  Spontaneous symmetry breaking (SSB) is crucial to the occurrence of phase
transitions. Once a phase transition occurs, a quantum system presents
degenerate eigenstates that lack the symmetry of the Hamiltonian. After
crossing the critical point, the system is essentially evolved to a quantum
superposition of these eigenstates until decoherence sets in. Despite the
fundamental importance and potential applications in quantum technologies, such
quantum-mechanical SSB phenomena have not been experimentally explored in
many-body systems. We here present an experimental demonstration of the SSB
process in the Lipkin-Meshkov-Glick model, governed by the competition between
the individual driving and intra-qubit interaction. The model is realized in a
circuit quantum electrodynamics system, where 6 Xmon qubits are coupled in an
all-to-all manner through virtual photon exchange mediated by a resonator. The
observed nonclassical correlations among these qubits in the symmetry-breaking
region go beyond the conventional description of SSB, shedding new light on
phase transitions for quantum many-body systems.
