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Recently, strong coupling regimes of superconducting qubits or quantum dots inside a micro-wave circuit cavity and BEC atoms inside an optical cavity were achieved experimentally. The strong coupling regimes in these systems were described by the Dicke model. Here, we solve the Dicke model by a $ 1/N $ expansion. In the normal state, we find a $ \sqrt{N} $ behavior of the collective Rabi splitting. In the superradiant phase, we identify an important Berry phase term which has dramatic effects on both the ground state and the excitation spectra of the strongly interacting system. The single photon excitation spectrum has a low energy quantum phase diffusion mode in {\sl imaginary time} with a large spectral weight and also a high energy optical mode with a low spectral weight. The photons are in a number squeezed state which may have wide applications in high sensitive measurements and quantum information processing. Comparisons with exact diagonization studies are made. Possible experimental schemes to realize the superradiant phase are briefly discussed.

Superradiance, Berry phase, photon phase diffusion, and number squeezed state in the U(1) Dicke (Tavis-Cummings) model