Downconversion of quantum fluctuations of photonic heat current in a circuit

We discuss the nonzero frequency noise of heat current with the explicit example of energy carried by thermal photons in a circuit. Instead of the standard circuit modeling that gives a convenient way of predicting timeaveraged heat current, we describe a setup composed of two resistors forming the heat baths by collections of bosonic oscillators. In terms of average heat transport this model leads to identical results with the conventional one, but besides this, it yields a convenient way of dealing with noise as well. The nonzero-frequency heat current noise does not vanish in equilibrium even at zero temperature, a result that is known for, e.g., electron tunneling. We present a modulation method that can convert the difficult-to-measure high-frequency quantum noise down to zero frequency.