The Quantum Neutron
The neutron, one of the most common building blocks of matter, is also a unique probe for studying materials and fundamental interactions. The only electrically-neutral nucleus, the neutron passes through most materials with ease, even at the lowest energies. Nowadays neutrons, even with their ~ 15 minute lifetime, are used to study problems ranging from current flow in common batteries to cosmological dark energy. I focus on the neutron as a quantum particle. We use a Mach-Zehnder neutron interferometer to fork a neutron into superposition states separated by decimeters, to exploit macroscopic quantum interference. I discuss recent experimental searches for dark energy, the use of quantum information concepts to enhance coherence, and the twisting of neutron waves.