Photonic Links for Spin-Based Quantum Sensors

A growing variety of optically accessible spin qubits have emerged in recent years as key components for quantum sensors, qubits, and quantum memories. However, the scalability of conventional spin-based quantum architectures remains limited by direct microwave delivery, which introduces thermal noise, electromagnetic cross-talk, and design constraints for cryogenic, high-field, and distributed systems. In this work, we present a unified framework for RF-over-fiber (RFoF) control of optically accessible spins through RFoF optically detected magnetic resonance (ODMR) spectroscopy of nitrogen-vacancy (NV) centers in diamond. The RFoF platform relies on an electro-optically modulated telecom-band laser that transmits microwave signals over fiber and a high-speed photodiode that recovers the RF waveform to drive NV center spin transitions. We obtain an RFoF efficiency of 1.81% at 2.90~GHz, corresponding to $P_{\mathrm{RF,out}}=-0.7$~dBm. The RFoF architecture provides a path toward low-noise, thermally isolated, and cryo-compatible ODMR systems bridging conventional spin-based quantum sensing protocols with emerging distributed quantum technologies.

💡 Research Summary

The paper presents a proof‑of‑concept demonstration of radio‑frequency‑over‑fiber (RFoF) control for optically accessible spin qubits, specifically nitrogen‑vacancy (NV) centers in diamond, and evaluates its performance against conventional coaxial microwave delivery. The motivation stems from the thermal load, electromagnetic cross‑talk, and design constraints that direct microwave delivery imposes on cryogenic, high‑magnetic‑field, and distributed quantum sensing platforms. To address these challenges, the authors construct an RFoF link that uses a 1310 nm telecom‑band continuous‑wave laser intensity‑modulated by a Mach–Zehnder electro‑optic modulator (EOM). The modulated optical carrier propagates through single‑mode fiber and is converted back to an electrical microwave tone by a high‑speed photodiode placed near the experiment. The recovered microwave drives a broadband “pinhole” microstrip antenna that couples magnetic fields to the NV ensemble.

Key experimental results include: (1) an optical‑to‑RF conversion efficiency η_O→RF = 1.81 % at 2.90 GHz, corresponding to an output microwave power of –0.7 dBm for an incident optical power of 47 mW on the photodiode; (2) ODMR contrast exceeding 2.2 % at this power level, comparable to the 11 % contrast obtained only with 25 dBm (≈300 mW) delivered via coaxial cable; (3) a contrast of 0.6–0.9 % across magnetic fields of 8–36 G when the RFoF output is –5.5 dBm, demonstrating reliable spin driving even at modest power. The authors also present conventional ODMR spectra recorded with coaxial delivery ranging from 0 dBm to 25 dBm, showing the expected increase in contrast and modest power‑broadening of linewidths. By keeping the antenna, sample placement, and optical detection identical for both delivery methods, the study isolates the effect of the delivery path on the spin response.

The discussion highlights several advantages of the RFoF approach. First, the microwave source and high‑power electronics can be located outside the cryogenic environment, eliminating direct thermal loading on the cold stage. Second, the fiber link provides intrinsic electromagnetic isolation, reducing cross‑talk and interference that can degrade sensor performance. Third, fiber‑based distribution is naturally compatible with long‑distance quantum networks, enabling scalable multi‑node architectures where microwave references, timing, and control signals are shared across kilometers of fiber.

Limitations are also identified. The current efficiency is constrained by the modulation depth of the EOM, insertion losses in the fiber‑to‑photodiode interface, and the linearity and saturation power of the photodiode. Consequently, the recovered microwave amplitude is lower than that achievable with direct coaxial feeding, requiring higher optical power or more efficient modulators to close the gap. The authors propose several routes for improvement: using high‑power, low‑noise lasers; employing low‑Vπ, high‑bandwidth EOMs; optimizing photodiode responsivity and biasing; and integrating optical phase‑stabilized heterodyne or photomixing schemes to reach frequencies well beyond 10 GHz, potentially into the sub‑THz regime required for high‑field NV or other spin‑defect transitions.

Finally, the paper positions RFoF as a enabling technology for next‑generation quantum sensing and quantum information platforms. By decoupling microwave delivery from the cryogenic node, RFoF opens pathways for (i) cryogenic NV magnetometry and thermometry with minimal thermal disturbance, (ii) high‑field (>10 T) experiments where spin transition frequencies exceed the bandwidth of conventional coaxial cables, and (iii) distributed quantum sensor networks where synchronized microwave control is delivered over fiber to many remote nodes. The demonstrated 1.81 % conversion efficiency and >2 % ODMR contrast at sub‑dBm microwave power illustrate that, even in its current form, RFoF can meet the basic requirements for coherent spin control while offering significant system‑level benefits. Future work focusing on higher efficiency, broader bandwidth, and integration with low‑noise optical frequency combs could further solidify RFoF as a cornerstone of scalable, low‑noise quantum technologies.