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optimizer is used to vary the parameters θ and ϕ. Remember, the Von Neumann entropy is invariant under unitary action, so the entropy of U(ϕ)ρ(θ)U†(ϕ) is equal to the Von Neumann entropy of ρ(θ). Thus, the loss function of the VQT is minimized when U(ϕ)ρ(θ)U†(ϕ) is equal to the density matrix of