Non-Markovian bath-induced coupling revealed by two-dimensional spectroscopy

Problems in the field of open quantum systems often involve an environment that greatly impacts excitation dynamics. Here we show that there can be coherent coupling between different system states of a form that only occurs in a non-Markovian treatment of the bath. Because this involves entangled system-bath states, we demonstrate that there are distinct signatures of this physics in simple absorption spectra and two-dimensional electronic spectroscopy. To do this we introduce a numerical method to simulate optical spectra of non-Markovian open quantum systems. The method employs a process tensor framework to efficiently compute multi-time correlation in a numerically exact way.