The properties of ground state in a simple photon phonon interactive model are studied by using a new coherent state function. The energy and wave concise approach, and some discussions on properties of ground state are given.
The coherent and nonlinear properties of a polaritonic crystal (PolC), formed by trapped two-level atoms in an optical cavity array and interacting with an optical field, are analyzed. The nonlinear Schrödinger equation is considered for the dynamics of coupled atom-light states and low-branch polaritons associated with PolCs in the continuous medium limit. The existence of a stable ground-state PolC wave function is predicted using the variational approach. For negative scattering lengths, the wave function collapses in the presence of small quintic nonlinearity.
The Pancharatnam phase in simple polariton model is investigated, and the influences of temperature, coupling strength, frequency difference of photon and phonon and average photon number on the evolution dynamics of Pancharatnam phase are also discussed. The results show that the Pancharatnam phase oscillates with time, the oscillating frequency and wave-form vary with time, and the variation increases with the enhancement of temperature, coupling strength and frequency difference of photon and phonon. The system Pancharatnam phase evolutes regularly when the average photon number of coherent light is small, while it becomes more chaotic as the average photon number increases.
Coherent properties of a two-dimensional spatially periodic structure, polaritonic crystal (PolC) formed by trapped two-level atoms in an optical cavity array interacting with a light field, are analyzed. By considering the wave function overlapping for both photonic and atomic states, a cubic-quintic complex nonlinear Schr\"odinger equation is derived for the dynamics of coupled atom-light states, wave function of low-branch polaritons, associated with PolC in the continuous limit. A variational approach predicts that a stable ground-state wave function of PolC exists but is accompanied by an oscillating width. For a negative scattering length, the wave function collapses in the presence of a small quintic nonlinearity appearing due to a three-body polariton interaction. By studying the nonequilibrium (dissipative) dynamics of polaritons with adiabatic approximation, we have shown that the collapse of PolC wave function can be prevented even in the presence of small decaying of a number of polariton particles.
The ultrastrong coupling between the elementary excitations of matter and microcavity modes is studied in a fully analytical quantum-mechanical theoretical framework. The elementary excitation could be phonons, excitons, plasmons, etc. From the diagonalization of the Hamiltonian, we obtain the ground state of the polariton Hamiltonian. The ground state belongs to the Gaussian class. Using the Gaussian property, we calculate the quantum entanglement in the ground state. We use two different measures for quantum entanglement---entanglement entropy and the logarithmic negativity parameter---and obtain rather simple analytical expressions for the entanglement measures. Our findings show that the amount of quantum entanglement in the ground state is quite significant in the ultrastrong-coupling regime. It can be obtained from the measurement of the polariton frequencies.
The second- and third-order coherence functions g(n)(0) (n=2 and 3) of an exciton-polariton condensate are measured and compared to the theory. Contrary to an ideal photon laser, deviation from unity in the second- and third-order coherence functions is observed, thus showing a bunching effect, but not the characteristics of a standard thermal state with g(n)(0)=n!. The increase in bunching with the order of the coherence function, g(3)(0)>g(2)(0)>1, indicates that the polariton condensate is different from a coherent state, a number state, or a thermal state. The measurement of third-order coherence has the advantage, compared to the second-order one, that the difference between a thermal state and a coherent state is more pronounced. The experimental results are in agreement with the theoretical model where polariton-polariton and polariton-phonon interactions are responsible for the loss of temporal coherence.