Fluctuations, Sensing, and Information in Open Quantum Systems

Today, many technological devices exist which are built on the principles of quantum mechanics. These are mainly based on the concept of energy quantization and include the laser and the transistor. Currently there is an active quest in developing novel quantum technologies which harness the more elusive features of quantum theory such as coherent superposition and entanglement. While there are still numerous open questions concerning the nature of future quantum devices, they will rely on systems that are out of equilibrium and couple to their surrounding environment. Therefore, a thorough understanding of open quantum systems driven far from equilibrium is paramount for the development of devices based on quantum technology. The emerging field of quantum thermodynamics investigates concepts such as heat, work, and temperature in the quantum regime, with a focus on determining fundamental limitations on physically allowed processes. This theory therefore provides an ideal framework for addressing the capabilities of open quantum systems. While quantum theory follows a different set of rules than classical theories, it is often a non-trivial task to determine if a quantum device is able to outperform an analogous classical device, i.e., exhibits a quantum advantage . A promising route to determining such an advantage is provided by the investigation of fluctuations. Quantum fluctuations behave fundamentally different from their classical counterparts and are still far from being fully understood. Their implications are tightly connected to the measurement process, implying a close interrelation with the established fields of quantum sensing and information thermodynamics . This project constitutes a unifying investigation of fluctuations, sensing, and information; concepts which have so far mostly been studied independently. The long-term goals of this project are twofold: The first objective is to expand our understanding of the tasks that can be performed by out-of-equilibrium open quantum systems , including the potential provided by quantum features such as superposition and entanglement. The second objective is to develop novel technologies in the fields of quantum thermodynamics and quantum metrology . To this end, the project is focused on thermodynamic processes, such as the conversion of heat into electrical work and the estimation of low temperatures. The research will rely on well established methods, such as Markovian quantum master equations, methods which I (co-)developed, such as the Keldysh quasi- probability distribution, as well as methods that will be developed during the project. In addition to a unified understanding of the role of fluctuations, sensing, and information in quantum thermodynamics , a number of results going well beyond the state of the art are expected to emerge from the project. These include advances in understanding fundamental properties of quantum devices. For example I intend to clarify the role of the particle-wave duality in quantum thermal machines . Furthermore, a number of practical tools for both theorists and experimentalists will be developed including tests to certify non-classical behavior, as well as theories such as a novel input-output theory based on Keldysh path integrals . A particular focus of the project lies on bridging the gap between theory and experiment with examples being the certification of non-classical fluctuations, as well as the implementation of low-temperature thermometry schemes that are only limited by fundamental constraints. With these results, the project is expected to have a deep impact on topics ranging from the description of driven-dissipative systems to the state of the art in thermometry measurements.