⚛️ quantum physics

Electro-optic conversion of itinerant Fock states

This paper demonstrates the first successful on-demand electro-optic conversion of itinerant non-Gaussian microwave Fock states from a superconducting qubit to telecom photons with negligible added noise, establishing a viable pathway for connecting modular cryogenic quantum nodes via optical fiber.

Original authors: Thomas Werner, Erfan Riyazi, Samarth Hawaldar, Rishabh Sahu, Georg Arnold, Paul Falthansl-Scheinecker, Jennifer A. Sánchez Naranjo, Dante Loi, Lucky N. Kapoor, Martin Zemlicka, Liu Qiu, Andrei Militar

Published 2026-02-03

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Thomas Werner, Erfan Riyazi, Samarth Hawaldar, Rishabh Sahu, Georg Arnold, Paul Falthansl-Scheinecker, Jennifer A. Sánchez Naranjo, Dante Loi, Lucky N. Kapoor, Martin Zemlicka, Liu Qiu, Andrei Militaru, Johannes M. Fink

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a super-fast, super-powerful computer brain made of metal that lives in a freezer colder than outer space. This is a superconducting quantum computer. It's incredibly fast at solving problems, but it has a major flaw: it can only "speak" a language called microwaves.

The problem is that microwaves are like whispers in a hurricane. If you try to send them out of the freezer into a normal room, the heat and noise of the room drown them out instantly. This means these powerful quantum computers are stuck in the freezer, unable to talk to each other or to the outside world.

On the other hand, the internet uses light (fiber optics) to send information. Light is like a shout that can travel across the world without losing its voice, even in a warm room.

The Big Challenge
Scientists have been trying to build a "translator" that can take the quantum computer's microwave whispers and turn them into light shouts, so they can travel through fiber optic cables. But there's a catch: quantum information is incredibly fragile. If the translator is too noisy or clumsy, it destroys the message. Until now, no one had successfully translated a single, specific quantum particle (a "Fock state") from microwave to light without losing its special quantum properties.

What This Paper Did
The researchers at the Institute of Science and Technology Austria built a new kind of translator and successfully pulled off this difficult trick. Here is how they did it, step-by-step:

Creating the Message: They used a tiny quantum bit (a qubit) inside a metal box to generate a single, perfect microwave photon. Think of this as a single, pure note played on a violin inside a soundproof room.
The Translator (The Transducer): They built a special device that acts like a bridge. It has a tiny spinning disk made of a special crystal (Lithium Niobate).
- They shine a strong laser (the "pump") onto this disk.
- When the single microwave note hits the disk, the laser helps "kick" it up in energy, turning it from a microwave whisper into a light shout (an infrared photon).
- Crucially, they did this so gently that the original quantum computer wasn't disturbed.
The Result: They successfully caught the new light photon on the other side. They proved it was the same "message" by showing that the light photon arrived exactly when the microwave note was sent, and that it retained its unique quantum shape.

The "Noise" Problem
In any translation, there's static. The researchers had to be very careful to ensure the translator didn't add its own "static" (noise) to the message.

They found that if they sent too many messages too quickly, the translator got slightly warm, which added static.
However, by sending messages slowly, they kept the static incredibly low. They achieved a "Signal-to-Noise Ratio" of about 5. This means the message was five times louder than the background static. In the world of quantum physics, this is a clear, loud voice.

Why This Matters (According to the Paper)
The paper claims this is a major step forward because:

It works on demand: They can create the message and translate it whenever they want.
It preserves the secret: The quantum nature of the message survived the trip from microwave to light.
It opens the door to networks: This proves we can eventually connect separate quantum computers (located in different freezers) using standard fiber optic cables, creating a "quantum internet."

The Bottom Line
Think of this as the first time someone successfully mailed a fragile, glowing crystal from a deep-freeze vault to a sunny garden without it melting or breaking. They built a special box (the transducer) that changed the crystal's form just enough to survive the journey, proving that we can finally connect these super-fast quantum computers to the world outside their freezers.

Technical Summary: Electro-optic conversion of itinerant Fock states

Problem Statement
Superconducting qubits are a leading platform for quantum computing due to their fast gate speeds and scalability via nanofabrication. However, their operation at microwave frequencies (e.g., 10 GHz) necessitates millikelvin temperatures to avoid thermal noise, creating a severe bottleneck for scaling and long-range quantum networking. While optical fiber links operating at telecom wavelengths (~193 THz) are ideal for distributing quantum information at room temperature, superconducting circuits lack coherent optical transitions. Consequently, converting non-classical microwave states (specifically itinerant Fock states) to the optical domain with high fidelity and low noise has remained an outstanding milestone. Previous demonstrations were limited to classical coherent states or did not successfully convert itinerant non-Gaussian states without affecting the generating qubit.

