  The ground state of the photon-matter coupled system described by the Dicke
model is found to be perfectly squeezed at the quantum critical point of the
superradiant phase transition (SRPT). In the presence of the counter-rotating
photon-atom coupling, the ground state is analytically expressed as a two-mode
squeezed vacuum in the basis of photons and atomic collective excitations. The
variance of a quantum fluctuation in the two-mode basis vanishes at the SRPT
critical point, with its conjugate fluctuation diverging, ideally satisfying
the Heisenberg uncertainty principle.
