ABSTRACT
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Processes involving quantum tunneling can be found in a huge variety of physical and chemical systems, ranging in length from the mesoscopic scale of a few microns to the subatomic scale of a few fermi. It has been realized already three decades ago that the interaction of the tunneling degree of freedom with environmental degrees of freedom plays a crucial role. Namely, any realistic description has to account for the fact that a purely isolated system is always an idealization, an issue which tends to become more and more relevant with the growing complexity of the system and particularly with increasing system size. A quantitative understanding of experimental observations must include the presence of, for example, electromagnetic modes in electrical circuits, vibrational modes in molecular aggregates, or phonon backgrounds in solid-state systems. Moreover, as a genuine quantum effect tunneling processes serve as paradigm to analyze the boundary between the microscopic and the macroscopic world or at least to elucidate how the latter one emerges from the former one.
