Agenda

Résumé : Recent advances in superconducting circuits have opened access to new and exotic regimes of quantum physics. By applying strong drives, individual or coupled artificial atoms can be pushed far from thermodynamic equilibrium, where coherent quantum evolution competes with external driving and dissipation. In this regime, non-thermal steady states emerge, providing an experimental platform to test out-of-equilibrium many-body theories that predict novel phase transitions—whose classification remains largely unresolved. In this talk, I will present an experiment on an array of 21 nonlinear oscillators operating close to the thermodynamic limit. The system exhibits a bistable first-order transition, revealed through spectroscopic measurements, emission power, and time-resolved dynamics. The first two are found to be in excellent agreement with Gaussian fluctuations around a mean-field theory, while the latter shows bistable lifetimes spanning four orders of magnitude, enabling the reconstruction of the system’s phase diagram. Interestingly, the location of this phase boundary has not been derived analytically so far, even approximately. This difficulty arises because, although the system displays many features of a genuine thermodynamic transition, no satisfactory thermodynamic potential has yet been formulated. In the second part of the talk, I will show how this issue can be understood and overcome using a semiclassical limit of the Keldysh path-integral formulation of the model. Within this framework, the phase boundary can be obtained via an instanton approach, paving the way for powerful semi-analytical methods to study driven-dissipative quantum phase transitions.