Essential ingredients for a thermodynamic machine. (a) Schematic machine where three baths , , may gain or lose heat, , 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 controlled by a heat bath at temperature , to a third qubit with energy splitting and temperature . The interaction is indicated by . The virtual qubit itself consists of two qubits , and temperatures . These parameters uniquely determine and (below).