  The vacuum (i.e., the ground state) of a system in ultrastrong light-matter
coupling contains particles that cannot be emitted without any dynamical
perturbation and is thus called virtual. We propose a protocol for inducing and
observing real mechanical excitations of a mirror enabled by the virtual
photons in the ground state of a tripartite system, where a resonant optical
cavity is ultrastrongly coupled to a two-level system (qubit) and, at the same
time, optomechanically coupled to a mechanical resonator. Real phonons are
coherently emitted when the frequency of the two-level system is modulated at a
frequency comparable to that of the mechanical resonator and, therefore much
lower than the optical frequency. We demonstrate that this hybrid effect is a
direct consequence of the virtual photon population in the ground state. Within
a classical physics analogy, attaching a weight to a spring only changes its
resting position, whereas dynamically modulating the weight makes the system
oscillate. In our case, however, the weight is the vacuum itself. We propose
and accurately characterize a hybrid superconducting-optomechanical setup based
on available state-of-the-art technology, where this effect can be
experimentally observed.
