Phonon pumping by modulating the ultrastrong vacuum

The vacuum, or ground state, of a system in ultrastrong light-matter coupling contains particles that cannot be directly detected, often referred to as virtual particles. We propose a protocol for inducing and observing real mechanical excitations in a mirror, enabled by the only presence of virtual photons. This setup includes a resonant optical cavity that is ultrastrongly coupled to a two-level system (qubit) and optomechanically coupled to a mechanical resonator. When the frequency of the two-level system is modulated at a frequency comparable to the mechanical resonator, real phonons are coherently emitted. Analogously in classical physics, attaching a weight to a spring merely shifts its equilibrium position, while dynamically modulating the weight causes the system to oscillate; in our case, the "weight" is the vacuum itself. We propose and thoroughly characterize a hybrid superconducting-optomechanical setup, based on current state-of-the-art technology, where this effect could be observed experimentally.