observed the interaction between color centers and phonons. Furthermore, toward the realization of a quantum interface, we successfully integrated a diamond optical cavity onto a photonic waveguide chip (Fig. 2). Using this device, we succeeded in observing the interaction between NV centers and the optical cavity via an optical fiber at low temperature, which represents an important step toward the demonstration of a quantum interface.

Subject 2: Quantum-Controlled Electronic Integrated Circuits

To achieve fast and high-fidelity quantum control of quantum circuits at cryogenic temperatures, we have developed a superconducting microwave pulse generator capable of generating microwaves of arbitrary amplitude (Fig. 3). The system utilizes a single flux quantum (SFQ) circuit to generate a 5 GHz SFQ pulse train, which is then shaped into a 5 GHz microwave pulse with variable amplitude using a superconducting filter.

Subject 3: Theoretical study of quantum interface

We theoretically analyzed the coherent conversion efficiency of a microwave photon (emitted from a superconducting qubit) into an optical photon (suitable for long-distance propagation), using a device composed of a microwave resonator, a phonon resonator, a diamond NV center, and an optical resonator, which is expected to be fabricated in this project. In this device, a drive pulse in the communication-wavelength band is applied to the NV center through the optical resonator. Switching to a rotating flame at the drive frequency, we derived a five-oscillator model (Fig. 4) that phenomenologically describes the whole setup and obtained an analytical expression for the optimal drive power. Furthermore, with the aid of the input-output formalism, we performed a rigorous quantum-mechanical analysis to the original setup. The figures of merit of the proposed device are evaluated as follows: total conversion efficiency 30.0%, coherent conversion efficiency (conversion efficiency without loss of coherence) 27.5%, and dark count probability 1.24%.