  The hybridization of distinct quantum systems is now seen as an effective way
to engineer the properties of an entire system leading to applications in
quantum metamaterials, quantum simulation, and quantum metrology. One well
known example is superconducting circuits coupled to ensembles of microscopic
natural atoms. In such cases, the properties of the individual atom are
intrinsic, and so are unchangeable. However, current technology allows us to
fabricate large ensembles of macroscopic artificial atoms such as
superconducting flux qubits, where we can really tailor and control the
properties of individual qubits. Here, we demonstrate coherent coupling between
a microwave resonator and several thousand superconducting flux qubits, where
we observe a large dispersive frequency shift in the spectrum of 250 MHz
induced by collective behavior. These results represent the largest number of
coupled superconducting qubits realized so far. Our approach shows that it is
now possible to engineer the properties of the ensemble, opening up the way for
the controlled exploration of the quantum many-body system.

Quantum Physics