Quantum electro-optic transducer hits 67% efficiency with light & millimeter waves

Researchers at École Polytechnique Fédérale de Lausanne have achieved 67% efficiency in a new transducer that converts signals between light and millimeter-wave frequencies. This record performance uses a device where lithium niobate and gold resonators are engineered to have comparable dimensions and minimal size, overcoming a key limitation in scaling electro-optic coupling. The work demonstrates coherent transduction with cooperativities approaching 0.3, also revealing electro-optic dynamical back-action, where measurement of one frequency influences the other, and establishing an architecture for sensitive quantum and classical networking.

Millimeter-Wave Transduction for Quantum and Classical Technologies

The demonstrated device reaches a vacuum electro-optic coupling rate six times larger than previous millimeter-wave transducers, a substantial increase enabling more efficient signal conversion between optical and microwave frequencies. This improvement stems from a triply resonant transducer integrating lithium niobate Fabry-Pérot cavities with a quarter-wave gold resonator operating at 263 GHz, a frequency band increasingly important for advanced communication systems. These coupled cavities possess free spectral ranges exceeding 650 GHz and quality factors reaching up to 10⁶, minimizing signal loss during transduction.

The architecture allows for room-temperature detection of thermal millimeter-wave photons at effective occupancies below one, a capability that is also a calibration method for determining transduction efficiency. Coherent transduction reaches cooperativities approaching 0.3, corresponding to internal efficiencies around 67 %, accompanied by electro-optic dynamical back-action that broadens or narrows the optical resonances. “These results demonstrate the advantage and feasibility of the frequency scaling of electro-optic coupling,” the authors state, “establishing an architecture for weak-signal sensing and quantum and classical networking.”

Achieving high efficiency in signal conversion between light and millimeter waves remains a significant challenge, yet a newly demonstrated device surpasses previous efforts by reaching efficiencies around 67% in this process. This performance relies on a transducer designed with optical and millimeter-wave resonators of comparable size and minimal volume, a configuration that had previously proven difficult to realize. This advancement occurs in a regime where millimeter-wave dissipation is almost two orders of magnitude greater than that of the optical components, a contrast to conventional electro-optic and optomechanical systems operating at microwave frequencies.

The transducer’s design, described as a photonic molecule, uses a shared, partially transmissive photonic crystal mirror to evanescently couple the Fabry-Pérot cavities. Coupled-mode description of the device reveals a three-mode interaction Hamiltonian, where the optical splitting is resonant with the millimeter-wave mode, enabling triply resonant operation.

The efficiency calculation peaks when the drive frequency matches the optical splitting, and uses thermal sidebands as a calibration tone. The device’s ability to operate with high efficiency and suppression of unwanted signals positions it as a promising component for advanced quantum and classical technologies.

This level of efficiency is not merely incremental. It represents an improvement over existing millimeter-wave transducers and validates the principle of frequency scaling for electro-optic coupling.

Room-Temperature Detection of Thermal Millimeter-Wave Photons

This performance leap allows for the direct optical detection of millimeter-wave photons emitted by thermal fluctuations at room temperature, even when those signals represent less than one excitation, an effective occupancy below one, and simultaneously provides a built-in calibration standard for assessing transduction efficiency. Analysis of the thermal photon detection reveals a consistent extraction of cooperativity values approaching 0.3, which directly corresponds to the reported internal efficiencies around 67 percent, as confirmed through independent analysis of averaged optical spectra. The system’s ability to detect these thermal photons above 100 GHz establishes a new reference point for calibrating transducer performance, using the inherent thermal signal as an absolute standard.

On-chip millimeter-wave-to-optical transduction efficiency reaches 1.6 x 10^-4, corresponding to an internal efficiency of 67 percent, a figure consistent with the analysis of thermal sidebands. Attenuation of the source power to avoid nonlinear frequency shifts during millimeter-wave frequency sweeps confirms the stability and reliability of the measurements.

