Witnessing entanglement in quantum magnets using neutron scattering

We demonstrate how quantum entanglement can be directly witnessed in the quasi 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 spectra simulated by finite temperature density matrix renormalization group DMRG and classical Monte Carlo methods. We find that each witness provides direct access to entanglement. Of these, quantum Fisher information is the most robust experimentally and indicates the presence of at least bipartite entanglement up to at least 50 K, corresponding to around 10 of the spinon zone boundary energy. We apply quantum Fisher information to higher spin S Heisenberg chains and show theoretically that the witnessable entanglement gets suppressed to lower temperatures as the quantum number increases. Finally, we outline how these results can be applied to higher dimensional quantum materials to witness and quantify entanglemen