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.

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Kenji Hayashida, Takuma Makihara, Nicolas Marquez Peraca, Diego Fallas Padilla, Han Pu, Junichiro Kono, Motoaki Bamba

Perfect Intrinsic Squeezing at the Superradiant Phase Transition Critical Point 

arXiv:2009.02630