When dephasing fails: Thermodynamic consequences of decoherence models in quantum transport

Understanding how decoherence influences heat and information flow is essential for realizing the promise of quantum technologies. Two widely used models for incorporating decoherence in quantum transport are the voltage probe (VP), which imposes local charge current conservation, and the voltage-temperature probe (VTP), which also conserves heat current. Although these models are often treated as functionally equivalent, we demonstrate that this equivalence actually exists only under highly symmetric conditions, which may be challenging to achieve experimentally. Under asymmetric coupling or thermal bias, the VTP respects thermodynamic constraints and enforces decoherence in both charge and heat channels, while the VP instead acts as a source or sink of heat. Strikingly, the VP can fail to model decoherence in the heat transport entirely, even with large probe coupling strengths. Using a benzene-based molecular junction as a realistic example, we show that these effects significantly impact the predicted heat transport. These results establish that the VP and VTP models are not interchangeable; only the VTP provides a thermodynamically consistent framework for modeling decoherence in quantum transport.