Pauli’s matrices
In 1927 the German physicist Wolfgang Pauli developed a theory of the electron based upon the physical quantity spin, which has no equivalent in classical mechanics, although it is sometimes described as the rotation of the electron on itself.The spin of the electron has a half integer value and the spin axis in general is time-dependent, as it can also interact with the electromagnetic field.
https://geometrica.vialattea.net/en/paulis-matrices/
