One of the principal objectives of quantum thermodynamics is to explore quantum effects and their potential beneficial role in thermodynamic tasks like work extraction or refrigeration. So far, even though several papers have already shown that quantum effect could indeed bring quantum advantages, a global and deeper understanding is still lacking. Here, we extend previous models of autonomous machines to include quantum batteries made of arbitrary systems of discrete spectrum. We establish their actual efficiency, which allows us to derive an efficiency upper bound, called maximal achievable efficiency, shown to be always achievable, in contrast with previous upper bounds based only on the Second Law. Such maximal achievable efficiency can be expressed simply in term of the apparent temperature of the quantum battery. This important result appears to be a powerful tool to understand how quantum features like coherence but also many-body correlations and non-thermal population distribution can be harnessed to increase the efficiency of thermal machines.
Recently, there has been significant interest in extending thermodynamic concepts—particularly thermal machines—into the quantum regime to better understand the exchange of physical quantities such as energy and information between a system and its environment. In this thesis, we present the theoretical framework necessary to develop the concept of a quantum thermal machine from a quantum mechanical perspective. Leveraging these tools, we propose a two-qubit system, which can be prepared either in a separable state or a correlated state, and subject it to a thermodynamic cycle by immersing the system in an environment while externally controlling its energy gap Δ. We evaluate the performance of the system in both configurations under ultra-weak and weak coupling regimes, employing an endoreversible cycle and dynamics governed by a Lindblad master equation, respectively. Our findings show that the efficiency η exhibits similar behavior in both regimes, indicating potential for optimization through correlations. However, the power output P displays distinct behaviors across the two regimes, attributed to the specific protocol applied in the weak coupling scenario. Despite this difference, our results suggest that correlations offer opportunities for enhancing efficiency and power in quantum thermal machines.
We present a comprehensive theoretical investigation about the operational regions of quantum systems, specifically examining their roles as working media functioning between two thermal reservoirs in quantum thermal machines (QTMs). This study provides relevant and novel insights, including a complete spectrum of QTMs within the operational region of these quantum systems, and introduces new QTM designs never before described in the literature. Additionally, this work introduces a standardized and cohesive classification scheme for QTMs, ensuring robustness in nomenclature and operational distinctions, which enhances both theoretical understanding and practical application. Notably, one of these designs directly addresses the need for a more appropriate explanation of the operation of a laser (or maser) as a QTM. Initial calculations were performed to achieve results applicable to any quantum system subjected to rules analogous to those used in classical thermal machine studies. These results were then used to analyze two-level quantum systems as the working medium of QTMs in the Otto cycle. In particular, we analyzed two specific quantum systems: the laser and a spinless electron in a one-dimensional quantum ring, yielding consistent and innovative results. Overall, this study offers valuable insights into the operation and classification of QTMs, establishing a clear and unified framework for their nomenclature while opening new avenues for the design and enhancement of these devices.