All-photonic millimeter-wave and terahertz signal core at room and cryogenic temperatures

arXiv:2610.30857 · physics.optics, physics.app-ph, quant-ph · Submitted 2026-10-22 · Read on arXiv

Listen

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: We're looking at the paper: "All-photonic millimeter-wave and terahertz signal core at room and cryogenic temperatures".

Mira: Here's the gist of the paper. Detailed Research Summary: All-Photonic Millimeter-Wave and Terahertz Signal Core This research presents a novel, monolithic lithium tantalate (LiTa) photonic chip architecture termed a "signal core," designed to serve as an end-to-end platform for generating, modulating, routing, and detecting continuous-wave signals across the millimeter-wave (mmWave) to terahertz (THz) spectrum (100 GHz to 2.5 THz).

Kai: The primary motivation is to overcome the limitations of existing electronic signal chains in this high-frequency regime, which are typically constrained by weak sources, noisy receivers, and lossy interconnects.

Mira: Core Architecture and Material Innovation: The signal core integrates high-frequency functional modules onto a single chip using micro-coplanar striplines (CPS) for interconnectivity.

Kai: A key innovation is the realization of this core in thin-film lithium tantalate, which provides a unique combination of strong chi(2) nonlinearity, low optical loss, a high optical damage threshold, engineered phase matching, and wafer-scale manufacturability.

Title and authors: Kai: The paper introduces this "signal core" which is essentially a single chip that can generate, modulate, route, and detect continuous-wave signals across that whole frequency spectrum of one hundred gigahertz to two point five terahertz <ref:2610.30857#pg2>.

Mira: It’s a monolithic lithium tantalate chip using micro-coplanar striplines for the interconnectivity; so you get these high-frequency functional modules all on one piece of silicon, which is pretty dense.

Lev: If we look at that integration, it means we’re trying to put the entire signal chain on a single die instead of having separate electrical components and optical links somewhere else, which really reduces the complexity for real hardware implementation.

Kai: Exactly; they highlight that this setup uses exclusively fiber-based optical interfaces, which is pretty cool because it lets you run this core at room temperature or even at cryogenic temperatures of four Kelvin through those same links.

Mira: That part about operating at four Kelvin through fiber-only links is significant because it pushes the all-photonic concept into environments that usually demand heavy active cooling, which makes it relevant for quantum hardware control.

The paper's summary: Kai: The core idea they are presenting is a two-resonantly enhanced module architecture where the generator creates the signal using difference-frequency generation within a lithium tantalate racetrack resonator.

Mira: So they pump it with two optical pumps at these angular frequencies, say omega one and omega two, to get that difference frequency Omega equal to omega two minus omega one inside the coplanar stripline cavity.

Lev: That DFG mechanism is interesting because it’s a way to generate the desired signal field where you have this resonance enhancement from both the generator and the detector modules coupled together.

Kai: And then they modulate that generated field using bidirectional coherent electro-optic conversion in their detector module, which creates sidebands at frequencies omega three plus or minus Omega.

Mira: The real technical hurdle they address is achieving simultaneous triply resonant conversion, which requires four specific spectral conditions to be met for the whole system to function optimally.

Lev: They also managed to extract a single-photon electro-optic coupling rate of g0 over two pi being about ten kilohertz, which confirms it’s viable for terahertz applications, even if that rate is still relatively low compared to what you might hope for <ref:2610.30857#pg2,single-photon electro-optic coupling>.

The paper's improvements: Kai: One major point they push is the pump loading efficiency; they show a high pump loading efficiency greater than ninety-five percent even when the intracavity pump powers are over seventy milliwatts in this lithium tantalate platform.

Mira: That’s a big deal because it’s better than what you see in lithium niobate devices where their maximum combined loading metric was limited to approximately eta load equal to seventy-five percent at higher power levels.

Lev: From a hardware standpoint, that high efficiency means less input power is wasted as heat or just not converted into the desired signal field, which simplifies designing the actual physical chip you’d build for real hardware.

Kai: And they also show that this system operates up to two point five terahertz at room temperature and can extend operation up to five hundred seventy-six gigahertz when it's cooled down to four Kelvin using those fiber-only interfaces.

Mira: They also provided a rigorous analysis of the loss, showing a crossover point where conductor loss starts dominating over the electro-optic interaction, moving from conductor loss dominated behavior near one hundred gigahertz to radiation loss dominated behavior near three hundred gigahertz <ref:2610.30857#pg3>.

Conclusion: Kai: So, this "All-photonic millimeter-wave and terahertz signal core at room and cryogenic temperatures" successfully establishes a compact, monolithic photonic platform that lets you control and read out millimeter-wave quantum hardware without needing external high-frequency electrical signal chains.

Mira: It shifts the instrumentation from being big rack assemblies to circuits that are co-located with the device under test, which really makes sense for dense integration.

