using Neutron Scattering*

We use neutron scattering to directly witness entanglement between quantum spins in the 1D Heisenberg antiferromagnet KCuF₃. We apply three entanglement witnesses---one-tangle, two-tangle, and quantum fisher information---to its inelastic neutron spectrum, and compare with simulated DMRG spectra. We find that each of these provides direct access to entanglement, but quantum fisher information is the most robust experimentally. This 1D system serves as a proof-of-principle that solid state quantum spin entanglement can be directly measured. We outline how these results can be applied to higher dimensional and higher spin materials to witness and quantify entanglement in quantum materials.