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The passage around this formula

Because $H_{SB}$ conserves total excitation number, the dynamics starting from a single excitation stays confined to the two-dimensional subspace spanned by |e,g$\rangle$ (system excited, bath in its ground state) and |g,e$\rangle$ (the reverse). On resonance, both basis states carry the same bare energy, +$\hbar$$\omega$/2 - $\hbar$$\omega$/2 = 0 , so $H_f$ restricted to this subspace is proportional to the swap operator between the two, and the Schrödinger equation solves in closed form. Starting from |$\psi(0)$$\rangle$ = |e,g$\rangle$ ,

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