Abstract: The 2025 Nobel Prize honors the discovery of Macroscopic Quantum Tunneling (MQT). However, standard superconducting qubits, such as the transmon, are engineered to suppress this phenomenon in a Josephson Junction (JJ), thereby localizing its phase to create a nonlinear inductor. We investigate the opposite regime where MQT proliferates, and the junction undergoes a quantum phase transition to an insulating state. Here, the phase delocalizes while its conjugate variable—charge—becomes localized, effectively turning the junction into a nonlinear "Bloch" capacitor.
We experimentally realize this underexplored insulating state by galvanically connecting a junction to a high-impedance transmission line composed of thousands of large-area JJs. This line achieves characteristic impedances of 5*Resistance quantum while creating a bath of accessible standing wave modes, for which the junction acts as a nonlinear capacitive termination. Consequently, the junction scatters incoming photons, inducing measurable frequency shifts and spectral broadening. We further combine microwave spectroscopy with DC excitation to probe charging effects, such as Coulomb blockade. With our hybrid DC-RF setup, we elucidate the quantum electrodynamics of dual superconducting circuits, which holds promise for advancing quantum circuit theory and metrological techniques.