  All physical systems are to some extent open and interacting with their
environment. This insight, basic as it may seem, gives rise to the necessity of
protecting quantum systems from decoherence in quantum technologies and is at
the heart of the emergence of classical properties in quantum physics. The
precise decoherence mechanisms, however, are often unknown for a given system.
In this work, we make use of an opto-mechanical resonator to obtain key
information about spectral densities of its condensed-matter heat bath. In
sharp contrast to what is commonly assumed in high-temperature quantum Brownian
motion describing the dynamics of the mechanical degree of freedom, based on a
statistical analysis of the emitted light, it is shown that this spectral
density is highly non-Ohmic, reflected by non-Markovian dynamics, which we
quantify. We conclude by elaborating on further applications of opto-mechanical
systems in open system identification.
