  The coherent control of a two-level system is among the most essential
challenges in modern quantum optics. Understanding its fundamental limitations
is crucial, also for the realization of next generation quantum devices. The
quantum coherence of a two level system is fragile in particular, when the two
levels are connected via an optical transition. When such quantum emitters are
located in solids the coherence suffers from the interaction of the optical
transition with the solid state environment, which requires the sample to be
cooled to temperatures of a few Kelvin or below. Here, we use a mechanically
isolated quantum emitter in hexagonal boron nitride to explore the individual
mechanisms which affect the coherence of an optical transition under resonant
drive. We operate the system at the threshold where the mechanical isolation
collapses in order to study the onset and temperature-dependence of dephasing
and independently of spectral diffusion. The new insights on the underlying
physical decoherence mechanisms reveals a limit in temperature until which
coherent driving of the system is possible. This study enables to increase the
operation temperature of quantum devices, therefore reducing the need for
cryogenic cooling.
