We show that if it is possible to manufacture a beam of spin-1/2 heavy atoms corresponding to a quantum-mechanical pure state, then the dark lines of the Stern-Gerlach effect should display a fine structure. The two lips of the open-mouth pattern will each be a doublet; the lighter area between the two lines of the doublet arises from an interference between silver atoms on opposite sides of the incoming beam. We obtain a simple classical model giving exactly the same angular distribution around the open mouth. However, this classical model does not give the fine structure.

• Received 9 December 1991

DOI:https://doi.org/10.1103/PhysRevA.46.2265