We derive the Hamiltonian of a superconducting circuit that comprises a single-Josephson-junction flux qubit and an LC oscillator. If we keep the qubits lowest two energy levels, the derived circuit Hamiltonian takes the form of the quantum Rabi Hamiltonian, which describes a two-level system coupled to a harmonic oscillator, regardless of the coupling strength. To investigate contributions from the qubits higher energy levels, we numerically calculate the transition frequencies of the circuit Hamiltonian. We find that the qubits higher energy levels mainly cause an overall shift of the entire spectrum, but the energy level structure up to the seventh excited states can still be fitted well by the quantum Rabi Hamiltonian even in the case where the coupling strength is larger than the frequencies of the qubit and the oscillator, i.e., when the qubit-oscillator circuit is in the deep-strong-coupling regime. We also confirm that some of the paradoxical properties of the quantum Rabi Hamiltonian in the deep-strong-coupling regime, e.g. the non-negligible number of photons and the nonzero expectation value of the flux in the oscillator in the ground state, arise from the circuit Hamiltonian as well.
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