Exciton–polaritons are among the strangest creatures in the solid-state menagerie. They arise when excitons — bound pairs of an electron and a hole — couple so strongly to photons trapped in an optical cavity that the distinction between light and matter blurs entirely. The resulting quasiparticles inherit the best of both worlds: the tiny effective mass and coherence of photons, and the strong mutual interactions of excitons. When enough of them crowd into a single quantum state, they undergo a form of Bose–Einstein condensation, spontaneously locking their phases together into a single macroscopic wavefunction that emits coherent light. These condensates have long been touted as candidates for quantum communication, cryptography and novel lasers, but a persistent problem has been control: in conventional semiconductor microcavities, the interactions are weak, and tuning the condensate typically means physically rebuilding the device.

The German-led collaboration, spearheaded by Heng Zhang, Niloufar Nilforoushan and Christian Weidgans at the University of Regensburg, together with colleagues in Munich, Prague and elsewhere, saw a way around this limitation by turning to a material in which the excitons themselves are intimately tied to magnetic order. CrSBr is an air-stable, A-type antiferromagnet in which the spins within each layer are aligned ferromagnetically, while adjacent layers are magnetized in opposite directions. Crucially, its excitons are quasi-one-dimensional, tightly bound, and strongly polarized along the crystallographic b axis, with an enormous oscillator strength. Because the electronic orbitals that form these excitons are fully spin-polarized, any change in the magnetic order of the crystal directly reshapes the exciton wavefunction — and with it, the polaritons built from that exciton.

To trap light and matter together, the researchers fabricated microscopic cavities of unusual elegance. Few-nanometer-thick flakes of CrSBr, exfoliated from bulk crystals, were sandwiched between two gold mirrors separated by thin spacers of polymethyl methacrylate. The flakes naturally cleave into elongated microwires, tens of micrometers long along the a axis but only about a micrometer wide along the b axis. This geometry acts as a slab waveguide with additional lateral confinement: light is squeezed tightly in the vertical direction, quantized into discrete modes across the narrow width, and left essentially free to propagate along the length of the wire. Modeling the polaritons as particles in an elongated two-dimensional rectangular potential well, the team predicted a ladder of discrete quantized states along the short axis, with energy splittings of tens of millielectronvolts — exactly what the experiments revealed.

When the researchers pumped one such microwire with femtosecond laser pulses, the angle-resolved photoluminescence showed a strikingly anisotropic picture. Along the narrow b axis, the emission resolved into discrete quantized states — a ground state and two excited states at 1.248, 1.265 and 1.286 electronvolts — while along the long a axis the modes dispersed parabolically, as expected for particles confined in only one lateral direction. A global fit with a coupled oscillator model confirmed that the system had entered the strong-coupling regime, with a vacuum Rabi splitting of 200 millielectronvolts, matching earlier observations of polaritonic effects in bulk CrSBr. Two additional, dispersionless resonances were identified as surface excitons, sitting slightly below the bulk exciton energy — a detail that would prove decisive for driving condensation.

The hallmarks of condensation appeared when the pump laser was tuned into resonance with those surface excitons. Above a threshold fluence of roughly 30 microjoules per square centimeter, the emission from the first excited quantized state surged by more than two orders of magnitude as the pump fluence rose by just a factor of three, while the emission linewidth collapsed from 19 to 8.7 millielectronvolts and the peak energy shifted upward by 5 millielectronvolts. Off-resonant pumping at higher photon energy, by contrast, produced only linear, lackluster emission. Interferometric measurements drove the point home: below threshold, the photoluminescence showed no interference fringes; above it, crisp fringes appeared, and the measured coherence time of 427 femtoseconds exceeded the polariton lifetime by an order of magnitude. Together, the superlinear intensity scaling, linewidth narrowing, energy shift and long-range spatial and temporal coherence constitute an unambiguous fingerprint of exciton–polariton condensation.

