Classical and quantum phase-space behavior of a spin-boson system

We study the phase-space behavior of the standard two-level system (spin 1/2) coupled to a harmonic oscillator (without the rotating-wave approximation), the classical counterpart of which is known to display deterministic chaos. We study the quantum-mechanical phase-space behavior by means of harmonic-oscillator coherent states (Husimi representation). Stationary quantum Poincar\'e sections may be defined; they show many parallels to their classical counterparts. We also study the time development of initially coherent states and observe a tendency towards a decay into many small wave packets in phase space for parameters in the chaotic regime.