Researchers are tackling the challenge of scaling up spin-based quantum sensors, which hold promise for advances in fields from materials science to biomedicine. M. Reefaz Rahman, Karsten Schnier, and Ryan Goldsmith, all from the Department of Electrical and Computer Engineering at The University of Alabama, alongside Benjamin J. Lawrie from Oak Ridge National Laboratory, Joseph M. Lukens from Purdue University, and Seongsin M. Kim from The University of Alabama and Seongsin M., demonstrate a novel approach using photonic links to deliver microwave control signals to these sensors. This work is significant because it overcomes limitations of traditional microwave delivery , namely thermal noise and design constraints , by utilising fibre optics, paving the way for quieter, more scalable and thermally isolated quantum sensing systems and potentially distributed quantum networks.
Remote RF Control of NV Centres via Fibre
Scientists have recently demonstrated a novel framework for remotely controlling Spin qubits using radio frequency (RF) signals transmitted via optical fiber, achieving a significant step towards scalable quantum technologies. This approach circumvents the constraints of traditional coaxial cables, which struggle to deliver high-frequency signals efficiently in cryogenic and high-magnetic-field environments. The ability to distribute control signals via optical fiber opens possibilities for creating interconnected quantum nodes and scaling up quantum sensing and computing architectures. The team’s work builds upon the growing interest in optically accessible spin qubits, like NV centers in diamond and boron vacancies in hexagonal boron nitride, which are increasingly utilized as components in quantum sensors, qubits, and quantum memories.
By addressing the microwave delivery bottleneck, this innovation unlocks opportunities for coherently addressing spins in challenging environments, paving the way for advanced spin-based quantum sensing applications. Furthermore, the RFoF system offers advantages for high-field cryogenic experiments, where conventional microwave delivery becomes increasingly difficult as spin transition frequencies reach the sub-THz range0.2 Tesla, scaling these efforts to cryogenic temperatures presents significant technical hurdles. This new approach provides a viable solution, enabling coherent spin control even at high magnetic fields and low temperatures. The recovered RF tone from the photodiode output was routed to a broadband microstrip “pinhole” antenna, fabricated on a 1.6mm thick FR-4 substrate with a copper top layer and metallic ground plane. The team positioned the diamond sample above the antenna aperture to maximize coupling to the NV spin transitions, verifying antenna response by measuring the reflection coefficient (S11), which confirmed broadband coupling in the 2.8, 3.0GHz range.
To characterize the RFoF link, scientists reported optical-to-RF power conversion efficiency based on optical power incident on the photodiode and the recovered RF power delivered to the antenna0.7 dBm. This breakthrough delivers a scalable pathway toward high-field (multi-10GHz to sub-THz), cryogenic, and networked-node operation for spin-based quantum systems0.2% at higher delivered power. The authors acknowledge limitations in recovered RF power and link linearity, but suggest improvements through enhanced modulation depth and optimised photodiode operation.
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🗞 Photonic Links for Spin-Based Quantum Sensors
🧠 ArXiv: https://arxiv.org/abs/2601.22011
The underlying physical process involves converting the quantum signal from the material system (like the NV center) into a measurable microwave photon. This necessitates highly efficient coupling mechanisms, often involving plasmonic structures or dedicated antenna arrays positioned precisely near the sample. Optimizing the geometry of the coupling region is critical, as coupling loss—the energy lost between the quantum emitter and the detection circuit—directly limits the achievable signal-to-noise ratio (SNR) for remote measurements.
Operating in high-magnetic-field regimes introduces substantial complications for microwave electronics, primarily due to Larmor frequency shifts and complex shielding requirements. Coaxial cables and conventional electronic links often exhibit significant stray electromagnetic interference (EMI) that can mask the subtle spin resonance signals. By isolating the microwave signal propagation path entirely within the dielectric properties of the optical fiber and coupling it only at the distal end, the RFoF approach effectively maintains signal integrity over distances previously considered prohibitively complex for sensitive quantum measurements.
Furthermore, the inherent scalability offered by fiber optics makes this technology highly adaptable for distributed quantum sensing arrays. Instead of integrating complex microwave lines into a dense sensor chip, one can deploy modular quantum nodes separated by kilometers of fiber. This capability supports the development of true quantum Internet infrastructure, allowing multiple, spatially separated diamond sensors to contribute coherently to a single, large-scale sensing measurement.
A major technical consideration for the system’s practical deployment is the maintenance of low-loss connectivity across varying temperatures. Traditional coaxial cables require specialized, complex thermal management systems when transitioning from room temperature to millikelvin regimes. The utilization of silica-based optical fibers, which maintain stable refractive indices and signal transmission properties across vast temperature ranges, significantly simplifies the cryogenic engineering challenge, opening up pathways for field-deployable quantum sensors.
