Frequency-renormalized Multipolaron Expansion for the Quantum Rabi Model*

We present a frequency-renormalized multipolaron expansion method to explore the ground state of the quantum Rabi model (QRM). The main idea is to take polaron as starting point to expand the ground state of QRM. The polarons are deformed and displaced oscillator states with variationally determined frequency renormalization and displacement parameters. This method is an extension of the previously proposed polaron concept and the coherent state expansion used in the literature, which shows high efficiency in describing the physics of the QRM. The proposed method is expected to be useful for solving other more complicated light-matter interaction models.