Seminarium Fizyki Materii Skondensowanej

The concept of a single mode of the electromagnetic field interacting with matter has been a paradigm in the field of light-matter interactions. For example, the single mode Jaynes-Cummings model and its many generalizations, such as the many-body Dicke model, have been indispensable tools in studying the quantum dynamics of various systems. In particular in cavity and circuit QED, where strong light-matter coupling is routinely achieved in experiment, such models have been tremendously successful. Recently, however, various experimental platforms have emerged where multi-mode effects, complex environments and the openness of the system constitute an essential part of the physics, and where the applicability of few-mode models has been debated. In this talk, we will present "ab initio few-mode theory", where these factors are treated systematically and which allows to include new physics into Jaynes-Cummings type models without abandoning their conceptual and computational simplicity. We will outline some implications, in particular for the new field of X-ray cavity QED as well as for ultra-strong coupling theory. From a practical perspective, our method connects the extensive toolbox of few-mode models to ab initio theory. From a more general perspective, it constitutes a non-perturbative expansion scheme that allows to extract the relevant degrees of freedom of a quantum system featuring continuum interactions. As such, it may find applications in a broader context ranging from transport theory to open quantum dynamics.