Witnessing Entanglement and Quantum Correlations in Condensed Matter

The identification of e.g. quantum spin liquid materials is a challenging task, yet it is vital for the development of magnetic based quantum information technologies. A principal problem is the lack of single, unambiguous measurements that can uniquely identify the quantum states involved without the costly and technically difficult step of creating quantum interferometers with the materials in question. To address this urgent need we have developed a protocol that uses innovations in quantum measurement theory, Hamiltonian determination with neutrons, and validation against theoretical predictions to down-select materials. This involves demonstrating the extraction of quantum entanglement witnesses from neutron scattering data that quantify the degree of entanglement of the spins in the material with the rest of the spins in the network as well as the entanglement depth of this network. Our results, and the application of the protocol, are illustrated on triangular quantum spin liquid candidates. A range of quantum materials and application of quantum measures are given and future prospects discussed.