Fig. 5: Transport processes relevant to the highest-power machines.

a Transport process between two reservoirs associated with a single-hole excitation. This type of processes lead to the activation of all the transfer edges in the state-transfer networks between the fully occupied state (i.e., the no-hole state) and the single-hole states. The blue and red reservoirs represent the cold and hot ones, respectively. b Transport processes between two reservoirs associated with two-hole excitations, which correspond to activated and inactivated state transitions, respectively. In the latter, the inactivation of the transitions originates from the energy mismatch in two-hole states due to the inevitable hole–hole interaction between two holes. Here, the white and black nodes represent the empty and occupied electron levels, respectively. The wavy lines indicate the interaction and the arrows represent the exchanges of electrons with reservoirs. Only a part of the transfer edges between single-hole states and double-hole states can be activated, where red (blue) edges correspond to activated (inactivated) edges. The fraction of the activated edges in the highest-power machines can asymptotically approach to the unity in the thermodynamic limit (see the figures in “Methods”).