Lev: For us on the error correction side, knowing you have this integrated source and detector means you don't have to worry about external noise or lossy links degrading your measurements as much when we try to run real hardware.

Kai: The authors flag that although the electro-optic interaction gets stronger with frequency, eventually conductor and radiation losses outweigh that advantage, which is a key design rule they established for future work.

Mira: They also pointed out that their method relies on those four critical spectral conditions aligning perfectly to work optimally, so tuning it up to run consistently across the whole band will be a major engineering task.

Lev: Overall, this paper lays out how to build a core component that could actually be used in systems where we need coherent control over qubits at these high frequencies.

Kai: And that’s where we are with the paper on the "All-photonic millimeter-wave and terahertz signal core at room and cryogenic temperatures".

Jiawen Liu, Chengli Wang, Aleksei Gaier, Tianyi Zhang, Gabriel Jülg, Yazan Lampert, André Garcia Primo, Alessandro Tomasino, Hua Li, & Xin Ou, & Ileana-Cristina Benea-Chelmus

Hybrid Photonics Laboratory, École Polytechnique Fédérale de Lausanne (EPFL) · Center for Quantum Science and Engineering (EPFL) · National Key Laboratory of Materials for Integrated Circuits, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences

physics.optics, physics.app-ph, quant-ph

Submitted: 2026-10-22

Updated: 2026-10-22

License: http://creativecommons.org/licenses/by-nc-nd/4.0/

The gist: Detailed Research Summary: All-Photonic Millimeter-Wave and Terahertz Signal Core This research presents a novel, monolithic lithium tantalate (LiTa) photonic chip architecture termed a "signal

Key concepts

Lithium Tantalate (LiTa): A specific thin-film material used to build the chip's core. It is chosen because it has strong nonlinear properties ($\chi^{(2)}$), low optical loss, and good manufacturability, making it superior to other materials for this high-frequency application.
Difference-Frequency Generation (DFG): The signal generation method where two optical pump beams at different frequencies ($\omega_1$ and $\omega_2$) interact within a lithium tantalate resonator. This interaction creates a new, desired output frequency ($\Omega = \omega_2 - \omega_1$), which is the base signal for the system.
Coherent Electro-Optic Conversion: The process used by the detector module to modulate and read out signals. It involves converting an optical signal into an electrical one, and then back again, allowing for precise modulation of sidebands at frequencies related to the generated difference frequency.

Terminology

Core — 1× in this paper · explained in 2 episodes

Summary

Detailed Research Summary: All-Photonic Millimeter-Wave and Terahertz Signal Core

This research presents a novel, monolithic lithium tantalate (LiTa) photonic chip architecture termed a signal core, designed to serve as an end-to-end platform for generating, modulating, routing, and detecting continuous-wave signals across the millimeter-wave (mmWave) to terahertz (THz) spectrum (100 GHz to 2.5 THz). The primary motivation is to overcome the limitations of existing electronic signal chains in this high-frequency regime, which are typically constrained by weak sources, noisy receivers, and lossy interconnects.

Core Architecture and Material Innovation:

The signal core integrates high-frequency functional modules onto a single chip using micro-coplanar striplines (CPS) for interconnectivity. A key innovation is the realization of this core in thin-film lithium tantalate, which provides a unique combination of strong chi(2) nonlinearity, low optical loss, a high optical damage threshold, engineered phase matching, and wafer-scale manufacturability. This material choice is highlighted as superior to lithium niobate control devices in terms of pump loading efficiency.

The system operates end-to-end using exclusively fiber-based optical interfaces, allowing the core to function at room temperature and even at cryogenic temperatures (4 K) via these same links, establishing a truly all-photonic platform for mmWave-THz systems.

Signal Generation and Modulation Mechanism:

The signal generation relies on a two-resonantly enhanced module architecture. The generator produces the desired signal via difference-frequency generation (DFG) within a lithium tantalate racetrack resonator. This process is driven by optical pumps (omega 1 and omega 2) at angular frequencies, resulting in the generation of a field at the difference frequency = omega 2 - omega 1 within the coplanar-stripline (CPS) cavity.

This generated field is subsequently modulated by the detector module via bidirectional coherent electro-optic conversion. This modulation produces sidebands at frequencies omega 3 plus or minus. The system achieves simultaneous triply resonant conversion, requiring four critical spectral conditions to function optimally:

Alignment of pump tones with optical racetrack resonances.
Matching of the difference frequency to the generator CPS resonance.
Alignment of sidebands with detector racetrack modes.
Coincidence of both generator and detector CPS resonances.

The coupled CPS cavities enable significant resonant field enhancement for both generation and detection, allowing the amplitude-transfer factor (E out det / E DFG) to exceed unity near coupled-cavity eigenfrequencies, effectively compensating for the inherently low single-photon electro-optic coupling rate (g 0/2 pi about 10 kHz).

