This article is about a concept in physics. For the psychiatric condition, see Mixed state (psychology).

The term mixed state refers to a concept in physics, particularly quantum mechanics. In quantum mechanics a state E of a quantum mechanical ensemble is represented by a density operator which can be decomposed as a randomization of two statistically different statistical ensembles, or a linear combination of pure states.

A mixed state is different from a pure state in that the density operator that describes a mixed state is not a projection operator. In the density operator formalism, this means that a mixed state has the following property

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If two mixed states are described by the same density matrix, any state of one of the mixtures can be expressed as a linear combination of states of the other mixture.