  We investigate theoretically how the spectroscopy of an ancillary qubit can
probe cavity (circuit) QED ground states containing photons. We consider three
classes of systems (Dicke, Tavis-Cummings and Hopfield-like models), where
non-trivial vacua are the result of ultrastrong coupling between N two-level
systems and a single-mode bosonic field. An ancillary qubit detuned with
respect to the boson frequency is shown to reveal distinct spectral signatures
depending on the type of vacua. In particular, the Lamb shift of the ancilla is
sensitive to both ground state photon population and correlations. Back-action
of the ancilla on the cavity ground state is investigated, taking into account
the dissipation via a consistent master equation for the ultrastrong coupling
regime. The conditions for high-fidelity measurements are determined.