Cooperativity of 0.3 Demonstrates Efficient Transduction

This level of interaction allows for control over the spectrum and phase of the interacting modes, mirroring techniques used in Brillouin lasers for functions like tunable filtering and amplification. The benefits of scaling down device dimensions to enhance frequency response are often offset by reductions in optical quality and spatial overlap, ultimately limiting cooperativity; however, this work circumvents that trade-off. A comparison of current microwave and millimeter-wave devices reveals that while transduction frequency increased eighty-fold, from approximately 4 to 310 GHz, the coupling rate only increased tenfold, falling short of the expected linear scaling.

This discrepancy highlights the challenges in maintaining performance during miniaturization, challenges now demonstrably overcome. Analysis of sideband power, measured through optical linewidth variation, confirms the impact of cooperativity on transduction bandwidth.

Conversely, at a higher cooperativity of 0. 25, the optical linewidth of sideband modes exhibited linear broadening and narrowing, consistent with predictions of dynamical back-action. “Consequently, gains from scaling the devices down are paid for with degraded optical quality factors and reduced overlap, with a net reduction in cooperativity,” the researchers write, acknowledging the inherent difficulties and their successful mitigation. The measured efficiency of the device corresponds to a maximum internal efficiency of approximately 0. 67, aligning with both the achieved cooperativity and thermal sideband analysis.

Further improvements are possible through material choices. The current gold resonator could be replaced with a superconductor like NbTiN or NbN, reducing the millimeter-wave loss rate by more than an order of magnitude. This substitution would increase the ratio of external to internal loss, and consequently, the cooperativity and transduction efficiency at a fixed pump power. The fabrication process is already compatible with these superconducting films, suggesting a clear path toward even higher performance in future iterations.

Electro-Optic Dynamical Back-Action Modifies Resonance Spectrum

The design matches optical and millimeter-wave resonator dimensions, a key step in realizing the predicted scaling of electro-optic coupling with frequency. This frequency scaling unlocks opportunities in areas where cooperativity, the ratio of coupling strength to loss, is the limiting factor, including transduction of quantum states between processors, advanced sensing applications and optical networks. Researchers observed that the transducer’s performance is accompanied by electro-optic dynamical back-action, a phenomenon where the measurement of one wave influences the other, modifying the damping rates of the interacting modes and allowing control over their spectrum and phase.

Beyond calibration, the observation of dynamical back-action demonstrates the ability to reshape the optical spectrum in situ, potentially switching an optical resonance between under- and overcoupling. “At the cooperativities demonstrated here, back-action already provides a tuning knob for the optical damping rate, as observed in our experiment,” the researchers report. The achievement of linear frequency scaling is a direct result of the matched resonator dimensions, according to the team led by Yazan Lampert.

EPFL Hybrid Photonics Laboratory Advances Transducer Architecture

The École Polytechnique Fédérale de Lausanne (EPFL) has demonstrated a transducer capable of detecting its own thermal environment at the sub-photon level, a feat enabled by a device that efficiently converts light and millimeter waves. This capability establishes that optical readout of quantum-level millimeter-wave cavity fields is possible, opening avenues for sensitive detection and potentially new quantum technologies. This transducer’s architecture allows for adaptability, with simple changes to resonator length enabling operation at different frequencies.

The flat electro-optic response of lithium niobate further positions the design as a potential route to quantum transduction and high-efficiency wireless optical interfaces. Researchers at EPFL are targeting the sub-terahertz bands relevant to emerging 6G communication standards with this technology, suggesting a path toward faster and more efficient wireless systems.

The optical and millimeter-wave responses of the transducer were characterized using a setup involving a tunable laser, polarization controller and optical spectrum analyzer, as detailed in their published work. Analysis of the device’s optical reflection spectrum revealed a sequence of doublets, where observed splittings between hybrid modes are determined by the fabrication-defined coupling between cavities and their detuning. The ability to adapt the device for operation at different frequencies, combined with its efficient energy conversion, is a key feature distinguishing it from previous designs.

The demonstrated ability to operate at these frequencies, coupled with the sensitivity achieved, could lead to improvements in signal processing and the development of new tools for analyzing millimeter-wave signals. The EPFL team’s work highlights a promising approach to bridging the gap between optical and millimeter-wave technologies, potentially unlocking new capabilities in both quantum and classical domains.

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