Short answers, pulled from the story.
Spin polarization is the degree to which the intrinsic angular momentum of elementary particles is aligned with a given direction. It applies to electrons in ferromagnetic metals such as iron, to nuclear spins in materials under a magnetic field, and to the photons that make up circularly polarized light.
Spin polarization of deuterium and tritium fuel is expected to increase fusion reactivity by as much as 50% for fully parallel D-T fuel, which is a direct quantum effect. The resulting rise in plasma temperature can push total power output up by 80-90% or more, and electric power output could reach as much as twice the raw fusion increase.
A Mott detector uses annular optics to decelerate back-scattered electrons and focus them onto a ring-shaped electron multiplier at roughly 15 degrees. Electrons hit different positions on the ring depending on their spin state. Only about 1% of electrons are scattered in the gold foil, and of those, about 1% are collected by the detector.
Spintronics is a branch of solid-state electronics that uses electron spin as a carrier of information alongside electric charge. Magnetic semiconductors are being researched as possible spintronic materials, and spin polarization is the fundamental property those materials must exhibit.
Circularly polarized electromagnetic radiation is produced by the spin polarization of its constituent photons. When the photons in a beam have their spins aligned, the field takes on a circular character; the polarization is a property of the individual particles, not just the wave as a whole.
Two main techniques are used: low-energy electron diffraction from a clean tungsten crystal (SPLEED) and an electron microscope built from electrostatic lenses with a gold foil sample. Both methods rely on spin-orbit coupling, which causes electrons with different spins to scatter at different angles.