  The coherent process that a single photon simultaneously excites two qubits
has recently been theoretically predicted by
[https://link.aps.org/doi/10.1103/PhysRevLett.117.043601 {Phys. Rev. Lett. 117,
043601 (2016)}]. We propose a different approach to observe a similar dynamical
process based on a superconducting quantum circuit, where two coupled flux
qubits longitudinally interact with the same resonator. We show that this
simultaneous excitation of two qubits (assuming that the sum of their
transition frequencies is close to the cavity frequency) is related to the
counter-rotating terms in the dipole-dipole coupling between two qubits, and
the standard rotating-wave approximation is not valid here. By numerically
simulating the adiabatic Landau-Zener transition and Rabi-oscillation effects,
we clearly verify that the energy of a single photon can excite two qubits via
higher-order transitions induced by the longitudinal couplings and the
counter-rotating terms. Compared with previous studies, the coherent dynamics
in our system only involves one intermediate state and, thus, exhibits a much
faster rate. We also find transition paths which can interfere. Finally, by
discussing how to control the two longitudinal-coupling strengths, we find a
method to observe both constructive and destructive interference phenomena in
our system.