Two peculiarities of the condensation process hint at the deep magnetic physics at play. First, condensates formed only under near-resonant excitation of the surface excitons, never under high-energy pumping, suggesting that relaxation into the condensate requires a carefully matched sequence of scattering events. Second, the condensates formed not in the ground state of the microwire but in excited quantized states — and in several samples, the energy gap between the pump photons and the condensate emission closely matched the energy of an optical magnon mode of CrSBr of about 45 millielectronvolts. Density functional theory calculations supported a complementary pathway through lattice vibrations: excitons in CrSBr localize electrons on chromium atoms and holes on sulfur atoms, inducing a structural distortion that increases the interlayer separation and couples efficiently to the interlayer breathing phonon. The picture that emerges is one of resonant phonon- and magnon-assisted scattering shuttling excitation energy from surface states into the condensate — a direct handshake between light, lattice and spin.

The centerpiece of the study, however, is the magnetic control. Placing a microwire in a magneto-optical cryostat with an out-of-plane magnetic field, the researchers tracked the polariton modes as the field swept from zero to 2 tesla. Both the polariton branches and the surface exciton resonance redshifted by more than 10 millielectronvolts, while defect emission stayed put — evidence that the shifts stem from a field-driven reconfiguration of the collective spin order rather than ordinary Zeeman splitting. The spectral shift saturated above 2 tesla, exactly where CrSBr is known to flip from its antiferromagnetic to a ferromagnetic configuration under out-of-plane bias. Most strikingly, the condensed state itself followed suit: the macroscopically coherent emission shifted by up to 10.5 millielectronvolts between the antiferromagnetic and ferromagnetic phases, with condensation achievable in both regimes.

What makes this tuning so powerful is its mechanism. Previous electric-field-based strategies perturbatively modify the potential experienced by polaritons; in CrSBr, the magnetic field instead changes the magnetic order and symmetry of the material itself, transforming the exciton from a quasi-one-dimensional species into a spatially extended three-dimensional wavefunction as the spins reorganize. Because the polaritons emerge from a manifold of spin-polarized electronic orbitals, spin order becomes a native control knob for the condensate — one that operates at the level of the quantum wavefunction rather than merely its environment. The authors even suggest that the fluence-dependent energy shifts they observed may involve not only repulsive exciton–exciton interactions but also magnon-induced magnetic disorder, which reduces the exciton oscillator strength, shrinks the Rabi gap, and pushes the lower polariton energy upward.

The implications stretch well beyond a single laboratory demonstration. Because the condensate is coupled to the magnetic order, the researchers envision modulating its coherent emission on ultrafast timescales by linking it to coherent, high-frequency magnons — spin waves that can be launched and manipulated with picosecond precision. Such a platform could enable magnetic-memory integration and efficient microwave-to-optical transduction for quantum information processing, with microwave photons coupling to condensate emission through the mediating magnons. For a field that has spent two decades searching for robust ways to control macroscopic quantum states of light, the message of this work is tantalizing: sometimes the most elegant control knob is simply a magnet, and the material that responds to it may be sitting in a vial of layered crystals, waiting to be exfoliated.

Subject of Research: Magnetically tunable exciton–polariton condensation in the van der Waals antiferromagnet CrSBr

Article Title: Magnetic control of an exciton–polariton condensate in a van der Waals magnet

Article References: Zhang, H., Nilforoushan, N., Weidgans, C., Inzenhofer, T., Liebich, M., Riepl, J., Hirschmann, J., Gronwald, I., Mosina, K., Sofer, Z., Tyagi, R., Wilhelm, J., Mooshammer, F., Dirnberger, F., & Huber, R. (2026). Magnetic control of an exciton–polariton condensate in a van der Waals magnet. Nature Materials. https://doi.org/10.1038/s41563-026-02751-y

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02751-y

Keywords: exciton–polaritons, condensate, CrSBr, van der Waals magnet, antiferromagnetism, magnons, microcavity, strong coupling, quantum light sources, spintronics, photoluminescence, coherence

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