The polaron picture is employed to investigate and elucidate the physics of the two-qubit quantum Rabi model, which describes two identical qubits coupled to a common harmonic oscillator. This approach enables us to obtain the ground-state energy and some other simpler physical observables with high accuracy in all regimes of the coupling strengths g, while there is no constraint to the ration of tunneling frequency Ω and field frequency ω, which is not simultaneously possible using previous methods. In addition, we also discover a phenomenon that is not present in the one-qubit Rabi model: with the increase of coupling strength g, there is a transition of the ground state of the system from a multipolaron state to a bipolaron state. However, the tunneling frequency Ω counteracts this process. Specifically, when the tunneling frequency Ω=0, the system always stays in the bipolaron state.
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Sun et al. (2020) studied this question.