Density Matrix Formalism

Abstract

In this note we develop an elegant and powerful formulation of quantum me- chanics, the density matrix formalism. This formalism provides a structure in which we can address such matters as: We typically assume that it is permissible to work within an appropriate subspace of the Hilbert space for the universe. Is this all right? In practice we often have situations involving statistical ensembles of states. We have not yet addressed how we might deal with this.