Pure and Mixed States
In quantum mechanics, a quantum system is represented by a state vector (or ket) . A quantum system with a state vector is called a pure state. However, it is also possible for a system to be in a statistical ensemble of different state vectors: For example, there may be a 50% probability that the state vector is and a 50% chance that the state vector is . This system would be in a mixed state. The density matrix is especially useful for mixed states, because any state, pure or mixed, can be characterized by a single density matrix.
A mixed state is different from a quantum superposition. In fact, a quantum superposition of pure states is another pure state, for example .
Read more about this topic: Density Matrix
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—Amanda Berry Smith (1837–1915)