Progress Report

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Quantum Cyberspace with Networked Quantum Computer[4] Superconducting networking technology

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Progress until FY2025

1. Outline of the project

This R&D project aims to realize a “quantum transducer,” a key technology for scaling up and networking superconducting quantum computers. The transducer converts microwave photons—used to carry quantum information in superconducting qubits—into optical photons within an ultra-low-temperature environment. We focus on using electron spin ensembles associated with nitrogen-vacancy (NV) centers in diamonds as the medium for this wavelength conversion. To achieve this, we have tackled several technical challenges: implementing optical cavities containing bulk diamond crystals, maintaining cavity stability at millikelvin temperatures, and integrating them with microwave resonators. We have designed and employed a custom low-vibration, cryogen-free dilution refrigerator to overcome these difficulties.

Fig.1
Figure 1 Cartoon of the spin ensemble-based quantum transducer developed by this project (Blue: microwave photon, Red: optical photon).

2. Outcomes so far

  1. We successfully stabilized an optical cavity in a cryogen-free dilution refrigerator at millikelvin temperatures.
  2. We achieved stable operation of an optical cavity even with a bulk diamond crystal coated with anti-reflection and high-reflection films placed inside.
  3. We designed, fabricated, and evaluated a hybrid microwave–optical resonator device under cryogenic conditions.
  4. We developed a theoretical model of microwave–optical photon conversion and performed numerical simulations of the conversion efficiency.
  5. We demonstrated the proof-of-principle of coherent-state microwave-to-optical conversion.
  6. We demonstrated a near-quantum-limited ultra-low-noise microwave amplifier using a spin maser.

Regarding 1, we successfully stabilized an optical cavity inside a cryogen-free dilution refrigerator, which is essential for superconducting- and semiconductor-based quantum computers. Regarding 2, we demonstrated stable operation of an optical cavity containing a bulk diamond crystal, showing the feasibility of the quantum transducer scheme proposed in this project. Regarding 3, we designed, fabricated, and evaluated the hybrid microwave–optical resonator device itself. We developed a hybrid resonator device to realize this architecture (Figure 2). Furthermore, using this device under millikelvin conditions, we performed electron spin resonance experiments and demonstrated strong coupling with spin ensembles as well as spin relaxation-time measurements. Regarding 4, we established a theoretical framework for microwave–optical photon conversion and simulated the conversion efficiency. Regarding 5, we achieved the initial proof-of-principle demonstration of coherent-state microwave-to-optical conversion. Regarding 6, we demonstrated near-quantum-limited ultra-low-noise microwave amplification using a diamond spin maser. Compared with existing superconducting technologies, the demonstrated device potentially offers orders-of-magnitude higher saturation power, making it promising as a scalable component for large-scale quantum computers.

Fig.2
Figure 2 Transducer device. (Inset) Piezo-attached mirror, enabling cavity stabilization.

3. Future plans

We plan to further improve the conversion efficiency of the quantum transducer device. We aim to demonstrate conversion of non-classical microwave quantum states generated by superconducting qubits. In addition, we will further enhance microwave and optical quantum-memory operations as well as ultra-low-noise microwave amplification technologies based on spin masers.

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