The Stern-Gerlach experiment, with results published in 1922, discovered that uncharged silver atoms passing through an inhomogeneous magnetic field would be deflected in two discrete directions depending upon an intrinsic quantised property of matter which came to be called “spin”. This was the first observation of a pure quantum effect which was unrelated to the emission or absorption of energy and became one of the foundations upon which the “new quantum theory” formulated later in the 1920s by Erwin Schrödinger, Werner Heisenberg, Paul Dirac, and others.
Oddly, this paper, reporting one of the most significant physics experiments of the twentieth century, was not translated into English until 2023, when physicist Martin Bauer of Durham University in the U.K. published “The Stern-Gerlach Experiment, Translation of: ‘Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld’ ” (full text [PDF]). The translation, including the original illustrations, is just four pages and is a “just the facts” report of how the experiment was conducted and the results observed. The German title refers to “Richtungsquantelung”, which translates to “space quantisation”, which indicates how theorists were groping toward an understanding of the behaviour of the ground state of electrons in the atom. (The term had nothing to do with modern speculation about space being quantised at the Planck length, which is 19 orders of magnitude smaller than the proton and 25 orders of magnitude smaller than the silver atoms used in the experiment.)
Today, a Stern-Gerlach type apparatus is used to prepare quantum states for many experiments exploring the fundamentals of quantum processes such as superposition and entanglement.