  We study ultrastrong-coupling quantum-phase-transition phenomena in a
few-qubit system. In the one-qubit case, three second-order transitions occur
and the Goldstone mode emerges under the condition of ultrastrong-coupling
strength. Moreover, a first-order phase transition occurs between two different
superradiant phases. In the two-qubit case, a two-qubit Hamiltonian with
qubit-qubit interactions is analyzed fully quantum mechanically. We show that
the quantum phase transition is inhibited even in the ultrastrong-coupling
regime in this model. In addition, in the three-qubit model, the superradiant
quantum phase transition is retrieved in the ultrastrong-coupling regime.
Furthermore, the N-qubit model with U(1) symmetry is studied and we find that
the superradiant phase transition is inhibited or restored with the
qubit-number parity.
