  Spontaneous symmetry breaking (SSB) plays a central role in understanding a
large variety of phenomena associated with phase transitions, such as
superfluid and superconductivity. So far, the transition from a symmetric
vacuum to a macroscopically ordered phase has been substantially explored. The
process bridging these two distinct phases is critical to understanding how a
classical world emerges from a quantum phase transition, but so far remains
unexplored in experiment. We here report an experimental demonstration of such
a process with a quantum Rabi model engineered with a superconducting circuit.
We move the system from the normal phase to the superradiant phase featuring
two symmetry-breaking field components, one of which is observed to emerge as
the classical reality. The results demonstrate that the environment-induced
decoherence plays a critical role in the SSB.
