Essential ingredients for a thermodynamic machine. (a) Schematic machine where three baths T1, T2, T3 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 Ev controlled by a heat bath at temperature Tv, 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 Tv and Ev (below).