  Matter-light interaction is at the center of diverse research fields from
quantum optics to condensed matter physics, opening new fields like laser
physics. A magnetic exciton is one such rare example found in magnetic
insulators. However, it is relatively rare to observe that external variables
control matter-light interaction. Here, we report that the broken inversion
symmetry of multiferroicity can act as an external knob enabling the magnetic
exciton in van der Waals antiferromagnet NiI2. We further discover that this
magnetic exciton arises from a transition between Zhang-Rice-triplet and
Zhang-Rice-singlet's fundamentally quantum entangled states. This quantum
entanglement produces an ultra-sharp optical exciton peak at 1.384 eV with a 5
meV linewidth. Our work demonstrates that NiI2 is two-dimensional magnetically
ordered with an intrinsically quantum entangled ground state.
