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General trace formula for heat flux fluctuations

Within the framework of macroscopic quantum electrodynamics and scattering theory, we derive the general expressions for the variance of radiative heat transfer between two arbitrarily shaped objects placed in an arbitrary environment in terms of their T-operators. The such derived expression is valid in the far- and near-field regime of thermal radiation as well as for reciprocal and non-reciprocal objects of any size within a reciprocal or non-reciprocal environment as long as the distances between the objects and their sizes are covered by the macroscopic approach. As special cases we discuss the variance of the radiative heat flux between two nanoparticles and between a nanoparticle and a substrate in near- and far-field regime.