Witnessing entanglement in quantum magnets using neutron scattering*

Quantum entanglement is a key driver of exotic physics in quantum materials, but it historically has been extremely hard to measure. I will discuss recent experiments showing how quantum spin entanglement can be extracted from neutron scattering data using the tools of quantum information theory. We applied these protocols to 1D spin chains KCuF₃, Cs₂CoCl₄, and triangular lattice delafossite KYbSe₂. Of the three protocols we tested, we found Quantum Fisher Information (QFI) to be the most experimentally robust. Crucially, these protocols do not require theoretical models, which are only sometimes available for 2D and 3D quantum systems. Entanglement witnesses in KYbSe₂ show evidence of a proximate quantum spin liquid phase, which is confirmed by comparison to numerical simulations of a 2D triangular lattice spin liquid.