Optical control over topological Chern number in moiré materials

Controlling quantum matter with light offers a promising route to dynamically tune its many-body properties, ranging from band topology^1,2 to superconductivity³. However, achieving such optical control for strongly correlated electron systems in the steady state has remained elusive. Here we demonstrate optical switching of the spin-valley degree of freedom of itinerant ferromagnets in twisted MoTe₂ (t-MoTe₂) homobilayers. This system uniquely features flat valley-contrasting Chern bands and exhibits a range of strongly correlated phases at various moiré lattice fillings, including Chern insulators and ferromagnetic metals^4-7. We show that the spin-valley orientation of all of these phases can be dynamically reversed by resonantly exciting the exciton-polaron⁸ transitions with circularly polarized light. These findings not only provide direct evidence for non-thermal optical switching of a ferromagnetic spin state at zero magnetic field but also demonstrate the possibility of dynamical control over a topological order parameter, paving the way for optical generation of chiral edge modes and topological quantum circuits.