We assess two different nonequilibrium quantum Landauer bounds: the traditional approach based on the change in entropy, referred to as the “entropic bound,” and one based on the details of the dynamical map, referred to as the “thermodynamic bound.” By first restricting to a simple exactly solvable model of a single two-level system coupled to a finite-dimensional thermal environment and by exploiting an excitation-preserving interaction, we establish the dominant role played by the population terms in dictating the tightness of these bounds with respect to the dissipated heat and clearly establish that coherences only affect the entropic bound. Furthermore, we show that sharp boundaries between the relative performance of the two quantities emerge and find that there are clear instances where both approaches return a bound weaker than Clausius' statement of the second law, rendering them ineffective. Finally, we show that our results extend to generic interaction terms.

• Received 1 August 2017

DOI:https://doi.org/10.1103/PhysRevA.96.042109