Quantum electrodynamics of a dissipative phase transition in a Josephson junction

It is conventional to rely on dc resistance and its scaling with temperature when assigning a system into the class of superconductor or insulator. However, for a Josephson junction in an ohmic environment, the conventional approach has failed to produce a definite answer. As a result, the existence of a dissipative phase transition between the superconducting and the insulating behavior of a Josephson junction remains controversial. Here, instead of measuring the junction's resistance, we study how the junction elastically and inelastically scatters the environmental photons. Instead of ohmic resistance, our environment is a Josephson transmission line with a non-dissipative wave impedance, the value of which we can tune. We probe the system at the single photon level and measure the junction's reflection coefficient at frequencies exceeding the temperature. Finally, we could follow the scaling of the reflection coefficient by tuning the probe's frequency, which is more reliable than the temperature. Our approach allows us to separate the superconducting and the insulating behavior of a Josephson junction. As the environment's impedance goes up, the junction looks more and more like a capacitor to the reflecting photons, revealing the junction's insulating nature.