  Light-matter interaction, and the understanding of the fundamental physics
behind, is the scenario of emerging quantum technologies. Solid state devices
allow the exploration of new regimes where ultrastrong coupling (USC) strengths
are comparable to subsystem energies, and new exotic phenomena like quantum
phase transitions and ground-state entanglement occur. While experiments so far
provided only spectroscopic evidence of USC, we propose a new dynamical
protocol for detecting virtual photon pairs in the dressed eigenstates. This is
the fingerprint of the violated conservation of the number of excitations,
which heralds the symmetry broken by USC. We show that in flux-based
superconducting architectures this photon production channel can be coherenly
amplified by Stimulated Raman Adiabatic Passage (STIRAP). This provides a
unique tool for an unambiguous dynamical detection of USC in present day
hardware. Implementing this protocol would provide a benchmark for control of
the dynamics of USC architectures, in view of applications to quantum
information and microwave quantum photonics.
