Progress Report

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

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Progress until FY2024

1. Outline of the project

This research and development project aims to realize a “quantum transducer,” a key technology for scaling 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 diamond 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. Outcome 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 placed inside.
  3. We designed, fabricated, and evaluated a hybrid microwave–optical resonator device under cryogenic conditions.
  4. We developed a theoretical model for the microwave–optical photon conversion and performed numerical simulations of the conversion efficiency.
  5. We realized the initial demonstration of coherent-state microwave-to-optical conversion.

Regarding 1, we could stabilize an optical cavity in a cryogen-free dilution refrigerator, where the pulse tube's cold-head induces vibrations, making optical cavity stabilization very challenging. Regarding 2, we managed to stabilize an optical cavity, even including a bulk diamond crystal. This manifests that the quantum transducer developed in this project is technically possible. Regarding 3, it is the transducer device. A diamond crystal must be placed in microwave and optical cavity modes. To this end, we designed and tested a combo-cavity (Figure 2). Regarding 4, we developed a theory of the microwave-optical photon conversion and simulated the conversion efficiency. Regarding 5, this marks a major milestone in our project. We are currently preparing experiments to convert truly non-classical photon states—such as those generated by superconducting qubits—into optical states.

Fig.2
Figure 2 Hybrid resonator device. (Inset) One of the mirrors in the optical cavity is mounted on a piezoelectric actuator, allowing precise adjustment of the distance between the two opposing high-reflectivity mirrors for cavity stabilization.

3. Future plans

Using the developed transducer device, we first convert classical weak microwave or optical signals to demonstrate the proof-of-concept of the quantum transduction. We will then convert non-classical quantum microwave photons prepared by a superconducting qubit to optical photons. We will collaborate with the "Development of Integration Technologies for Superconducting Quantum Circuits" team for these objectives.

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