EPFL Demonstrates Millimeter-Wave-to-Optical Transducer with 67% Internal Efficiency

EPFL’s Millimeter‑Wave‑to‑Optical Transducer Hits 67 % Internal Efficiency

A team at the École Polytechnique Fédérale de Lausanne (EPFL) has demonstrated a compact electro‑optic device that directly links high‑frequency millimeter‑wave signals to telecom‑band light. The integrated transducer, built from thin‑film lithium‑niobate (TFLN) photonic cavities coupled to a sub‑millimeter metallic resonator, achieves an internal photon‑conversion efficiency of 67 %—a notable step toward low‑loss quantum interconnects.

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Architecture and Key Parameters

The device combines two Fabry–Pérot micro‑cavities etched into a 400 nm TFLN membrane with a quarter‑wave gold resonator (≈165 µm long). The optical cavities (110 µm and 119 µm) provide free‑spectral ranges >650 GHz and quality factors up to 10⁶ (linewidth 186–230 MHz). The millimeter‑wave resonator operates at 263 GHz, matching the frequency of many cryogenic quantum processors and atomic Rydberg transitions.

• Vacuum electro‑optic coupling rate (g₀/2π): ≈ 60 kHz, roughly 60× higher than typical microwave‑domain electro‑optic devices.
• Electro‑optic cooperativity (C): up to 0.27, yielding the reported 67 % internal conversion efficiency.
• Thermal photon detection: 0.21 ± 0.08 photons, enabling sub‑single‑quantum background measurement via optical sidebands.

The design resolves a longstanding mode‑volume mismatch by shrinking the electromagnetic resonator to sub‑millimeter dimensions, allowing strong spatial overlap with the optical fields.

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Operational Regime and Performance

Because the optical linewidth is two orders of magnitude narrower than the millimeter‑wave dissipation (γₘₘw/2π ≈ 15 GHz), the transducer operates in the reversed dissipation regime (κ ≪ γₘₘw). In this regime, the optical filter isolates the narrow slice of thermal millimeter‑wave photons that fall within its bandwidth, providing an internal standard for cooperativity measurement.

When driven coherently, electro‑optic dynamical back‑action manifests as linewidth modulation: the beam‑splitter up‑conversion interaction broadens the optical resonance, while parametric down‑conversion narrows it. These effects confirm coherent frequency conversion and open pathways to quantum‑limited transduction.

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Outlook and Challenges

The current off‑chip wireless efficiency is limited by free‑space coupling (≈1.6 × 10⁻⁴). The authors suggest that replacing the gold resonator with a high‑gap superconductor such as NbTiN could suppress internal ohmic loss and raise total transduction efficiency at liquid‑helium temperatures.

If these improvements materialize, the transducer could serve as a low‑noise bridge between cryogenic quantum processors—whether superconducting qubits, neutral‑atom arrays, or Rydberg platforms—and long‑distance optical fiber networks, enabling distributed quantum computing architectures without adding thermal load to the cold stage.

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Takeaway: EPFL’s integrated lithium‑niobate transducer demonstrates that high‑frequency millimeter‑wave signals can be coherently up‑converted to telecom‑band photons with a respectable 67 % internal efficiency, marking a practical advance toward scalable quantum interconnects.