  The most important difference between ultrastrong and non-ultrastrong
coupling regimes is that the ground state contains excitations. We consider a
qubit-plasmon-phonon ultrastrong coupling (USC) system with a three-level atom
coupled to the photon and phonon via its upper two energy levels and show that
spontaneous emission of the atom from its intermediate to its ground state
produces photon and phonon pairs. It is shown that the current system can
produce a strong photon/phonon stream and the atom-phonon coupling plays the
active role, which ensures the experimental detection. The emission spectrum
and various high-order correlation functions confirm the generation of the
pairs of photons and phonons. Our study has important implications for future
research on virtual photon and phonon pairs creation in the ground state of the
USC regime.
