Researchers at the École Polytechnique Fédérale de Lausanne (EPFL) Hybrid Photonics Laboratory and Center for Quantum Science and Engineering have demonstrated an integrated electro-optic transducer bridging the millimeter-wave (mm-wave) and telecom optical domains. Detailed in the paper “Coherent transduction and dynamical back-action between light and millimeter waves,” the architecture couples a thin-film lithium niobate (TFLN) photonic molecule to a sub-millimeter metallic resonator, achieving a vacuum electro-optic coupling rate of approximately 60 kHz and an internal conversion efficiency of 67%.
| EPFL Integrated Millimeter-Wave-to-Optical Transducer Specifications | ||
|---|---|---|
| Parameter | Measured Value | Architectural Significance |
| Operating Frequencies | 263 GHz (mm-wave) / 192 THz (1,561 nm telecom optical) | Interfaces high-frequency cryogenic modes and atomic Rydberg transitions directly with telecom C-band optical infrastructure. |
| Optical Resonators | Coupled Fabry–Pérot cavities in TFLN (lengths 110 μm and 119 μm) | Achieves free spectral ranges exceeding 650 GHz with optical quality factors up to 106 (κ± / 2π ≈ 186–230 MHz). |
| Electromagnetic Resonator | Quarter-wave gold resonator (length ≈ 165 μm) | Replaces macroscopic lines to equalize optical and mm-wave mode volumes, eliminating spatial overlap penalties. |
| Vacuum Coupling Rate (g0 / 2π) | 60 ± 8 kHz | Scales g0 linearly with frequency, exceeding microwave-domain electro-optic coupling rates by a factor of 60. |
| Electro-Optic Cooperativity (C) | Up to 0.27 ± 0.03 | Yields 67% internal photon conversion efficiency and drives observable dynamical back-action linewidth modulation. |
| Effective Thermal Detection | 0.21 ± 0.08 photons | Resolves thermal background occupancies below the single-quantum limit directly via optical sideband detection. |
Coherent frequency conversion between high-frequency electric fields and optical carriers serves as a critical interface for distributed quantum architectures, linking cryogenic processors and neutral-atom registers over optical fiber without introducing thermal load. While vacuum electro-optic coupling scales with operating frequency according to g0 ∝ (ωmmw / Vmmw)1/2, conventional implementations suffered from mode-volume mismatch: sub-millimeter electromagnetic fields were paired with millimeter-scale optical racetrack resonators, degrading field overlap and reducing net cooperativity.
The EPFL design resolves this spatial discrepancy by defining two compact Fabry–Pérot microcavities via photonic crystal mirrors etched into a suspended 400 nm TFLN membrane. Integrated DC tuning electrodes adjust the residual detuning between the resulting symmetric and anti-symmetric modes to align the doublet splitting with the adjacent quarter-wavelength resonator. Complete polarization alignment along the crystalline Z-axis maximizes interaction with lithium niobate’s primary Pockels tensor component (r33).
Because the optical linewidth is roughly two orders of magnitude narrower than the dissipative mm-wave resonance (γmmw / 2π ≈ 15 GHz), the transducer operates in the reversed dissipation regime (κ± ≪ γmmw). The optical filter isolates the narrow spectral slice of room-temperature thermal mm-wave photons falling within its bandwidth, enabling the thermal sideband to function as an uncalibrated internal standard for measuring device cooperativity.
Under coherent drive, electro-optic dynamical back-action modulates the optical response: the beam-splitter up-conversion interaction introduces an additional dissipation path that broadens the optical linewidth, while parametric down-conversion anti-damps and narrows the mode. Although total off-chip wireless efficiency remains constrained by free-space beam coupling into the integrated antenna (1.6 × 10−4), transitioning from a normal-metal gold resonator to high-gap superconductors such as niobium titanium nitride (NbTiN) is projected to suppress internal ohmic damping and increase total transduction efficiency at liquid-helium temperatures.
The research is detailed in the technical preprint here.
In GQI Portal
The players behind the news
The team that writes QCR tracks every company, deal and technology in the GQI Factory, GQI's verified database of the quantum industry. Next up: every story linked to its players, coming to QCR's paid plans.
- Players Companies and institutions across the quantum industry, by segment.
- Scorecards How the players compare on hardware, software, funding and more.
Newsletter
QCR Alerts in your inbox
The latest reporting and analysis from Quantum Computing Report. Free, unsubscribe any time.