Observation of a collective mode of an array of transmon qubits

Arrays of transmon qubits coupled to a $\lambda/2$ superconducting coplanar waveguide resonator have been studied by microwave spectroscopy. The emergence of a collective mode has been discovered for a cluster of $N>5$ qubits, whose coupling constant to the electromagnetic field in the resonator is $\sqrt{N}$ times greater compared to a single qubit. In addition, the emergence of collective multiphoton transitions exciting higher levels of a qubit cluster has been demonstrated and the interaction of an individual qubit with such a cluster has been investigated.