Quantum thermodynamics with coherence: from fundamental laws to quantum heat engine

Quantum thermodynamics serves as a vital bridge between quantum technologies, statistical mechanics, and quantum information theory. With the rapid advancements in experimental techniques for controlling nanoscale systems, there is an urgent need to explore the thermodynamics of small quantum systems. A key challenge is understanding the fundamental laws governing these systems, such as the first law of thermodynamics in the quantum regime, which requires a nuanced division of internal energy into heat and work, particularly regarding energy changes induced by quantum coherence. Heat is associated with entropy-related energy changes, whereas work refers to usable energy that can be stored and extracted. A deeper understanding of the role coherence plays is critical for the development of high-performance quantum engines. Additionally, unraveling the mechanisms behind entropy production in arbitrary non-equilibrium quantum processes is essential. In the open quantum systems framework, entropy production arises from the reduction of relative entropy between the current and equilibrium states, and the interaction between a subsystem and its environment illuminates how irreversibility emerges from global unitary evolution, offering valuable insights into the quantum information paradox. Further exploration of quantum thermodynamics in curved spacetime seeks to integrate gravitational effects into quantum theory. By designing quantum heat engines, such as those based on the quantum Otto cycle within black hole spacetimes, spacetime parameters are incorporated into thermodynamic considerations, creating new intersections between quantum theory and general relativity.