Performance Metrics and Validation:

The realization of the signal core validates several key benchmarks:

Coupling Rate: The single-photon electro-optic coupling rate (g 0/2 pi) was successfully extracted to the 10 kHz scale, confirming its viability for terahertz applications.
Pump Loading Efficiency: The system demonstrates high pump loading efficiency (> 95%) even when intracavity pump powers exceed 70 mW in the LiTa platform, a significant advantage over lithium niobate devices where the maximum combined loading metric was limited to approximately eta load = 75% at higher power.
Frequency Operation: The system operates up to 2.5 THz at room temperature and extends operation up to 576 GHz when cooled to 4 K via fiber-only interfaces.

Loss Analysis and Characterization:

The paper provides a rigorous analysis of propagation loss, establishing a central design rule: although the electro-optic interaction strengthens with frequency, increasing conductor and radiation losses eventually outweigh this advantage. The extracted loss coefficients (alpha) show a crossover from conductor-loss-dominated behavior (e.g., alpha about 1.8 dB/mm near 100 GHz) to radiation-loss-dominated behavior (e.g., alpha about 3 dB/mm near 300 GHz). Furthermore, an in situ photonic probing method based on the signal core architecture allows for the measurement of on-chip mmWave-THz propagation loss by analyzing sideband power reduction between co-propagating and counter-propagating fields.

Cryogenic Operation Details:

At 4 K, end-to-end measurements confirm that transmission changes are less than 1 dB compared to ambient conditions, although optical resonances exhibit a slight shift toward shorter wavelengths. The measured round-trip field-transmission coefficients (alpha rt = 0.9888 plus or minus 0.0003 for the generator and alpha det = 0.9948 plus or minus 0.0004 for the detector) are very high, supporting low intrinsic loss within the chip structure itself.

Conclusion and Future Directions:

The signal core successfully establishes a compact, monolithic photonic platform capable of controlling and reading out mmWave quantum hardware without requiring external high-frequency electrical signal chains. This architecture shifts high-frequency instrumentation from rack-level assemblies to fiber-addressed circuits co-located with the device under test. Future research will focus on spectral agility through reconfigurable photonic molecules and mitigating propagation loss further by improving conductor geometry or utilizing low-refractive-index substrates. The platform is highly relevant for next-generation applications, including mmWave superconducting qubits and sub-THz transducers.

Improvements for AI systems

Improve high-frequency signal processing in communication systems by using a monolithic lithium tantalate chip as a signal core to generate, modulate, route, and detect continuous-wave signals between 100 GHz and 2.5 THz. This allows for integration of generation and detection functions on a single chip to mitigate the constraints of weak sources and lossy off-chip links.
Develop ultra-compact cryogenic systems for quantum hardware control by implementing an all-photonic platform that operates at 4 K through fiber-only optical interfaces, enabling the coherent generation, routing, and detection of millimeter-wave and terahertz signals required for dispersive readout of superconducting qubits.
Create programmable mmWave-THz waveform synthesizers that allow for nanosecond-scale control over the amplitude, phase, and instantaneous frequency of high-frequency fields by modulating optical pump tones on chip using arbitrary waveform generators. This capability is crucial for coherent communications and pulsed control of high-frequency quantum systems.
Implement in situ photonic probing methods to measure on-chip mmWave-THz propagation loss by analyzing the contrast between co-propagating and counter-propagating optical sidebands, allowing for direct characterization of conductor and radiation loss across the frequency spectrum.
Design novel architectures for integrated high-frequency devices that embed a device under test directly within the CPS link, using it as a local source and coherent analyzer to synthesize, deliver, and recover high-frequency fields in situ without external instrumentation.

Abstract

Millimeter-wave and terahertz technologies offer new opportunities for high-capacity communications, sensing, and quantum systems, but electronic signal chains above 100 GHz are limited by weak sources, noisy receivers, and lossy interconnects. Photonic approaches mitigate these constraints by enabling chip-scale high-frequency components, yet existing implementations generally retain lossy off-chip electrical or free-space links, hindering dense and compact integration for cryogenic or space-constrained settings. Here, we show a monolithic lithium tantalate photonic chip, termed a signal core, which generates, modulates, routes, and detects continuous-wave signals at discrete frequencies between 100 GHz and 2.5 THz. The architecture integrates high-frequency functional modules on chip and uses exclusively fiber-based optical interfaces. Bidirectional coherent electro-optic conversion enables signal generation, detection, and nanosecond-scale amplitude and phase modulation, while micro-coplanar striplines interconnect the modules into an end-to-end signal chain on a single chip. Our modules achieve single-photon electro-optic coupling rates of g 0/2π=10 kHz and peak drive-power-normalized photon-flux conversion efficiencies of 1.0%/mW for generation and 0.5%/mW for detection. It also operates at 4 K in a dilution refrigerator through the same fiber-only optical links. Together, these capabilities establish an all-photonic platform for millimeter-wave-to-terahertz systems and offer a broadly applicable architecture for applications constrained by electrical-interconnect loss, heat load, and footprint.