  Superradiant phase transitions (SPTs) are important for understanding
light-matter interactions at the quantum level, and play a central role in
criticality-enhanced quantum sensing. So far, SPTs have been observed in
driven-dissipative systems, but the emergent light fields did not show any
nonclassical characteristic due to the presence of strong dissipation. Here we
report an experimental demonstration of the SPT featuring the emergence of a
highly nonclassical photonic field, realized with a resonator coupled to a
superconducting qubit, implementing the quantum Rabi model. We fully
characterize the light-matter state by Wigner matrix tomography. The measured
matrix elements exhibit quantum interference intrinsic of a photonic mesoscopic
superposition, and reveal light-matter entanglement
