Which experiment demonstrated the quantization of electron spin?

The Stern–Gerlach experiment demonstrated the quantization of electron spin. In the experiment, a beam of silver atoms passed through a nonuniform magnetic field and split into two distinct paths instead of forming a continuous spread.

Otto Stern conceived the experiment in 1921, and Walther Gerlach carried it out with him in Frankfurt in 1922. The separated beam showed that the measured component of angular momentum could take discrete values. Although the experiment used silver atoms and predates the 1925 proposal of electron spin, the result is understood through the atom’s unpaired electron and its spin magnetic moment.

The experiment is often described as direct evidence of spin quantization, but it did not show a tiny classical sphere literally rotating. It measured quantum angular momentum through magnetic deflection. The Franck–Hertz experiment instead demonstrated quantized atomic energy levels, making it a related but different result.