Physicists Steer a Quantum Light Condensate With a Magnetic Field

In a development that could reshape how scientists build quantum light sources, researchers have created a condensate of hybrid light–matter particles inside a magnetic material and shown that the condensate can be steered with nothing more exotic than an external magnetic field. The work, published in Nature Materials, demonstrates magnetically tunable exciton–polariton condensation in chromium sulfide bromide, or CrSBr, a layered van der Waals magnet that has rapidly become one of the most closely watched quantum materials of the past few years. By applying a modest magnetic field of just 2 tesla, the team shifted the energy of their macroscopically coherent quantum state by up to 10.5 millielectronvolts — a remarkably large tuning range for a condensate whose emission is normally fixed by the geometry of the device that hosts it.

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