In this project, we focus on transduction between optical and microwave electromagnetic waves and matter-based quantum systems, aiming for the realization of quantum repeaters.
The technologies under development include quantum memory systems that interface with visible photons, such as rare-earth-doped material like Pr:YSO and rubidium atoms; quantum mechanical memories that couple to microwave fields; and enabling technologies such as frequency stabilization and quantum transducers that link quantum memories with optical signals. These efforts aim to establish entanglement between distant quantum memories and to advance quantum repeater technology, ultimately enabling the integration of superconducting quantum computers with long-distance quantum communication networks.

Development of stabilization techniques for light sources used in a quantum memory

Frequency stabilization techniques have been developed for the light sources used in quantum memory experiments. In this year, we performed laser frequency stabilization for the laser source used to transfer the entangled photons. The frequency variation of the laser source used to transfer the entangled photons was suppressed to less than 1 kHz. We also developed an optical frequency comb with high reliability. We have realized the phase lock of the carrier-envelope offset frequency of the developed comb.

Research on Enhancing the Reliability of Entangled Photon Generation via Spin Waves

By cooling and trapping rubidium atoms in a magneto-optical trap and precisely controlling their internal states using laser fields, we have achieved the atomic control necessary for the operation of a quantum memory. Furthermore, we have successfully detected photons emitted from an ensemble of atoms using a single-photon detector, a process essential for the operation of quantum memory. Additionally, to enable efficient exchange of quantum information between light and atoms, we constructed an optical resonator to confine photons using low-loss mirrors. We also established multiple independent write modes to store multiple quantum entangled states in a single quantum memory, enabling up to 18 write modes.

Development of Quantum Mechanical Memory

We have developed thin-film lithium niobate (LN) devices incorporating a variety of optical and acoustic waveguide geometries and resonator structures, aimed at device integration and miniaturization (Fig. 2 left). Since device performance is largely limited by optical and acoustic losses, we have continued to improve and optimize the fabrication processes, as in our previous work.
For acoustic devices, we also conducted several low-temperature measurements (T = 300 mK), and in thin-film acoustic resonators we observed long-lived phonon excitations with lifetimes of 10 μs (Q~2×10^5) (Fig. 2 right) [1]. In addition, we carried out measurements of various Brillouin scattering processes in multimode waveguides [2].
With the development of electro-optic modulators for quantum transducer applications in mind, we designed and characterized ultra-high-frequency, ultra-wideband modulators operating beyond 200 GHz [3]. Furthermore, we developed and evaluated chip platforms integrating electrical wiring and optical input coupling mechanisms (grating couplers) for operation within a cryogenic refrigerator environment.

[Related Publications]

[1] arXiv:2603.11293 (2026)

[2] Optics Express 34, 12304 (2026)

[3] 2024 IEEE Photonics Conference (IPC) (2024)

We aim to advance toward quantum repeater technologies, including the demonstration of entanglement between quantum memories. Furthermore, through the development of quantum mechanical memories and quantum transducers, we will work toward interfacing superconducting quantum computers with quantum communication networks.