  Two-dimensional electronic spectroscopy (2DES) is one of the most powerful
spectroscopic techniques, capable of attaining a nearly complete picture of a
quantum system including its couplings, quantum coherence properties and its
real-time dynamics. While successfully applied to a variety of condensed phase
samples, high precision experiments on isolated quantum systems in the gas
phase have been so far precluded by insufficient sensitivity. However, such
experiments are essential for a precise understanding of fundamental mechanisms
and to avoid misinterpretations, e.g. as for the nature of quantum coherences
in energy trans-port. Here, we solve this issue by extending 2DES to isolated
nanosystems in the gas phase prepared by helium nanodroplet isolation in a
molecular beam-type experiment. This approach uniquely provides high
flexibility in synthesizing tailored, quantum state-selected model systems of
single and many-body properties. For demonstration, we deduce a precise and
conclusive picture of the ultrafast coherent dynamics in isolated high-spin Rb2
molecules and present for the first time a dynamics study of the system-bath
interaction between a single molecule (here Rb3) and a superfluid helium
environment. The results demonstrate the unique capacity to elucidate
prototypical interactions and dynamics in tailored quantum systems and bridges
the gap to experiments in ultracold quantum science.
