  Inelastic neutron scattering (INS) is one powerful technique to study the
low-energy single-spin dynamics of magnetic materials. A variety of quantum
magnets show novel magnetic correlations such as quantum spin liquids. These
novel magnetic correlations are beyond the direct detection of INS. In this
paper we propose a coincidence technique, coincidence inelastic neutron
scattering (cINS), which can detect the two-spin magnetic correlations of the
magnetic materials. In cINS there are two neutron sources and two neutron
detectors with an additional coincidence detector. Two neutrons from the two
neutron sources are incident on the target magnetic material, and they are
scattered by the electron spins of the magnetic material. The two scattered
neutrons are detected by the two neutron detectors in coincidence with the
coincidence probability described by a two-spin Bethe-Salpeter wave function.
Since the two-spin Bethe-Salpeter wave function defines the momentum-resolved
dynamical wave function with two spins excited, cINS can explicitly detect the
two-spin magnetic correlations of the magnetic material. Thus, it can be
introduced to study the various spin valence bond states of the quantum
magnets.

Strongly Correlated Electrons