  We study heat transport in a pair of strongly coupled spins. In particular,
we present a condition for optimal rectification, i.e., flow of heat in one
direction and complete isolation in the opposite direction. We show that the
strong-coupling formalism is necessary for correctly describing heat flow in a
wide range of parameters, including moderate to low couplings. We present a
situation in which the strong-coupling formalism predicts optimal rectification
whereas the phenomenological approach predicts no heat flow in any direction,
for the same parameter values.
