  The spin of the neutron allows neutron scattering to reveal the magnetic
structure and dynamics of materials over nanometre length scales and picosecond
timescales. Neutron scattering is particularly in demand in order to understand
high-temperature superconductors, which lie close to magnetically ordered
phases, and highly correlated metals with giant effective fermion masses, which
lie close to magnetic order or pass through a mysterious phase of hidden order
before becoming superconducting. Neutron scattering also is the probe of choice
for revealing new phases of matter and new particles, as seen in the surprising
behaviour of quantum spin chains and ladders where mass gaps and excited
triplons replace conventional spin waves. Examples are given of quantum
phenomena where neutron scattering has played a defining role that challenges
current understanding of condensed matter.
