A new paradigm in open quantum systems: a transmon qubit coupled to a mesoscopic and tunable environment*

At ultra-strong coupling, an open quantum system forms a non-perturbative hybridized state with its environment. In the past few years, this regime has been reached for superconducting qubits coupled to single mode resonators or to the continuum of modes in a superconducting transmission. Here we go a step further down this route by realizing an in-situ tunable multimode environment, the degrees of which can be individually monitored. To do so we interface for the first time a transmon qubit to a superconducting metamaterial consisting of a one-dimensional array of 4700 SQUIDs. We demonstrate ultra-strong coupling with a qubit decay rate of up to 10% of its natural frequency, and hybridization with many (up to 10) environmental modes at a time. Owing to the simplicity of the transmon qubit design, we are able to obtain quantitative agreement between experiment and a theoretical model with no unknown parameters, over a broad parameter range.