Nonequilibrium phase transitions pose many questions and challenges, in part due to their complexity in theoretical descriptions, in part due to the experimental difficulties in systematically controlling systems out of equilibrium. In work now published in a new PRX article, the Houck Lab has studied a chain of 72 microwave cavities, each coupled to a superconducting qubit. This 1d circuit QED lattice can be driven coherently and thus brought into a nonequilibrium steady state.

In our collaboration with the Houck group we have helped identify experimental evidence for a dissipative phase transition in the system in which the steady state changes dramatically as the mean photon number is increased. Near the boundary between the two observed phases, the system demonstrates bistability, with characteristic switching times as long as 60ms – far longer than any of the intrinsic rates known for the system.

The experiment paves the way for a host of future experiments exploring nonequilbrium physics of many-body photon states in circuit QED systems.