record-quality natural crystal of copper oxide found at the Tsumeb mine in Namibia using laser light of tunable frequency and ultralow spectral linewidth

An exciton is a quasiparticle in a solid-state system comprising an electron and a hole — the absence of an electron. It has an energy spectrum akin to that of the hydrogen atom, so it may be considered as an artificial hydrogen atom in a solid-state environment, with the hole playing the part of the proton. The concept of excitons has first been formulated in the early 1930s by Yakov Frenkel, who predicted their existence in molecular crystals. A few years later, Gregory Wannier 2 and Nevil Mott 3 described these electron–hole bound states for inorganic semiconductors. In 1952 Evgeniy Gross and Nury Karryjew discovered 4 such Wannier–Mott excitons experimentally in a copper oxide semiconductor. On page 000 of this issue, Kazimierczuk et al. 5 now report how they have redesigned this historic experiment to find excitons in a natural crystal of copper oxide that extend more than tens of billions of lattice sites of the crystal.