Atlas of Fundamental Concepts

Pure states, mixed states, wavefunctions, and density operators are related but not interchangeable descriptions. Observables and the Born rule connect states to probabilities; unitary evolution and superposition govern isolated dynamics. Tensor products introduce composite systems and entanglement, EPR sharpens the locality problem, Bell turns it into a theorem, experiments test the inequalities, and partial trace plus decoherence explain the emergence of reduced mixed states without by itself selecting an interpretation.