Quantum Translator: A Layered Magnet Turns Microwave Signals Into Light

The quest to build a functional quantum internet has long been hindered by an awkward mismatch: the best quantum processors, memories and sensors operate at microwave frequencies, while the only practical way to move quantum information across kilometers of fiber is with photons of visible or near-infrared light. Bridging these two worlds requires a transducer that can convert microwave photons into optical ones coherently, efficiently and without drowning out the fragile quantum signals in noise. Now, a team led by researchers at the City College of New York reports a strikingly simple route to that bridge, one that exploits the simultaneous talents of a single layered magnetic semiconductor rather than engineering a complicated hybrid device from multiple materials.

Writing in Nature Materials, Pratap Chandra Adak, Vinod M. Menon and colleagues demonstrate coherent microwave-to-optical transduction in chromium sulfide bromide, or CrSBr, a van der Waals antiferromagnet that has quickly become one of the most studied materials in condensed matter physics. The work, performed in collaboration with researchers at the CUNY Advanced Science Research Center, Columbia University, the University of Chemistry and Technology Prague, the University of Chicago and Rheinland-Pfälzische Technische Universität Kaiserslautern, shows that the magnetic and optical excitations of CrSBr are coupled strongly enough that a microwave signal applied to the crystal is faithfully written onto an optical beam reflected from its surface. Remarkably, the demonstration required no optical cavity, no cryogenic microwave resonator and no mechanical oscillator.

The central trick lies in the material’s excitons, tightly bound electron-hole pairs that dominate the optical response of CrSBr. In this layered magnet, excitons are not passive spectators; they interact intimately with the ordered array of chromium spins that gives the crystal its antiferromagnetic character. Previous studies had shown that coherent spin waves, known as magnons, can shift and modulate exciton resonances in van der Waals magnets, and that exciton-polaritons, hybrid light-matter quasiparticles, can be magnetically dressed in CrSBr. The New York-led team turned this magnon-exciton dialogue into an engineering resource: by driving the antiferromagnetic resonance of the crystal with microwaves, they modulate the resonant excitonic susceptibility, effectively making the material’s optical properties oscillate at the microwave frequency.

That oscillating susceptibility has a precise consequence in the language of nonlinear optics. When a continuous laser beam impinges on the crystal, the microwave-driven modulation scatters light into sidebands shifted up and down in frequency by exactly the microwave frequency, a process analogous to frequency conversion in an electro-optic modulator but mediated by collective spin dynamics rather than an applied electric field. The team detected these sidebands using homodyne interferometry, a phase-sensitive technique that mixes the converted light with a reference beam derived from the same laser. Because homodyne detection preserves phase information, it verifies that the conversion is coherent, meaning the quantum state of a microwave excitation would, in principle, be preserved as it crosses the frequency divide, an essential requirement for quantum networking applications.

The breadth of the conversion window is one of the most notable technical results. Many cavity-based transducers operate over narrow bandwidths, trading conversion efficiency against the range of microwave frequencies they can handle, a limitation that complicates interfacing with real superconducting qubits whose transition frequencies vary from device to device. The CrSBr platform exhibited coherent conversion over an intrinsically broadband window of roughly 300 megahertz, even in a plain bulk crystal with no cavity enhancement of any kind. This bandwidth arises naturally from the magnon-exciton coupling mechanism rather than from careful impedance engineering, suggesting it can be retained, and potentially widened, as devices shrink and become more sophisticated.

Equally significant is what the team observed when they tuned the probe laser away from the bare exciton resonance. Multiple exciton-polariton resonances, the hybrid modes that emerge when excitons couple strongly to photonic modes, all inherited the magnon-coupled response. In other words, the spin-driven transduction is not confined to a single narrow spectral line; it persists across several polariton branches. This inheritance points to a strategy for broadening the usable optical detuning range and for mitigating optical dissipation, since different polariton resonances offer different trade-offs between light-matter coupling strength and absorption loss. It also hints that polariton engineering, a specialty of the Menon laboratory, could become a design tool for future transducers rather than an afterthought.

