Magnons in the subkelvin range: an optical probe for quantum networks ✛ ✦
Context
Advancing quantum technologies demands the integration of disparate physical platforms—superconducting circuits, optical fibers, microwave resonators. Quantum states of magnons—collective spin excitations in magnetically ordered media—could bridge this gap. As predicted by Einstein and Bose, magnons obey Bose–Einstein statistics, which at room temperature results in a huge thermal background. Reaching the quantum detection limit requires millikelvin temperatures, achievable in dilution refrigerators already used to cool superconducting qubits. Until now, optical access to magnons under these conditions was missing, hampering the development of microwave-to-optical transducers for quantum optics.
Methods
The experiment combined Brillouin light scattering spectroscopy with a dilution refrigerator. An yttrium iron garnet (YIG) film, containing iron as the magnetic ion, was placed on microwave antennas that allowed electrical excitation and detection of spin waves. A 532 nm laser beam, whose energy—described by the Planck–Einstein relation discovered by Planck—was focused through a transparent substrate onto the film. Light scattered from magnons underwent a frequency shift and polarization rotation, detected by a tandem Fabry-Pérot interferometer and a single-photon avalanche detector. Electrical transmission spectra were recorded simultaneously for comparison. Numerical modeling of laser heating using Lumerical and COMSOL packages allowed estimation of the local temperature.
Results
Under continuous laser illumination with (7.9±1) μW incident on YIG, the global sample temperature measured by a thermometer was 510 mK. According to simulations, the local temperature in the focal spot reached about 900 mK due to the poor thermal conductivity and low heat capacity of YIG at cryogenic temperatures. Despite this, a clear magnon resonance at 7.85 GHz was observed, with the optical signal precisely matching the electrical one. At these temperatures, the thermal occupation of the magnon level, governed by Bose–Einstein statistics, is still significant but greatly reduced compared to room temperature. Thermodynamic analysis shows that transitioning to nanoscale waveguides will enable the single-magnon regime at ~200 mK.
Implications
Demonstrating optical detection of magnons at subkelvin temperatures is a significant step for quantum sensors and hybrid systems. It enables quantum frequency conversion—turning microwave quanta used by superconducting qubits into optical photons for fiber transmission—crucial for quantum networks. Moreover, the BLS spectroscopy method inside a dilution refrigerator is also applicable to phonons, broadening the study of magnon-phonon coupling.
Future development
The next step will optimize the optical setup: increasing the magnon operating frequency, using thinner films and nanostructures to reduce heat load and effective volume. Cryogenic objectives and adaptive optics will enhance resolution and sensitivity. Integration with materials of higher magneto-optical activity, such as bismuth-substituted YIG, will enable single-magnon detection. In the future, such systems could become components of quantum computers and repeaters.
Impact
The development will impact quantum telecommunications, quantum computing, and fundamental many-body physics research at ultralow temperatures.
Next steps
Immediate next steps include experiments with pulsed laser heating to reduce average heat load and demonstrating single-magnon detection in nanoscale waveguides.
Key open problems
This work brings us closer to solving a key challenge in quantum technology—creating an efficient, coherent interface between microwave and optical photons, essential for scaling quantum networks and linking distant quantum processors.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study demonstrates optical detection of magnons in a YIG film at sub-kelvin temperatures using Brillouin light scattering, simultaneously with electrical measurements. The local temperature in the focal spot was about 900 mK, bringing the system close to the single-particle quantum regime. The optical and electrical signals matched precisely at a frequency of 7.85 GHz.
Where its strength is
The key strength lies in combining optical access with millikelvin cooling, paving the way for quantum transducers between microwave and optical domains. The experiment was conducted under conditions where the thermal magnon background is significantly suppressed, which is crucial for quantum applications.
Where it could go next
- It might be interesting to compare the achieved thermal regime with work where magnons in a sphere were squeezed to the quantum limit, to assess how close the current system is to the single-particle limit and which parameters (size, temperature) are critical.based on: Magnons in a sphere squeezed to quantum limit arXiv:2602.19671
- A related approach is cooling a mechanical oscillator nearly to its ground state; perhaps similar active feedback or reservoir engineering techniques could help further reduce the effective magnon temperature in this geometry.based on: Tiny Seesaw Cooled Almost to Absolute Zero arXiv:2510.24199
- To advance further toward the quantum limit, one could consider transitioning to nanoscale magnonic waveguides, as noted in the work, and compare with experiments that observed quantum behavior in mesoscopic systems.based on: A Shivering Disk Cooled to Quantum Stillness arXiv:2511.15492
Worth trying
- Use nanoscale YIG waveguides to reduce the local temperature and thermal magnon population.
- Apply reservoir engineering or feedback methods to further cool the magnon mode.
- Perform measurements at lower laser power or in pulsed mode to reduce heating and approach the single-particle limit.
Why this may matter
If the quantum regime of magnons with optical access can be achieved, it would form the basis for hybrid quantum networks linking superconducting qubits to optical photons via magnonic interfaces, which is critical for scalable quantum communications.
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Original abstract
as written by the authors arXiv:2606.25834
Coupling light to magnetic excitations in the form of spin waves underpins both the optical study of magnetism and emerging schemes for quantum transduction, positioning the quanta of these excitations, magnons, as promising carriers for hybrid quantum networks. However, exploiting them in the quantum regime requires millikelvin temperatures to suppress thermal magnon populations, thereby confining such experiments to dilution refrigerators. There, magnons can already be excited and read out electrically, yet an optical interface required for microwave-to-optical photon conversion has been missing. Here, we demonstrate the first optical detection of coherently driven, propagating spin waves via Brillouin Light Scattering (BLS) spectroscopy inside a dilution refrigerator. By simultaneously recording the optical and electrical responses of the same spin-wave mode in a yttrium iron garnet film, we find that the BLS spectra track the electrically measured transmission across a range of applied magnetic fields. For the lowest optical power of 7.9 {\mu}W that still enabled spin-wave detection, we measured a global equilibrium sample temperature of 510 mK via a resistance thermometer, while numerical modelling of the laser-induced heating yields a maximum local temperature of 900 mK at the focal spot. This brings free-space optical access to magnons into the sub-kelvin regime, representing a milestone towards magnon-mediated quantum transduction in hybrid quantum systems.
To reach millikelvin temperatures, mixtures of helium-3 and helium-4 isotopes are used; they separate at low temperatures much like oil separates from water.
Key numbers
- global temperature: 510 mK
- local temperature: 900 mK
- laser power: 7.9 μW
- magnon frequency: 7.85 GHz
- focal spot diameter: 5 μm