Photon-blockade breakdown as a first-order dissipative phase transition in zero dimension

I present the recent concept of first-order dissipative phase transitions, that can occur in meso- and even microscopic quantum systems. One of the first examples of this phenomenon was the photon-blockade breakdown (PBB) effect. It occurs most simply in the driven-dissipative Jaynes-Cummings model describing the prototypical cavity QED scenario: a coupled system of a bosonic mode and a qubit.

For PBB, an abstract thermodynamic limit has been identified [1], where the coupling g between the subsystems goes to infinity without affecting the system size (hence the designation zero-dimensional), and this limit was studied in a finite-size scaling approach [2], with scaling exponents determined numerically. I discuss the microscopic mechanism and the fully quantum solution, and assess the connection with optical bistability via the neo- and semiclassical models.

I describe the experimental studies: PBB was first observed in a circuit QED system [3], and the thermodynamic limit could also be modeled on this platform with a bespoke device [4]. Even though the system remains microscopic, its behavior becomes increasingly macroscopic as a function of g/κ (κ – resonator linewidth). For the highest realized g/κ≈287, the system switches with a characteristic timescale as long as 6 s between a bright coherent state with approximately 10⁴ intracavity photons and the vacuum state. This exceeds the microscopic timescales by 6 orders of magnitude.

The numerical modelling [5] of this latter set of experiments highlighted the role of the higher lying transmon levels and the phase noise.