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Line 2: {{See also\|Quantum statistical mechanics}} {{Quantum mechanics\|cTopic=Advanced topics}} A '''density matrix''' is a [[matrix (math)\|matrix]] that describes the statistical state of a system in [[quantum mechanics]]. ~~The density~~Density matrix is especially helpful for dealing with ''mixed states'', which consist of a [[statistical ensemble]] of several different quantum systems. The opposite of a mixed state is a [[pure state]]. [[Quantum state\|State vectors]], also called [[bra-ket notation\|kets]], describe only pure states, whereas a density matrix can describe both pure and mixed states. Describing a quantum state by its density matrix is a fully general alternative formalism to describing a quantum state by its ket (state vector) or by its statistical ensemble of kets. However, in practice, it is often most convenient to use density matrices for calculations involving mixed states, and to use kets for calculations involving only pure states.

Density matrix: Difference between revisions