  Manipulating the propagation of electromagnetic waves through sub-wavelength
sized artificial structures is the core function of metamaterials. Resonant
structures, such as split ring resonators, play the role of artificial "atoms"
and shape the magnetic response. Superconducting metamaterials moved into the
spotlight for their very low ohmic losses and the possibility to tune their
resonance frequency by exploiting the Josephson inductance. Moreover, the
nonlinear nature of the Josephson inductance enables the fabrication of truly
artificial atoms. Arrays of such superconducting quantum two-level systems
(qubits) can be used for the implementation of a quantum metamaterial. Here, we
perform an experiment in which 20 superconducting flux qubits are embedded into
a single microwave resonator. The phase of the signal transmitted through the
resonator reveals the collective resonant coupling of up to 8 qubits. Quantum
circuits of many artificial atoms based on this proof-of-principle experiment
offer a wide range of prospects, from detecting single microwave photons to
phase switching, quantum birefringence and superradiant phase transitions.
