Density matrices are mathematical representations used to describe the statistical state of a quantum system, especially when that system is in a mixed state. They provide a way to encapsulate all possible information about a quantum state, including probabilities of various outcomes, thus playing a crucial role in understanding the behavior of quantum systems like light. The density matrix formalism is essential for analyzing systems where quantum superposition and entanglement are present, allowing for a more comprehensive description than pure state vectors alone.
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