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Developments of a Microwave-to-optical Quantum Transducer Based on a Chip-integrated Diamond Color Center

H. Kurokawa, K. SATO, T. Kobayashi, K. Goto, S. Hashimoto, M. Yamamoto, S. Ishida, H. Matsukiyo, Natthajuks Pholsen, Masao Nishioka, S. ji, H. Otsuki, M. Kamata, S. Hachuda, M. Kunii, T. Tamanuki, K. Takaki, M. Setoura, T. Makino, Michele Diego, B. Kim, Kimura Kosuke, Takenaka Kouta, A. Kamimaki, Y. Sekiguchi, H. Kato, H. Terai, K. Koshino, Onoda Shinobu, Masahiro Nomura, S. Iwamoto

Published Nov 12, 2025• 0 citations
OpenAlex

Abstract

Toward the realization of a large-scale fault-tolerant quantum computer, the development of microwave-to-optical quantum transducers has been actively underway over the past 10 years. Optical connections between refrigerators enable wide-bandwidth, flexible, room-temperature transmission of quantum information, extending the computing capacity of superconducting quantum processors. In this research, we explore the possibility of a diamond color-center-based quantum interface. We report the current status of developments in our fiber-integrated nanophotonic diamond devices.

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Toward the realization of a large-scale fault-tolerant quantum computer, the development of microwave-to-optical quantum transducers has been actively underway over the past 10 years. Optical connections between refrigerators enable wide-bandwidth, flexible, room-temperature transmission of quantum information, extending the computing capacity of superconducting quantum processors. In this research, we explore the possibility of a diamond color-center-based quantum interface. We report the current status of developments in our fiber-integrated nanophotonic diamond devices.
Scaling up superconducting quantum processors remains a central challenge for realizing fault-tolerant quantum computation. Although distributed architectures based on optical photons offer a promising route to scalability, they require an efficient microwave-to-optical quantum transducer that operates at cryogenic temperatures. Existing approaches typically rely on strong optical pumping, which induces undesirable heating and degrades single-photon coherence. Here, we propose a microwave-to-optical quantum transducer based on resonant scattering at a single color center embedded in a diamond optomechanical resonator. We show that strong coupling between the color center and the optical cavity enables coherent conversion at extremely weak pump powers on the order of 10 pW. The proposed scheme enables remote entanglement generation at a kilohertz-scale rate with a fidelity exceeding 0.9, demonstrating a viable pathway toward ultra-weak-pump and high-efficiency quantum transducers based on a solid-state defect.
The first quantum revolution has brought us the classical Internet and information technology. Today, as technology advances rapidly, the second quantum revolution quietly arrives, with a crucial moment for quantum technology to establish large-scale quantum networks. However, solid-state quantum bits (such as superconducting and semiconductor qubits) typically operate in the microwave frequency range, making it challenging to transmit signals over long distances. Therefore, there is an urgent need to develop quantum transducer chips capable of converting microwaves into optical photons in the communication band, since the thermal noise of optical photons at room temperature is negligible, rendering them an ideal information carrier for large-scale spatial communication. Such devices are important for connecting different physical platforms and efficiently transmitting quantum information. This paper focuses on the fast-developing field of optomechanical quantum transducers, which has flourished over the past decade, yielding numerous advanced achievements. We categorize transducers based on various mechanical resonators and discuss their principles of operation and their achievements. Based on existing research on optomechanical transducers, we compare the parameters of several mechanical resonators and analyze their advantages and limitations, as well as provide prospects for the future development of quantum transducers.
Superconducting quantum circuits are a leading hardware candidate for realizing a large-scale fault-tolerant quantum computer. However, scaling superconducting quantum computers will soon approach the physical limits imposed by their cryogenic operating environments. Consequently, interconnects between multiple processor modules will become increasingly necessary for scaling quantum computers. Optical fiber is a particularly promising candidate for these interconnects due to its low loss and negligible thermal noise at room temperature. However, interfacing microwave-frequency superconducting circuits with optical fibers presents several challenges. In this thesis, we present a modular architecture which addresses many of the current challenges in building this interface. We develop an optomechanical quantum transducer which is capable of generating microwave- and optical-frequency photons in pairs, and demonstrate quantum entanglement between these microwave-optical photon pairs. Further, we develop a superconducting qubit module optimized for efficient absorption of itinerant microwave photons, and demonstrate a high-efficiency photon catch protocol. Finally, we connect the superconducting qubit module to the transducer module via a superconducting microwave cable. We drive the microwave-optical photon pair generation process in the transducer module and employ the photon catch protocol in the qubit module to facilitate state transfer between the qubit and the flying microwave photon from the transducer. We observe correlations between the final qubit state and the detection of an optical photon, demonstrating a crucial step toward establishing quantum entanglement between superconducting qubits and light. We discuss ongoing progress and challenges in realizing hybrid entanglement and in extending this approach to entangle remote superconducting qubits over optical networks.
Connecting cryogenic solid-state quantum processors with room-temperature optical links is a key step toward a scalable quantum computer. This thesis presents two technological advancements toward the realization of a spin-ensemble-based quantum transducer using impurity spins in diamond. First, a low-loss dielectric microwave resonator based on rutile (TiO2) was developed, achieving an internal quality factor of Qint ∼ 104 despite incorporating large optical apertures. Second, an optical Fabry–Pérot cavity was stabilized at millikelvin temperatures with sub-0.1 nm root-mean-square cavity length fluctuations, enabling stable cavity locking to the laser in a dilution refrigerator. These components were integrated into a hybrid resonator enabling spatial mode overlap within a diamond sample containing an ensemble of nitrogen-vacancy centers. Measurements at 10 mK revealed spin coherence times of T1 = 1930 s and T2 = 470 µs, and characterized the inhomogeneous broadening of the optical transition. We estimate the current microwave-to-optical conversion efficiency and discuss potential improvements through device and cavity optimization. This work establishes a platform for spin ensemble-based transducers operating at millikelvin temperatures.
Chip integration of quantum emitters is a crucial milestone for scalable quantum photonic information processing. Among optically active defect centers for quantum photonics, diamond color centers are promising because of their long spin coherence times and high photon emission rates. However, for a coherent-photon emission, they typically require a cryogenic environment to protect optical coherence from thermal phonons, which makes chip integration challenging. In this paper, we develop a chip-integrated diamond photonic crystal cavity embedding an ensemble of nitrogen-vacancy (NV) centers. We confirm cryogenic operation by observing Purcell enhancement of NV-center emission via an edge-coupled optical fiber. This result demonstrates successful integration of diamond color centers, a photonic crystal cavity, and an optical waveguide-fiber package, representing a key step toward scalable diamond-based quantum communication platforms.