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Brillouin Light Scattering Spectroscopy of Propagating Magnons at Sub-Kelvin Temperatures

This paper reports the first optical detection of coherently driven, propagating spin waves via Brillouin Light Scattering spectroscopy inside a dilution refrigerator, successfully establishing a sub-kelvin optical interface for magnons that is essential for hybrid quantum transduction.

Original authors: David Schmoll, Nikolai Kuznetsov, Phillip Rehberger, Franz Vilsmeier, Roman Verba, Denys Slobodianiuk, Rostyslav O. Serha, Khrystyna O. Levchenko, Sebastiaan van Dijken, Andrii V. Chumak, Sebastian Kn

Published 2026-06-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: David Schmoll, Nikolai Kuznetsov, Phillip Rehberger, Franz Vilsmeier, Roman Verba, Denys Slobodianiuk, Rostyslav O. Serha, Khrystyna O. Levchenko, Sebastiaan van Dijken, Andrii V. Chumak, Sebastian Knauer

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 a world where information isn't just carried by electricity or light, but by tiny ripples in a magnetic field. Scientists call these ripples "spin waves," and the individual packets of energy that make them up are called magnons. Think of magnons like the individual notes in a musical chord; they are the fundamental building blocks of magnetic information.

For a long time, scientists have been able to "listen" to these notes using a technique called Brillouin Light Scattering (BLS). It's like shining a flashlight at a vibrating guitar string and analyzing the color change in the reflected light to hear the note. However, there's a catch: to hear the quietest, purest notes (the quantum level), you have to freeze the guitar string until it's almost completely still. This requires temperatures colder than outer space, known as "sub-Kelvin" temperatures.

The Problem: The Cold Room vs. The Flashlight
Scientists have been working in these ultra-cold "freezers" (called dilution refrigerators) for years, but they could only "listen" to the magnons using electrical wires. They wanted to use light (lasers) because it's better for connecting different types of quantum computers, but shining a laser into a super-cold freezer was a nightmare. The laser beam acts like a tiny, focused heater. If you shine it too brightly, it warms up the sample, ruining the experiment. If you shine it too dimly, you can't see anything.

The Breakthrough: A New Kind of Microscope
This paper describes the first time scientists successfully used a laser to "see" these magnetic ripples inside a super-cold freezer. They managed to build a special setup where a laser beam could enter the freezer, hit a magnetic film, and bounce back out to be analyzed, all while keeping the sample incredibly cold.

Here is how they did it, using some simple analogies:

The Stage: They used a very thin film of a special material called Yttrium Iron Garnet (YIG), sitting on a sapphire-like base. Think of this as a trampoline.
The Drummer: They used a tiny electrical antenna to "drum" on the trampoline, creating the magnetic ripples (magnons).
The Flashlight: They shined a green laser beam through the bottom of the stage to look at the ripples.
The Thermometer: They had a super-sensitive thermometer to check the temperature.

The Results: Seeing the Invisible
The team proved their setup worked by showing that the "sound" they heard with the laser matched perfectly with the "sound" they heard with the electrical wires. It was like having two different microphones recording the same song, and both recordings were identical. This confirmed they were actually seeing the same magnetic ripples.

The Temperature Challenge
The biggest hurdle was heat. Even a tiny bit of laser light can warm up the sample.

The Global Temperature: The whole freezer was kept at a bone-chilling 510 millikelvin (that's 0.51 degrees above absolute zero).
The Local Temperature: However, right where the laser hit the sample, the heat built up a little bit, raising the temperature to about 900 millikelvin (0.9 degrees).

To put this in perspective: The paper claims that even with this tiny "hotspot" created by the laser, the sample is still in the "sub-Kelvin" regime (colder than 1 Kelvin). This is a massive milestone because it means we can finally use light to talk to quantum magnetic systems without melting the ice.

The "Pulse" Trick
To make things even colder, the scientists tried a trick: instead of keeping the laser on like a steady lightbulb, they turned it on and off very quickly (like a strobe light).

When they did this, the average temperature of the whole freezer dropped to 240 millikelvin.
However, the spot where the laser hit still got hot (up to 1.68 Kelvin) for a split second every time the laser flashed, because the material is a poor conductor of heat at these temperatures. It's like hitting a frozen pond with a hammer; the spot you hit gets warm instantly, but the rest of the pond stays frozen.

What This Means (According to the Paper)
The paper concludes that they have successfully opened a "window" into the quantum world of magnets using light. They haven't built a quantum computer yet, but they have built the first pair of "glasses" that allow scientists to look at these magnetic particles in the extreme cold required for quantum work.

They note that to reach the "single-magnon" level (where only one single ripple exists at a time), they would need to get even colder or use smaller samples, but their current setup proves the door is open. They have shown that it is possible to combine the precision of light with the extreme cold of quantum physics, paving the way for future experiments where light and magnetism work together to carry quantum information.

