  Anomalous heat transfer (AHT), a process by which heat spontaneously flows
from a cold system into a hot one, superficially contradicts the Clausius
statement of the second law of thermodynamics. Here we provide a full
classification of mechanisms of the AHT in nonequilibrium quantum systems from
a quantum-information perspective. For initial states in local equilibrium, we
find that the AHT can arise from three resources: initial correlation,
intrasystem interaction, and intrasystem temperature inhomogeneity. In
particular, for qubit systems, we prove that initial quantum coherence is
necessary for AHT if the intersystem interactions are limited to the two-body
type. We explicitly show the AHT dominated by each of the mechanisms in a
three-qubit system. Our classification scheme may offer a guideline for
developing high-efficiency quantum heat pump.
