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Link visio: https://univ-grenoble-alpes-fr.zoom.us/j/95485055031?pwd=Kn7lPiQXM0bQQb0GLB3Gmh967GR01w.1
The defence will be in English.
Abstract: This thesis explores the dynamics of a small Josephson junction coupled to a high-impedance, finite-size transmission line, focusing on how such a system can emulate correctly the dissipative dynamics of a Josephson junction. While dissipation is traditionally modeled via idealized thermal baths, the discrete mode structure and low losses of a finite transmission line raise the question: can it still act as a good dissipative reservoir? Leveraging the flexibility of superconducting circuits, we embed a small junction in a chain of larger Josephson junctions acting as a high-impedance transmission line. Using a combination of DC and microwave measurements, we access both transport properties and the finite-frequency response, enabling a full characterization of the system. We first study the appearance of dual Shapiro steps in Josephson junctions, where a quantized current is observed under microwave irradiation. This experiment represents the first observation of dual Shapiro steps in Josephson junctions since their theoretical prediction more than thirty years ago. While studying dual Shapiro steps, we find signatures of a previously unaccounted-for current at low voltages. This leads us to identify a new effect, which we call the photonic Joule effect: the junction emits photons into the line, which, due to its finite size and weak coupling to the outside world, causes the modes to overheat. At higher bias, this thermal state becomes unstable, and the system undergoes a transition into a coherent state, similar to a lasing transition, where energy condenses into the lowest modes of the system. We observe and characterize this transition experimentally. Altogether, these results provide a detailed view of the out of equilibrium dynamics of a junction coupled to a structured environment, and highlight the role of finite-size effects in engineered dissipation.
