  In the spin-boson model, a continued fraction form is proposed to
systematically resum high-order quantum kinetic expansion (QKE) rate kernels,
accounting for the bath relaxation effect beyond the second-order perturbation.
In particular, the analytical expression of the sixth-order QKE rate kernel is
derived for resummation. With higher-order correction terms systematically
extracted from higher-order rate kernels, the resummed quantum kinetic
expansion (RQKE) approach in the continued fraction form extends the Pade
approximation and can fully recover the exact quantum dynamics as the expansion
order increases.
