Density Operator and Quantum States
At first sight the notion of density operator (or density matrix, as it is called in physics) should not lead to any particular difficulty: mathematically, a density operator is just a positive trace class operator with trace equal to one. It turns out that, perhaps somewhat unexpectedly, it is the positivity property which is the most delicate to establish.
KeywordsQuantum State Pure State Density Operator Uncertainty Principle Trace Class
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