Though one of the simplest quantum systems, with only two states, it was nonetheless critical to the development of quantum mechanics, demonstrating the nature of measurement as a process that fundamentally altered the system.
The machine used in this experiment can be represented as a projection operator into orthogonal subspaces. Usually modeled using electrons, the original experiment actually used silver atoms, and later ones verified it with hydrogen atoms. Though one of the simplest quantum systems, with only two states, it was nonetheless critical to the development of quantum mechanics, demonstrating the nature of measurement as a process that fundamentally altered the system. FTP, name this 1922 experiment devised by a pair of German physicists in which particles were deflected through an inhomogeneous magnetic field, demonstrated that they have intrinsic spin.
The machine used in this experiment can be represented as a projection operator into orthogonal subspaces.
{Stern-Gerlach} experiment