  One aspect of solid-state photonic devices that distinguishes them from their
atomic counterparts is the unavoidable interaction between system excitations
and lattice vibrations of the host material. This coupling may lead to
surprising departures in emission properties between solid-state and atomic
systems. Here we predict a striking and important example of such an effect. We
show that in solid-state cavity quantum electrodynamics, interactions with the
host vibrational environment can generate quantum cavity-emitter correlations
in regimes that are semiclassical for atomic systems. This behaviour, which can
be probed experimentally through the cavity emission properties, heralds a
failure of the semiclassical approach in the solid-state, and challenges the
notion that coupling to a thermal bath supports a more classical description of
the system. Furthermore, it does not rely on the spectral details of the host
environment under consideration and is robust to changes in temperature. It
should thus be of relevance to a wide variety of photonic devices.
