  Waveguide quantum electrodynamics offers a wide range of possibilities to
effectively engineer interactions between artificial atoms via a
one-dimensional open waveguide. While these interactions have been
experimentally studied in the few qubit limit, the collective properties of
such systems for larger arrays of qubits in a metamaterial configuration has so
far not been addressed. Here, we experimentally study a metamaterial made of
eight superconducting transmon qubits with local frequency control coupled to
the mode continuum of a waveguide. By consecutively tuning the qubits to a
common resonance frequency we observe the formation of super- and subradiant
states, as well as the emergence of a polaritonic bandgap. Making use of the
qubits quantum nonlinearity, we demonstrate control over the latter by inducing
a transparency window in the bandgap region of the ensemble. The circuit of
this work extends experiments with one and two qubits towards a full-blown
quantum metamaterial, thus paving the way for large-scale applications in
superconducting waveguide quantum electrodynamics.
