Utilizing an adiabatic approximation method, a bipartite qudit-oscillator Hamiltonian is studied for low-spin values in both strong and ultrastrong coupling regimes. The quasiprobability densities on the hybrid factorized phase space are introduced. Integrating over a sector of the composite phase space, the quasiprobability distributions of its complementary subsystem are recovered. In the strong coupling regime, the qudit entropy displays a pattern of quasiperiodic collapses and revivals, where the latter coincide with locally minimum entropy configurations. Starting with a bipartite factorizable initial state, we observe that almost pure spin kitten type states dynamically develop at near-null values of entropy. The Hilbert-Schmidt distance measure of these states puts them metrically far away from the initial state. Other localized spin states form at locally minimum but significantly large values of entropy. The evolution to the nonclassical transitory spin states is displayed via the diagonal spin ${\mathrm{P}}_{\mathcal{Q}}$ representation. As another manifestation of nonclassicality the emergence of the spin-squeezed states during the bipartite evolution is observed. In the ultrastrong coupling domain, a large number of interaction-dependent modes and their harmonics are generated. The consequent randomization of the phases eliminates the quasiperiodicity of the system which is now driven towards a stabilization of the entropy that also undergoes stochastic fluctuations around a suitably stabilized value. In both the strong and ultrastrong coupling realms, antibunching of the photoemission events is realized particularly for the small spin values.

• Received 6 May 2020
• Revised 21 April 2021
• Accepted 27 April 2021

DOI:https://doi.org/10.1103/PhysRevA.103.063703