ABSTRACT
In dealing with systems whose eigenstates are known, such as ideal spin systems in a magnetic field, it is sometimes useful to introduce a formalism involving the density matrix. Knowledge of the density matrix operator permits the expectation values of other operators to be determined in a way that is straightforward in principle. In the presence of a magnetic field or some other polarizing mechanism, the form of the density matrix will change, and the expectation value, for example, may be nonzero. Although the situation that has been considered involves an electron beam, a related but somewhat more complicated approach is used for polarized light beams.