Technical Summary: Brillouin Light Scattering Spectroscopy of Propagating Magnons at Sub-Kelvin Temperatures

Problem Statement
Magnons, the quanta of spin waves, are promising carriers for hybrid quantum networks and quantum transduction due to their broad frequency range and ability to couple with photons, phonons, and superconducting qubits. However, exploiting magnons in the quantum regime requires suppressing thermal magnon populations, necessitating millikelvin temperatures achievable only in dilution refrigerators (DR). While electrical excitation and readout of spin waves have been successfully adapted to DR environments, a critical optical interface for microwave-to-optical photon conversion has been missing. Existing optical detection techniques, such as Brillouin Light Scattering (BLS) spectroscopy, have historically been limited to liquid-helium temperatures and could not be implemented inside the complex thermal stages of a dilution refrigerator.

Methodology
The authors developed an experimental setup integrating a dilution refrigerator with a conventional backscattering BLS spectroscopy assembly.

Sample and Excitation: Spin waves were excited in a 18 µm-thick Yttrium Iron Garnet (YIG) film grown on a Gadolinium Gallium Garnet (GGG) substrate. A gold-plated copper microstrip antenna on an Aluminum Nitride (AlN) substrate was used to electrically excite Magnetostatic Surface Spin Waves (MSSW) via microwave signals from a Vector Network Analyser (VNA). A second antenna, separated by 2 mm, enabled electrical detection.
Optical Setup: A 532 nm Continuous-Wave (CW) laser was focused through the GGG substrate onto the YIG film, positioned between the two antennas. The system utilized a Tandem Fabry Pérot Interferometer (TFPI) and a Single Photon Avalanche Detector (SPAD) to analyze inelastically backscattered light.
Cryogenic Integration: The optical path entered the DR through anti-reflection coated windows at the 50 K and 4 K stages to suppress thermal radiation. The laser beam could be attenuated via neutral-density filters or pulsed using an Acousto-Optic Modulator (AOM) to manage heat load.
Thermal Modeling: To characterize the local temperature at the laser focal spot ( $T_{local}$ ), the authors performed 3D numerical simulations using Ansys Lumerical (for optical absorption) and COMSOL Multiphysics (for heat transfer). These simulations accounted for the temperature-dependent thermal conductivity and heat capacity of YIG and GGG.

Key Contributions

First DR-Coupled BLS: This work reports the first optical detection of coherently driven, propagating spin waves via BLS spectroscopy inside a dilution refrigerator, extending free-space optical access to the sub-kelvin regime.
Simultaneous Readout: The study demonstrates the simultaneous recording of optical and electrical responses for the same spin-wave mode, providing unambiguous confirmation that the BLS signal corresponds to the electrically driven magnon mode.
Thermal Characterization: The authors quantified the thermal impact of laser illumination in a DR environment, distinguishing between the global equilibrium temperature ( $T_{global}$ ) measured by a resistance thermometer and the local temperature ( $T_{local}$ ) at the focal spot.

Results

Spectral Correlation: The BLS spectra tracked the electrically measured transmission across a range of magnetic fields (200 mT to 250 mT). The peak transmission shifted from ~7.9 GHz to ~9.6 GHz as the field increased, matching the expected dispersion relations.
Temperature Regimes:
- Continuous Wave (CW): At the lowest optical power enabling detection (7.9 ± 1 µW), the global temperature was 510 mK. Numerical modeling indicated a local temperature of approximately 900 mK at the focal spot.
- Pulsed Operation: By pulsing the laser (1 µs pulse, 4 µs period), the average heat load was reduced. Under pulsed conditions with 20 µW average power, the global temperature dropped to 240 mK, while the peak local temperature reached 1.68 K.
Thermal Dynamics: Simulations revealed that due to the low thermal conductivity of YIG at cryogenic temperatures, heating is highly localized. Upon laser extinction, the heat diffuses primarily into the GGG substrate. The local temperature rises rapidly (~110 ns) during the pulse and decays during the off-period.

Significance and Claims
The paper claims this work represents a milestone toward magnon-mediated quantum transduction in hybrid quantum systems by establishing a direct optical interface for magnons at sub-kelvin temperatures.

Quantum Regime Proximity: The authors note that while the achieved local temperature (~~900 mK) is close to the theoretical limit where thermal occupation of a single magnon level drops below one (~~530 mK), the current setup still contains a significant thermal background (approx. $10^7$ thermal magnons in the probed volume).
Path Forward: The paper modestly asserts that reaching the single-magnon regime is within practical reach. It suggests that this could be achieved by reducing the probed mode volume (e.g., using nanoscale waveguides), operating at higher magnon frequencies, or minimizing absorbed laser power through longer wavelengths or thinner films.
Future Potential: The successful integration of BLS into a DR opens the door for future hybrid opto-magnonic experiments, coherent magnon-photon transduction, and the utilization of propagating magnons in quantum telecommunication networks, provided that spatially separated sources and detectors can be utilized.

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