Signature of resonant modes in radiative heat current noise spectrum

Radiative heat transfer between bodies is often dominated by a narrow resonance in the transmission, e.g., due to a cavity mode or a surface excitation. However, this resonant character is not visible in the average heat current. Here, we show that the noise spectrum of heat current can serve as a direct probe of the heat-carrying excitations. Namely, the resonant mode produces a sharp peak in the noise spectrum with a width related to the mode lifetime. We demonstrate that heat transfer in realistic superconducting circuits or between two-dimensional metals can realize our predictions. We also arrive at a general conclusion that the Landauer-type description for the radiative heat transfer breaks down on the level of the noise, while it holds for the average.