Coherent Thermal Machines: Fluctuations and Performance

Abstract

Quantum Engineering is a rapidly evolving discipline, promising the development of groundbreaking new technologies. To overcome the challenges posed by the nanoscopic scales and ultra-cold temperatures that are required for the implementation of quantum devices, understanding the thermodynamic interaction of these devices with their environment is important. Improving our knowledge of the laws of quantum thermodynamics enables engineers to exert better control over quantum circuits and it paves the way for more effective thermal management solutions on the nanoscale. In this dissertation, we focus on the study of thermal machines such as heat engines or refrigerators. These devices have recently been miniaturized in labs to the point where quantum effects can influence their performance. Due to the well-known advantages of quantum over conventional computers, it is natural to wonder whether coherence effects might be able to give a similar performance enhancement to thermal machines. Here, we approach this question on three different levels. First, we study the theory of periodically controlled open quantum systems in order to better understand the mathematical foundations of reciprocating thermal devices. Second, we investigate transport phenomena in open quantum systems, enabling us to discuss the fluctuations of thermodynamic currents and how they affect the performance of thermal machines. Third, and finally, we examine whether the performance of quantum heat engines is subject to universal bounds and how these bounds change in different operation regimes. The results of this thesis shed light on the optimal design of quantum devices, which will soon be ready for experimental implementation and investigation. Overall, we find that coherence leads to quantum friction, that is, to an increase in thermodynamic irreversibility with associated performance losses. In the adiabatic weak-coupling regime, these losses outweigh the benefits stemming from the increased number of degrees of freedom that are accessible in quantum setups. In other words, we find that coherence should generally be kept as small as possible in order to optimize the performance of nanoscale thermal machines in this regime. A promising direction for future investigations is to explore the effects of strong coupling and fast driving, where quantum advantages may be achievable.