Superconducting circuits are currently developed as a versatile platform for the exploration of many-body physics, both at the analog and digital levels. Their building blocks are oftenidealized as two-level qubits, drawing powerful analogies to quantum spin models. For a charge qubit that is capacitively coupled to a transmission line, this analogy leads to the celebrated spin-boson description of quantum dissipation. We put here into evidence a failure of the two-level paradigm for realistic superconducting devices, due to electrostatic constraints which limit the maximum strength of dissipation. These prevent the occurence of the spin-boson quantum phase transition for transmons, even up to relatively large non-linearities. A different picture for the many-body ground state describing strongly dissipative transmons is proposed, showing unusual zero point fluctuations.
Electromagnetic fields possess zero point fluctuations (ZPF) which lead to observable effects such as the Lamb shift and the Casimir effect. In the traditional quantum optics domain,these corrections remain perturbative due to the smallness of the fine structure constant. To provide a direct observation of non-perturbative effects driven by ZPF in an open quantum system we wire a highly non-linear Josephson junction to a high impedance transmission line, allowing large phase fluctuations across the junction. Consequently, the resonance of the former acquires a relative frequency shift that is orders of magnitude larger than for natural atoms. Detailed modelling confirms that this renormalization is non-linear and quantum. Remarkably, the junction transfers its non-linearity to about 30 environmental modes, a striking back-action effect that transcends the standard Caldeira-Leggett paradigm. This work opens many exciting prospects for longstanding quests such as the tailoring of many-body Hamiltonians in the strongly non-linear regime, the observation of Bloch oscillations, or the development of high-impedance qubits.
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 show that the non-local polarization response in a multimode circuit-QED setup, devised from a Cooper pair box coupled to a long chain of Josephson junctions, provides an alternativeroute to access the elusive Kondo screening cloud. For moderate circuit impedance, we compute analytically the universal lineshape for the decay of the charge susceptibility along the circuit, that relates to spatial entanglement between the qubit and its electromagnetic environment. At large circuit impedance, we numerically find further spatial correlations that are specific to a true many-body state.