Observation of the Photon-Blockade Breakdown Phase Transition*

Phase transitions, such as the change of liquid water into ice, are prominent features of the complex behavior of systems composed of many particles. Recently, theorists have predicted that a cavity containing only a single strongly coupled atom should transition from opaque to transparent when the input photon flux reaches a critical number [1]. And just as water and ice can coexist at the melting point temperature, the cavity was predicted to be both opaque and transparent close to the critical point, stochastically switching between the two states. This coexistence is a hallmark for a first-order phase transition.
We report the observation of this first-order dissipative quantum phase transition in a circuit QED system [2]. It takes place when the photon blockade of a driven resonator-qubit system is broken by increasing the drive power. The observed experimental signature is a bimodal phase space distribution with varying weights controlled by the drive strength. Our measurements show an improved stabilization of the classical attractors up to the millisecond range when the size of the quantum system is increased from one to three supercondcuting qubits. Controlling the formation of such robust pointer states is a first step to further investigate multiphoton phases of finite-size, nonlinear, open quantum systems.
[1] H. J. Carmichael, Phys. Rev. X 5, 031028 (2015)
[2] J. M. Fink, A. Dombi, A. Vukics, A. Wallraff, and P. Domokos, Phys. Rev. X 7, 011012 (2017)