Quantum Thermodynamics: A Case Study for Emergent Behavior

Abstract

Any attempt to provide a foundation of thermodynamics faces this central question: how come that a qualitatively different type of behavior emerges (as an effective description) from the underlying physical substrate?

Quantum thermodynamics is able to show that the partitioning of a closed quantum system into a smaller and a significantly larger part typically gives rise to thermal properties of the former, even though the system as a whole continues to exhibit unitary motion. Being based on entanglement, this feature may show up already in rather small total quantum systems, the dynamics of which can still be solved exactly. Furthermore, it allows for nano-thermodynamics, an entirely self-contradictory concept in the classical regime.

This picture differes substantially from the classical (statistical) description: It is not the system as such, which is thermal; rather it is made thermal by its environment. Thermal behavior is thus “apparent” only, i.e. dependent on the way the observer chooses to look. A much closer look would make the thermal properties disappear – just like a portrait will become unrecognizable after focusing on individual pixels. However, that very type of “looking” has to be included as part of the detailed modeling. Operational quantum thermodynamics establishes an intuitive link between the new quantum and the old classical description.