Mode-Entangled Neutron Beams: A possible new tool for exploring Quantum Materials

An entangled neutron beam is an intriguing potential probe of the properties of quantum materials. We have generated bipartite (spin-path) and tripartite (spin-path-energy) mode-entangled neutrons at both pulsed and continuous neutron sources. The subsystems of individual neutrons are entangled as proven by the violation of a contextuality inequality (similar to the Bell inequality). The entanglement length (i.e. path separation) was varied between 85 nm and 1600 nm, much smaller than the separations available to traditional neutron interferometry. The entanglement is reproducible across different beamlines, different beam collimations, different neutron wavelengths and with different mode entangling devices, with the primary limitation arising from the finite polarization of the neutron beam. A recent theoretical investigation has shown that neutron scattering by entangled electron spins in a dimer show unique scattering signatures for particular parameter values, encouraging us to believe that entangled neutron scattering may be a promising novel technique to probe highly-correlated, frustrated magnetic systems.