The Decrease of Mutual Information as a Necessary and Sufficient Condition for Anomalous Heat Flow Between Two Qubits.

It is known that a decrease in correlation is a necessary condition for heat to flow anomalously from a colder to a hotter body. Nevertheless, a sufficient condition for this phenomenon remains elusive due to the difficulty in assigning a proper temperature-like parameter to a fully dynamical system.

In this work, we investigate a sufficient condition for anomalous heat flow, particularly between two interacting qubits in an isolated setting. Unlike previous studies that use initial temperatures to define the flow direction, we instead employ the concept of local effective temperatures. As a result, the decrease in mutual information becomes both a necessary and sufficient condition in this specific setup. We further implement this scenario on an IBM quantum computer as an experimental demonstration.