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 entanglement.

Automated summary

The study demonstrates how quantum entanglement can be directly observed in a quasi-1D Heisenberg antiferromagnet; using three entanglement witnesses on inelastic neutron spectrum, quantum Fisher information proves to be the most robust experimentally; theoretical analysis shows that witnessable entanglement in higher spin Heisenberg chains gets suppressed at lower temperatures as the quantum number increases, with potential applications in higher dimensional quantum materials.