Schematic representation of the respective protocols. In the forward protocol, the system is initially prepared at equilibrium with the thermal bath at a temperature higher ( T h ) or lower ( T c ) than the target ( T w ) temperature. T h and T c are chosen to be TE from T w with T h > T w > T c . During the backward protocol, the system relaxes at the respective TE temperatures T h and T c , starting from a common initial condition that is the equilibrium at T w . In a third situation, only two temperatures are compared, and the evolution of the system upon heating and cooling between them is assessed. In b and c , solid and dashed arrows stand for the forward and backward process, respectively, and thick lines indicate faster evolution than thin ones.
