Exploring the quantum world often starts by drawing a sharp boundary between a microscopic subsystem and the bath to which it is invariably coupled. In most cases, knowledge of thephysical processes occuring in the bath is not required in great detail. However, recent developments in circuit quantum electrodynamics are presenting regimes where the actual dynamics of engineered baths, such as microwave photon resonators, becomes relevant. Here we take a major technological step forward, by tailoring a centimeter-scale on-chip bath from a very long metamaterial made of 4700 tunable Josephson junctions. By monitoring how each measurable bosonic resonance of the circuit acquires a phase-shift due to its interaction with a transmon qubit, one can indirectly measure qubit properties, such as transition frequency, linewidth and non-linearity. This new platform also demonstrates the ultra-strong coupling regime for the first time in the context of Josephson waveguides. Our device combines a large number of modes (up to 10 in the present setup) that are simultaneously hybridised with the two-level system, and a broadening dominated by the artificial environment that is a sizeable fraction of the qubit transition frequency. Finally, we provide a quantitative and parameter-free model of this large quantum system, and show that the finite environment seen by the qubit is equivalent to a truly macroscopic bath.

The quest to understand interaction between light and matter stretches back to the ray optics of Euclid and Ptolemy. In recent decades, studies at the quantum scale were performed bycoupling an isolated emitter to a single mode of the electromagnetic field, standard quantum optics providing a complete toolbox for describing such a setup. Current efforts aim to explore the coherent dynamics of systems containing an emitter coupled to several electromagnetic degrees of freedom. Combining superconducting metamaterials and qubits could allow for the observation of genuinely macroscopic quantum effects such as a giant Lamb shift or non-classical states of multimode optical fields. In this work, we couple a transmon qubit to a high impedance, centimeter-scale, metamaterial waveguide, made of 4700 in-situ tunable Josephson junctions. Our device combines three essential properties required to go beyond the standard quantum optics paradigm and reach the multi-mode, many-body regime, namely: a tunable waveguide with a high density of electromagnetic modes, a qubit non-linearity comparable to the other relevant energy scales, and ultrastrong coupling between the qubit and the waveguide modes. Besides providing experimental evidence for these non-trivial requirements, we also develop a quantitative theoretical description that does not contain any phenomenological parameters and that accurately takes into account vacuum fluctuations of our large scale quantum circuit in the regime of ultrastrong coupling and intermediate non-linearity. Furthermore, we show that the influence on the transmon of our fully controllable on-chip environment well approximates that of the macroscopic bath envisioned in the celebrated work of Caldeira and Leggett. Our work demonstrates that Josephson waveguides offer a promising platform to explore many-body quantum optics.

We present experimental results in which the unexpected zero-two transition of a circuit composed of two inductively coupled transmons is observed. This transition shows an unusualmagnetic flux dependence with a clear disappearance at zero magnetic flux. In a transmon qubit the symmetry of the wave functions prevents this transition to occur due to selection rule. In our circuit the Josephson effect introduces strong couplings between the two normal modes of the artificial atom. This leads to a coherent superposition of states from the two modes enabling such transitions to occur.