The Density Matrix
The concept of the density matrix is introduced so that observables, or expectation values of an operator, can be calculated in a straightforward way for an ensemble of microsystems with time-dependent properties. To follow the presentation of this Chapter, the reader should be familiar with the material of Sections A.1 to A.3 and of Chapter V. A more detailed account of the subject is found in a monograph by Lynden-Bell ; the definitions and the nomenclature used here are similar to those in that monograph.
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