  We show that gravity and matter fields are generically entangled, as a
consequence of the local Poincar\'e symmetry. First, we present a general
argument, applicable to any particular theory of quantum gravity with matter,
by performing the analysis in the abstract nonperturbative canonical framework,
demonstrating the nonseparability of the scalar constraint, thus promoting the
entangled states as the physical ones. Also, within the covariant framework, we
show explicitly that the Hartle-Hawking state in the Regge model of quantum
gravity is necessarily entangled. Our result is potentially relevant for the
quantum-to-classical transition, taken within the framework of the decoherence
programme: due to the gauge symmetry requirements, the matter does not
decohere, it is by default decohered by gravity. Generically, entanglement is a
consequence of interaction. This new entanglement could potentially, in form of
an "effective interaction", bring about corrections to the weak equivalence
principle, further confirming that spacetime as a smooth four-dimensional
manifold is an emergent phenomenon. Finally, the existence of the
gauge-protected entanglement between gravity and matter could be seen as a
criterion for a plausible theory of quantum gravity, and in the case of
perturbative quantisation approaches, a confirmation of the persistence of the
manifestly broken gauge symmetry.
