Abstract
By analytically solving the quantum Rabi model, we investigate the photonic properties of its ground eigenstate. In particular, we find that in the deep strong coupling regime, where the coupling strength g exceeds the mode frequency ω, the photonic state is effectively squeezed in one of its quadratures. The squeezing reaches its maximum at the curve corresponding to the quantum phase transition of the quantum Rabi system, and decreases rapidly on both sides of the phase transition. Notably, for g/ω≈ 3, which is experimentally testable in existing trapped-ion platforms, the achievable squeezing parameter can reach approximately r≈ 0.8. Intriguingly, the photonic state is squeezed while its number distribution follows a super-Poissonian distribution, with the largest deviation from Poissonian behavior occurring at the phase transition between the normal and superradiant phases. In other words, the ground state of the quantum Rabi model contains super-Poissonian quantum squeezed photons.