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Some essential questions to be able to answer in preparation for the Physics 130C final exam Lecturer: McGreevy Last updated: 2014/03/19, 11:27:22 1. Axiom 3: Given a state |ψ(0)i at time t = 0, and a Hamiltonian H, what is the state at time t? 2. Axiom 4: Given a state |ψi and a Hermitian operator A with eigenvalue a, what is the probability that will get a if we measure A in the state |ψi? 3. Axiom 4 for mixed states: Given a density matrix ρ and a Hermitian operator A with eigenvalue a, what is the probability that will get a if we measure A in the state ρ? 4. Reduced density matrix: Given a state ρ of a composite system HA ⊗ HB , construct the reduced density matrix ρA ≡ trHB ρ . What information does this encode? Under what circumstances does the resulting ρA describe a pure state? 5. The density matrix encodes a probability distribution on state vectors: In its spectral representation X ρ= pa |aiha| a (the states |ai are orthonormal since ρ is Hermitian), pa is the probability that the system is in the state |ai. 6. More generally, what are the connections between the notions of entanglement, interactions, pure states, mixed states? 7. Ehrenfest’s Theorem: Given an observable A and the Hamiltonian H, what is ∂t hAi ? 8. Path integral: From the point of view of quantum mechanics, what is special about the classical trajectory? 1