Phase transitions, such as the change of liquid water into ice, help elucidate the complex behavior of systems composed of many particles and occur in all areas of physics. Recently, theorists have predicted that a cavity containing only a single atom should transition from opaque to transparent when the input photon flux reaches a critical number. 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 so-called first-order phase transition, which has been observed for the first time in a dissipative quantum system.

Our setup consists of a microchip with a superconducting microwave resonator acting as the cavity and a few superconducting qubits—the quantum equivalent of a digital bit—acting as the atoms. We cool the chip to 0.01 K and send a coherent and continuous microwave tone to the input of the resonator on the chip. On the output side we amplify and detect the transmitted tone. For certain input powers, we detect a real-time telegraph signal between full transmission and zero transmission. Interestingly, the lifetimes of the two states have been observed to be much longer than the coherence times of the individual qubits and the cavity, which points to a significant stabilization of the two phases.

The high photon number limit in systems like ours is relatively poorly understood, and we hope to shed some light on the underlying physics. Our techniques can also be extended to multicavity systems and lattices forming artificial crystals of light. Such setups could be used to simulate certain quantum systems orders of magnitude faster than numerical simulations on a supercomputer cluster.