Double Rabi model in the ultra-strong coupling regime: entanglement and chaos beyond the rotating wave approximation

This work studies the relationship between entanglement and quantum chaos, by investigating the dynamical generation, transfer, and decay of quantum entanglement in a quadripartite system consisting of two qubits and two cavity modes, termed as the double Rabi model (DRM). In particular, it is found that the entanglement profile of DRM eigenstates changes qualitatively as the system is tuned from quasi-integrable regimes to quantum chaos regimes. Results indicate that in the chaotic regime non-local quantum correlations tend to involve all degrees of freedom in the system, with the least robust being the pairwise entanglement. The entanglement dynamics for various parts of the system is then studied in detail, with emphasis placed on the shortcoming of the rotating wave approximation. For example, how the counter rotating terms significantly change the qualitative features of the entanglement transfer picture and decoherence-free states is emphasized. Finally, we revisit the issue regarding whether chaos may enhance or suppress entanglement generation, offering a picture different from those in previous studies.