The off-diagonal Peierls coupling in Eq. 21a, on the other hand, is the main cause of pure decoherence when one does not consider population trans- fer between |+⟩ and |-⟩ or population transfer to the dark states. This term is also the main cause of decoherence when N = 1 (as there is no dark state in this case). One can also estimate the rate constant for the process of |+⟩ → |-⟩ using FGR, and this rate constant is