  Entanglement in hybrid quantum systems comprised of fundamentally different
degrees of freedom, such as light and mechanics is of interest for a wide range
of applications in quantum technologies. Here, we propose to engineer bipartite
entanglement between traveling acoustic phonons in a Brillouin active solid
state system and the accompanying light wave. The effect is achieved by
applying optical pump pulses to state-of-the-art waveguides, exciting a
Brillouin Stokes process. This pulsed approach, in a system operating in a
regime orthogonal to standard optomechanical setups, allows for the generation
of entangled photon-phonon pairs, resilient to thermal fluctuations. We propose
an experimental platform where readout of the optoacoustics entanglement is
done by the simultaneous detection of Stokes and Anti-Stokes photons in a
two-pump configuration. The proposed mechanism presents an important feature in
that it does not require initial preparation of the quantum ground state of the
phonon mode.
