Quantum Noise of Electron–Phonon Heat Current

We analyze heat current fluctuations between electrons and phonons in a metal. In equilibrium we recover the standard result consistent with the fluctuation–dissipation theorem. Here we show that heat current noise at finite frequencies remains non-vanishing down to zero temperature. From the experimental point of view, it is a small effect and up to now elusive. We briefly discuss the impact of electron–phonon heat current fluctuations on calorimetry, particularly in the regime of single microwave-photon detection.