  Superradiant phase transition (SPT) in thermal equilibrium, as a fundamental
concept bridging the statistical physics and electrodynamics, can offer the key
resources for quantum information science. Notwithstanding its fundamental and
practical significances, equilibrium SPT has never been observed in experiments
since the first proposal in the 1970s. Furthermore, the existence of
equilibrium SPT in the cavity quantum electrodynamics (QED) systems is still
subject of ongoing debates, due to the no-go theorem induced by the so-called
A2 term. Based on the platform of nuclear magnetic resonance (NMR), here we
experimentally demonstrate the occurrence of equilibrium SPT beyond no-go
theorem by introducing the antisqueezing effect. The mechanism relies on the
antisqueezing that recovers the singularity of the ground state via
exponentially enhancing the zero point fluctuation (ZPF) of system. The strong
entanglement and squeezed Schrodinger cat states of spins are achieved
experimentally in the superradiant phase, which may play an important role in
fundamental tests of quantum theory, implementing quantum metrology and
high-efficient quantum information processing. Our experiment also shows the
antisqueezing-enhanced signal-to-noise rate (SNR) of NMR spectrum, providing a
general method for implementing high-precision NMR experiments.