Methodology
The authors constructed a hybrid experimental setup integrating a superconducting qubit source with an electro-optic (EO) transducer.

Source: A transmon qubit embedded in a 3D aluminum cavity (qubit-cavity) generates itinerant single microwave photons (SP) at 8.9 GHz. This is achieved by driving a blue sideband (BSB) transition between the joint states $|g, 0\rangle$ and $|e, 1\rangle$ , causing the photon to leak into an output waveguide.
Transducer: The microwave photon is directed to an EO transducer based on the Pockels effect. The device consists of a 3D superconducting microwave cavity containing an embedded disk-shaped lithium niobate (LiNbO $_3$ ) whispering gallery mode resonator (WGMR). Thin-film aluminum electrodes are evaporated directly onto the LiNbO $_3$ surfaces to enhance mode overlap.
Conversion Process: The system operates in a "load-and-convert" regime. First, the itinerant microwave photon loads into the transducer's microwave mode. Once the occupation is maximized, a synchronized 200 ns optical pump pulse (1.22 mW at 193.4 THz) is applied to upconvert the microwave photon to the optical domain via a beam-splitter interaction.
Detection: The upconverted optical signal is filtered using a chain of four cascaded Fabry-Perot cavities to suppress the pump and Brillouin noise. The resulting single infrared photons are detected by a superconducting nanowire single-photon detector (SNSPD) with ~85% efficiency.
Characterization: The team performed state tomography in the microwave domain (using a Josephson parametric amplifier and heterodyne detection) and optical domain (photon counting) to verify the non-classical nature of the states and quantify the signal-to-noise ratio (SNR).

Key Contributions

First Demonstration of Itinerant Fock State Conversion: The work demonstrates the on-demand generation and tomographic reconstruction of itinerant single microwave photons from a superconducting qubit and their subsequent upconversion to the optical domain.
Non-Destructive Conversion: By upconverting the photon after it has left the qubit-cavity, the process ensures the conversion mechanism does not disturb the superconducting qubit used for generation.
Low-Noise Performance: The transducer achieves an added noise of below 0.012 quanta (at 1 kHz repetition rate) and a signal-to-noise ratio of up to $5.1 \pm 1.1$ for the converted telecom photons.
Comprehensive Noise Analysis: The authors distinguish between optical noise (filter leakage, dark counts) and upconverted thermal photons, identifying the latter as the primary limitation at higher repetition rates due to optical absorption heating.

Results

State Fidelity: The reconstructed Wigner functions for the generated half-photon (HP) and single-photon (SP) states show fidelities of 99.7% and 97.6%, respectively, confirming the non-Gaussian nature of the states.
Signal-to-Noise Ratio (SNR): In the optical domain, the SNR reaches $5.1 \pm 1.1$ at low repetition rates (250 Hz), decreasing to $1.2 \pm 0.1$ at 20 kHz due to thermal population of the microwave mode. The SNR exceeds unity by more than five standard deviations.
Rabi Oscillations: The authors successfully performed an optical single-photon Rabi measurement, observing correlations between the qubit's rotation angle and the detected optical photon counts, demonstrating phase coherence preservation.
Efficiency: The total external conversion efficiency is $\eta_{ext} \approx 2.2 \times 10^{-4}$ , with an internal efficiency of $\eta_{int} \approx 1.6 \times 10^{-3}$ . The single-photon detection efficiency is measured at $(1.32 \pm 0.03) \times 10^{-5}$ .
Noise Characterization: The input-referred added noise ranges from 0.012 quanta (at 1 kHz) to 0.12 quanta (at 20 kHz), placing the device within the quantum limit with a positive quantum channel capacity.

Significance and Claims
The paper claims to establish a viable path toward heralded entanglement distribution and gate teleportation between superconducting nodes and optical networks. By demonstrating that non-classical itinerant states can be converted with high SNR and low added noise, the work overcomes a major hurdle in quantum interconnects for superconducting circuits. The authors assert that these results empower existing superconducting devices to play a key role in distributed quantum technologies and heterogeneous quantum systems, bridging the gap between fast superconducting processors and long-range optical communication. They note that while current throughput is limited, the platform is independent of the specific qubit type (applicable to NV centers and spin qubits as well), suggesting a scalable route for integrating superconducting nodes into existing photonic quantum networks.

Technical Summary: Electro-optic conversion of itinerant Fock states

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