Witnessing entanglement in quantum magnets using neutron scattering

TL;DR

This paper demonstrates how neutron scattering can be used to directly detect quantum entanglement in magnetic materials, specifically in a quasi-1D Heisenberg antiferromagnet, using various entanglement witnesses including quantum Fisher information.

Contribution

It introduces a method to witness entanglement in quantum magnets through neutron scattering and compares different witnesses, highlighting quantum Fisher information's robustness and applicability to higher spins and dimensions.

Findings

Quantum Fisher information detects entanglement up to 50 K in KCuF3.

Entanglement witnesses can be applied directly to neutron spectra.

Higher spin chains show suppressed entanglement at lower temperatures.

Abstract

We demonstrate how quantum entanglement can be directly witnessed in the quasi-1D Heisenberg antiferromagnet KCuF$_3$. 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.