Microwave radiometry of a quantum-critical, hybrid Josephson array
Arrays of Josephson junctions can be tuned through anomalous metallic, quantum-critical, and insulating regimes. We introduce a new experimental probe, capturing microwave radiation across all three regimes, using a two-dimensional array of superconductor-semiconductor hybrid Josephson junctions as a model system. Our approach allows in-situ calibration of the sample's circuit parameters and provides isolation from measurement back-action effects. We measure the radiation temperature of the anomalous metal, and find that it is hotter than both the quantum-critical and insulating regimes. We further show that the anomalous-metallic regime is more susceptible to additional heating than other regimes, explaining its emergence in otherwise thermalized systems. Turning to the quantum-critical regime, we discover nonlinear scaling of radiative noise with applied bias, consistent with theoretical predictions of universal non-equilibrium behavior at quantum critical points.