Figure 1: Essential ingredients for a thermodynamic machine. (a) Schematic machine where three baths $T_1$, $T_2$, $T_3$ may gain or lose heat, $Q$, because they are connected by a “black box” (BB) node. (b) Brunner et al. describe this black box as a three-qubit interconnecting node. The action of this node can effectively be described as the resonant coupling of a virtual qubit, with energy splitting $E_v$ controlled by a heat bath at temperature $T_v$, to a third qubit with energy splitting $E(3)$ and temperature $T(3)$. The interaction is indicated by $H(v,3)$. The virtual qubit itself consists of two qubits $E(1)$, $E(2)$ and temperatures $T(1),T(2)$. These parameters uniquely determine $T_v$ and $E_v$ (below).