  We investigate the microscopic mechanism of quantum energy transfer in the
nonequilibrium spin-boson model. By developing a nonequilibrium
polaron-transformed Redfield equation based on fluctuation decoupling, we
dissect the energy transfer into multi-boson associated processes with even or
odd parity. Based on this, we analytically evaluate the energy flux, which
smoothly bridges the transfer dynamics from the weak spin-boson coupling regime
to the strong-coupling one. Our analysis explains previous limiting predictions
and provides a unified interpretation of several observations, including
coherence-enhanced heat flux and absence of negative differential thermal
conductance in the nonequilibrium spin-boson model. The results may find wide
applications for the energy and information control in nanodevices.
