  Superconducting qubits acting as artificial two-level atoms allow for
controlled variation of the symmetry properties which govern the selection
rules for single and multiphoton excitation. We spectroscopically analyze a
superconducting qubit-resonator system in the strong coupling regime under one-
and two-photon driving. Our results provide clear experimental evidence for the
controlled transition from an operating point governed by dipolar selection
rules to a regime where one- and two-photon excitations of the artificial atom
coexist. We find that the vacuum coupling between qubit and resonator can be
straightforwardly extracted from the two-photon spectra where the detuned
two-photon drive does not populate the relevant resonator mode significantly.
