Researchers reveal decoherence impacts heat and information flow, challenging equivalence of voltage probe models

The flow of heat and information at the nanoscale relies on understanding how quantum systems lose coherence, a process called decoherence, but accurately modelling this remains a significant challenge. Eren Erdogan and Justin P. Bergfield, from Illinois State University, investigate the commonly used voltage probe and voltage-temperature probe models for incorporating decoherence into calculations of quantum transport. Their work reveals that these models, often considered interchangeable, actually produce different results unless conditions are highly symmetrical, a situation rarely found in real experiments. The researchers demonstrate that the voltage probe can fail to accurately model decoherence in heat transport, even with strong measurement settings, while the voltage-temperature probe consistently respects fundamental constraints, establishing it as the more reliable framework for understanding these crucial nanoscale processes.

For incorporating decoherence in quantum transport, two common approaches 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, this research demonstrates that this equivalence 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.

Local Temperature and Entropy in Molecular Junctions

This research provides a detailed exploration of quantum transport in molecular junctions, employing advanced theoretical methods and Büttiker probes. The core message is that understanding and accurately measuring local temperature and entropy within these junctions is crucial for interpreting transport phenomena. Traditional methods that assume a uniform temperature across the junction can be misleading, especially when strong interactions, quantum interference, and out-of-equilibrium conditions are present. The Büttiker probe technique, combined with sophisticated theoretical modeling, offers a powerful tool for probing these local properties.

The research emphasizes that temperature within a molecular junction isn’t necessarily uniform; local hot spots or cold spots can form due to quantum interference, interactions, and current flow. Ignoring these local variations can lead to incorrect interpretations of conductance and thermopower. Büttiker probes provide a valuable method for measuring local quantities like temperature and entropy without significantly disturbing the system. The study also highlights the role of local entropy in determining transport behavior, as changes in entropy can drive current flow and affect the efficiency of molecular devices.

Accurate modeling requires incorporating the spatial variations of temperature and entropy, challenging the validity of simplified models that assume uniformity or neglect interactions. These findings have important implications for the design and optimization of molecular electronic devices, as understanding and controlling local temperature and entropy can lead to improved performance and efficiency. This work provides a more nuanced and accurate picture of electron behavior in molecular junctions, moving beyond simplistic assumptions and highlighting the importance of local properties, contributing significantly to the field of molecular electronics.

Voltage Probe and Temperature Probe Differ Significantly

Researchers have established a critical distinction between two widely used models for simulating decoherence in nanoscale electronic devices: the voltage probe (VP) and the voltage-temperature probe (VTP). The findings reveal that while often treated as equivalent, the VP and VTP impose fundamentally different thermodynamic constraints on the system being studied. The investigation centers on accurately modeling decoherence, a process that degrades quantum properties and limits the performance of emerging technologies. Both the VP and VTP introduce “probes” to mimic the effects of environmental interactions that cause decoherence.

However, the team discovered that the VP can act as a source or sink of heat, failing to accurately model decoherence in heat transport even with strong coupling. In contrast, the VTP consistently enforces decoherence in both charge and heat channels by respecting thermodynamic constraints and conserving both charge and heat currents. Through detailed calculations and simulations using a benzene-based molecular junction as an example, researchers demonstrated significant discrepancies between the two models. The results show that the VP can produce inaccurate predictions for heat transport, while the VTP provides a consistent framework for modeling decoherence under asymmetric conditions, such as uneven coupling or thermal bias. This is crucial because real-world devices rarely exhibit perfect symmetry. The team’s analysis establishes the VTP as the more reliable model for accurately simulating and predicting the behavior of nanoscale devices where decoherence plays a significant role, paving the way for improved designs and enhanced performance in future quantum technologies.

VP and VTP Models Differ in Decoherence

This research demonstrates a fundamental difference between two commonly used models for simulating how decoherence affects heat and charge flow in molecular junctions: the voltage probe (VP) and the voltage-temperature probe (VTP). The findings reveal that these models are not interchangeable, despite often being treated as equivalent. Specifically, the VTP consistently accounts for decoherence in both charge and heat channels by adhering to constraints that conserve both energy and heat, while the VP can act as a source or sink of heat and may fail to model decoherence in heat transport altogether. The study highlights that the apparent equivalence between the VP and VTP arises only under highly symmetrical conditions, such as balanced thermal biases and tunnel coupling.

When these symmetries are broken, as is often the case in real experimental setups, the VP model produces inaccurate predictions regarding heat transport. Using a benzene-based molecular junction as an example, the researchers show that these discrepancies can significantly impact predicted heat transport characteristics. The authors acknowledge that their analysis focuses on locally coupled probes, but preliminary results suggest that these conclusions hold true for spatially extended probe couplings as well. Future work could explore the implications of these findings for more complex molecular systems and experimental designs, potentially refining the accuracy of simulations used to understand and optimize nanoscale devices.