  The ultrastrong-coupling regime, where the atom-cavity coupling rate reaches
a considerable fraction of the cavity or atom transition frequencies, has been
reported in a flux qubit superconducting quantum circuit coupled to an on-chip
coplanar resonator. This regime enables the possibility of manipulating the
cavity quantum electrodynamic ground state with controllable physical
properties and situations may arise where the resonator field $\hat X= \hat
a+\hat a^\dagger$ acquires a nonzero expectation value in the system ground
state. We demonstrate that in this case the parity symmetry of an additonal
artificial atom with an even potential is broken by the interaction with the
resonator. Such mechanism is in close analogy with the Higgs mechanism where
the gauge symmetry of the weak force's gauge bosons is broken by the nonzero
vacuum expectation value of the Higgs field. The results here presented open
the way to controllable experiments on symmetry breaking mechanisms induced by
nonzero vacuum expectation values. Moreover the here proposed mechanism can be
used as a probe of the ground state macroscopic coherence emerging from quantum
phase transitions with vacuum degeneracy.
