Magnetic control of an exciton-polariton condensate in a van der Waals magnet.

Quasiparticle condensates are among the most striking solid-state manifestations of quantum physics. Coupling macroscopic real-space wavefunctions to additional degrees of freedom, such as the electron spin, would add valuable control knobs for quantum applications. While creating spin-carrying superconducting condensates has attracted enormous attention, condensates of light-matter hybrids known as exciton-polaritons have lacked an analogous spin-based perspective. Here we demonstrate magnetically tunable exciton-polariton condensation in the van der Waals magnet CrSBr. Under photoexcitation, CrSBr microwires embedded in an optical cavity show the hallmarks of polariton condensation. Owing to the strong coupling between the spin order and excitonic correlation, the energy of the condensate can be tuned by up to 10.5 meV by an external magnetic field of 2 T. Our results establish CrSBr microcavities as a powerful platform for exploring magnetic control of polariton condensates and mark an essential step towards spin-controlled coherent quantum light sources.