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arXiv:2605.15988

Ultra-Weak-Pump Microwave-to-Optical Quantum Transduction via a Single Color Center

Kyosuke Goto, Hodaka Kurokawa, Hideo Kosaka, Kazuki Koshino

quant-phSubmitted May 15, 2026• Announced May 18, 2026• 0 citations• Revised Sep 11, 2026 (v2)
v1 · May 15, 2026v2 · Sep 11, 2026
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Abstract

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.

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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.
Quantum transduction between microwave and optical photons plays a key role in quantum communication among remote qubits. Although the quantum transduction schemes generating communication photons have been successfully demonstrated by using optomechanical interfaces, the low conversion efficiency remains an obstacle to the implementation of a quantum network consisting of multiple qubits. Here, we present an efficient quantum transduction scheme using a one-dimensional diamond optomechanical crystal cavity tuned at a color-center emission without optomechanical coupling. The optomechanical crystal cavity incorporates a thin aluminum nitride (AlN) pad piezoelectric coupler near the concentrator cavity region, while retaining ultrasmall mechanical and optical mode volumes of about 1.5 \times 10^{-4} and ~0.2(λ/n)^3, respectively. The energy level of a coherent color-center electron is manipulated by a strong mechanical-mode-color-center electron-coupling rate up to 16.4MHz. In our system, we theoretically predict that the population-conversion efficiency from a single microwave photon into an optical photon can reach 15% combined with current technologies. The coherent conversion efficiency is over 10% with a reasonably pure decay time of T^2_{*} > 10 ns. Our results imply that an atomic color center strongly coupled to the optomechanical crystal cavity will offer a highly efficient quantum transduction platform.
The coherent conversion between microwave and optical photons, known as quantum transduction, is critical for connecting superconducting processors to optical networks. Existing methods are limited by complications associated with optical pumping. We propose a pump-free microwave-optical Bell-pair source for teleportation-based transduction. Using a spin or atomic system resonantly coupled to optical and microwave cavities, the scheme generates loss-robust heralded Bell pairs. Across color centers, atomic ensembles, and phonon-mediated systems, this assembly achieves kilohertz-range heralding rates with high fidelity.
Quantum transduction between microwave and optical photons plays a key role in quantum communication among remote qubits. Although the quantum transduction schemes generating communication photons have been successfully demonstrated by using optomechanical interfaces, the low conversion efficiency remains an obstacle to the implementation of a quantum network consisting of multiple qubits. Here, we present an efficient quantum transduction scheme using a one-dimensional diamond optomechanical crystal cavity tuned at a color-center emission without optomechanical coupling. The optomechanical crystal cavity incorporates a thin aluminum nitride (AlN\mathrm{Al}\mathrm{N}AlN) pad piezoelectric coupler near the concentrator cavity region, while retaining ultrasmall mechanical and optical mode volumes of about $1.5\phantom{\rule{0.1em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.1em}{0ex}}{10}^{\ensuremath{-}4}({\mathrm{\ensuremath{\Lambda}}}_{p}{)}^{3}$ and $0.2(\ensuremath{\lambda}/n{)}^{3}$, respectively. The energy level of a coherent color-center electron is manipulated by a strong mechanical-mode--color-center electron-coupling rate up to 16.4 MHz. In our system, we theoretically predict that the population-conversion efficiency from a single microwave photon into an optical photon can reach 15% combined with current technologies. The coherent conversion efficiency is over 10% with a reasonably pure decay time of ${T}_{2}^{\ensuremath{\ast}}>10\phantom{\rule{0.25em}{0ex}}\mathrm{ns}$. Our results imply that an atomic color center strongly coupled to the optomechanical crystal cavity will offer a highly efficient quantum transduction platform.
A microwave-optical converter with high efficiency (>50%) and low added noise ( ≪ 1 photon) could enable the creation of scalable optical quantum networks. However, integrated converters demonstrated to date are too lossy or weakly non-linear to provide this performance. Here we develop a theory of microwave-optical conversion employing an ensemble of spin-bearing color centers strongly coupled to a photonic resonator and/or a superconducting microwave resonator. We find a counterintuitive operating point where microwave and optical photons are tuned to bare center/cavity resonances, which, compared to the weak-coupling limit, has much stronger non-linearity, offering high efficiency with reduced pump- and center-induced losses, and that is robust to inhomogeneous broadening. Taking color center and optical pump-induced losses into account, we predict ~ 95% efficiency and added noise ≪ 1 quanta at low ( μ W) pump powers for both Er- and T-centers in Si. Our results open new pathways to build quantum networks.
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.
Abstract Frequency conversion between microwave and optical photons is a key enabling technology to create links between superconducting quantum processors and to realize distributed quantum networks. We propose a microwave-optical transduction platform based on long-coherence time superconducting radio-frequency (SRF) cavities coupled to electro-optic optical cavities to mitigate the loss mechanisms that limit the attainment of high conversion efficiency. We optimize the microwave-optical field overlap and optical coupling losses in the design while achieving long microwave and optical photon lifetime at milli-Kelvin temperatures. This represents a significant enhancement of the transduction efficiency up to 50% under incoming pump power of 140 μW, which allows the conversion of few-photon quantum signals. Furthermore, this scheme exhibits high resolution for optically reading out the dispersive shift induced by a superconducting transmon qubit coupled to the SRF cavity. We also show that low microwave losses enhance the fidelity of heralded entanglement generation between two remote quantum systems. Finally, high precision in quantum sensing can be reached below the standard quantum limit.