  In the ultrastrong-coupling regime, the quantum Rabi model can exhibit
quantum phase transition (QPT) when the ratio of the qubit transition frequency
to the frequency of the cavity field approaches infinity. However, it is
challenging to control the QPT in few-body systems because of the limited
coupling strength and the A^2 terms. Here, we propose a practical scheme to
manipulate the QPT of quantum Rabi model in the strong-coupling regime. By
applying a periodic frequency modulation to the two-level system in a standard
quantum Rabi model in the strong-coupling regime, an anisotropic quantum Rabi
model with ultrastrong and tunable coupling strengths for rotating and
counter-rotating terms is obtained. The ground-state and excitation energy of
this model in terms of the modulation parameters are studied. We find that the
QPT of quantum Rabi model can be observed in the strong-coupling regime and
externally controlled by the modulation.
