  When an atom is strongly coupled to a cavity, the two systems can exchange a
single photon through a coherent Rabi oscillation. This process enables precise
quantum-state engineering and manipulation of atoms and photons in a cavity,
which play a central role in quantum information and measurement. Recently, a
new regime of cavity QED has been reached experimentally where the strength of
the interaction between light and artificial atoms (qubits) becomes comparable
to the atomic transition frequency or the resonance frequency of the cavity
mode. Here we show that this regime can strongly modify the concept of vacuum
Rabi oscillations, enabling multiphoton exchanges between the qubit and the
resonator. We find that experimental state-of-the-art circuit- QED systems can
undergo two- and three-photon vacuum Rabi oscillations. These anomalous Rabi
oscillations can be exploited for the realization of efficient Fock-state
sources of light and complex entangled states of qubits.
