  We explore theoretically the physics of a collection of two-level systems
coupled to a single-mode bosonic field in the non-standard configuration where
each (artificial) atom is coupled to both field quadratures of the boson mode.
We determine the rich phase diagram showing 'superradiant' phases with
different symmetries. We demonstrate that it is possible to pass from a
discrete, parity-like $\mathbb{Z}_2$ symmetry to a continuous U(1) symmetry
even in the ultrastrong coupling regime where the rotating wave approximation
for the interaction between field and two-level systems is no longer
applicable. By applying this general paradigm, we propose a scheme for the
experimental implementation of such continuous U(1) symmetry in circuit QED
systems, with the appearance of photonic Goldstone and amplitude modes above a
critical point.
