The concept of a density matrix originated about 1930 in the process of adapting to quantum mechanics the statistical descriptions of classical phenomena. A new light was cast on this concept when Felix Bloch formulated his NMR equation, inclusive of dissipative terms: This equation, ostensibly the time-dependent Schrödinger equation for a spin’s density matrix, governs in fact the expectation values of evolving observables, namely, the components of the spin’s polarization vector. It serves… CONTINUE READING