The choice of material is doing heavy lifting here. CrSBr is an easy-axis van der Waals antiferromagnet whose gigahertz antiferromagnetic resonances sit squarely in the microwave band relevant to superconducting circuits. Unlike ferromagnetic insulators used in earlier magnon-based conversion proposals, CrSBr is also a semiconductor with sharp, strongly absorbing exciton resonances, so the magneto-optical interaction is not limited to the intrinsically weak, off-resonant Faraday-type effects that have historically hampered magnon transducers. Instead, the conversion leverages strong light-matter interactions at the exciton resonance, where even modest spin-driven changes in the dielectric function translate into large changes in optical phase and amplitude. The layered crystal structure is an additional bonus: CrSBr can be exfoliated to atomically thin flakes, and its magnetic order survives down to the two-dimensional limit, making the platform a natural candidate for integration into nanoscale photonic and spintronic circuits.

The authors are candid that the current demonstration is a proof of principle rather than a finished quantum link. The transduction efficiency observed in bulk crystals remains far from the unity conversion needed for lossless quantum state transfer, and the measurements were performed without the cavity enhancement that defines state-of-the-art electro-optic and optomechanical transducers. But the pathway to higher performance is spelled out by the physics itself: the transduction strength scales with the cooperativity between magnons and excitons, which can be increased by reducing the magnetic volume of the device and by adding optical cavities that lengthen the interaction time between light and the exciton-polariton modes. Shrinking the crystal to a nanoscale flake on a photonic chip, while keeping the microwave field concentrated, would boost the coupling per photon dramatically. Cavity integration would additionally sharpen the collection efficiency and could enable operation in the quantum-coherent regime where single microwave photons are converted into single optical photons.

If those engineering steps succeed, the implications extend across the quantum technology landscape. Superconducting quantum processors, arguably the leading platform for scalable quantum computing, produce and manipulate quantum information in the microwave domain but cannot easily communicate with one another over distance, since microwave photons are hopelessly lossy in fiber and even in free space at room temperature. A compact, broadband, fiber-compatible transducer would allow these processors to be linked into distributed networks, enabling modular quantum computing and long-baseline quantum sensing. Magnetic memory and spintronic devices, which operate naturally at microwave frequencies, could likewise be networked through optical links. Because CrSBr transduction is broadband, a single device could conceivably serve several qubits operating at slightly different frequencies, simplifying system architectures considerably.

The work also adds momentum to a broader renaissance in magnon-based quantum technologies. Theorists have recently proposed antiferromagnet-based microwave-to-optical quantum transduction schemes, and experimentalists have demonstrated coherent magnon-photon coupling in CrSBr itself, as well as magnon-mediated interactions between excitons. The new results tie these threads together into a concrete transduction demonstration, showing that a single van der Waals crystal can host the magnetic, electronic and optical degrees of freedom needed for frequency conversion simultaneously. For a field accustomed to stitching together transducers from superconducting circuits, piezoelectric resonators, electro-optic crystals and atomic vapors, the appeal of a monolithic, two-dimensional magnetic semiconductor that does the whole job is obvious. The CrSBr crystal sitting on the researchers’ microwave waveguide is, in effect, an entire quantum translator etched by nature down to a few atomic layers, and the race is now on to see how far this elegantly simple platform can be pushed.

Subject of Research: Coherent broadband microwave-to-optical quantum transduction via magnon-exciton coupling in the layered antiferromagnet CrSBr

Article Title: Microwave-to-optical transduction using magnon–exciton coupling

Article References: Adak, P. C., McDaniel, I. E., Paul, S., Heuvel-Horwitz, C., Das, B., Kozlov, V., Mosina, K., Ramanathan, A., Roy, X., Sofer, Z., Zhong, T., Kamra, A., Thielens, A., Alú, A., & Menon, V. M. (2026). Microwave-to-optical transduction using magnon–exciton coupling. Nature Materials. https://doi.org/10.1038/s41563-026-02748-7

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02748-7

Keywords: CrSBr, microwave-to-optical transduction, magnon-exciton coupling, quantum networks, exciton-polaritons, antiferromagnetism, van der Waals magnets, magneto-optics, spintronics, quantum transducers, two-dimensional materials, homodyne detection