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The passage around this formula

Begin with an equal superposition of two localized alternatives, $|\Psi(0)\rangle= \frac{|0\rangle+|1\rangle}{\sqrt 2}\otimes|h_{\rm in}\rangle$. The labels zero and one denote paths, not energy eigenstates. After the system’s long-range field interacts with the inaccessible degrees of freedom, an idealized final pure state has the form

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