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Quantum Mechanics by Giorgio Parisi, Mauro Fortunato, Gennaro Auletta

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5

Density matrix

The density matrix is a very useful tool that enlarges the concept of state from a vector in a Hilbert space to a true operator from which all properties of a quantum system can be extracted. This generalization is particularly relevant because it allows compound systems to be treated in a natural way, especially when dealing with specific properties of one subsystem. For this reason, as we shall see in Secs. 9.4 and 14.2, it plays a central role in the dynamics of open systems (non-unitary evolutions).

In this chapter we shall first deal with the formalism of the density matrix (Sec. 5.1). Then, we shall apply this formalism to calculations for expectation values and outcomes of a measurement (Sec. 5.2) and present the time ...

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