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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—Henry David Thoreau (1817–1862)
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In the pure air, the agent
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—John Ashbery (b. 1927)
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—For the City of New Orleans, U.S. public relief program (1935-1943)
“When some one remarked that, with the addition of a chaplain, it would have been a perfect Cromwellian troop, he observed that he would have been glad to add a chaplain to the list, if he could have found one who could fill that office worthily. It is easy enough to find one for the United States Army. I believe that he had prayers in his camp morning and evening, nevertheless.”
—Henry David Thoreau (1817–1862)