Observation of spin-correlated exciton-polaritons in a van der Waals magnet

Strong coupling between light and material excitations is emerging as a powerful tool to engineer the properties of solid-state systems. Spin-correlated excitations in magnetic crystals that couple strongly to optical cavities promise control over collective quantum phenomena, but their suitable electronic resonances have yet to be found. Here we report strong light-matter coupling in nickel phosphorus trisulfide ($\textrm{NiPS}_3$), a van der Waals magnet with strongly correlated magnetic and electronic degrees of freedom. A previously unobserved class of polaritonic quasiparticles emerges from the strong coupling between its spin-correlated excitons and the photons inside a microcavity. The hybridization with light offers unique opportunities to study the origin and interactions of magnetically coupled excitations in antiferromagnetic insulators. Our work introduces van der Waals magnets to the field of strong light-matter physics and provides a path towards the design and control of correlated quantum materials via cavity quantum electrodynamics