Extended Data Fig. 1: Methodology of power measurement.
Measurement and analysis steps required to measure the heat flow and extract \({\kappa }_{{\rm{xx}}}\). As an example, we present \(\nu =2\) data, measured at \(B=6.1{\rm{T}}\), and base temperature of \({T}_{0}=15{\rm{mK}}\). (a) Raw noise data. The excess noise measured at \({A}_{{\rm{S}}}\) and \({A}_{{\rm{PM}}}\) as a function of the sourced current \(I\) sourced from \({S}_{1}\) (while current \(-I\) is simultaneously sourced from \({S}_{2}\)). (b) Power-metre’s temperature as a function of the source’s temperature extracted from (a) using Eq. M7. The heating of the source from \({T}_{0}=15{\rm{mk}}\) to a temperature \({T}_{{\rm{S}}} \sim 40{\rm{mK}}\) causes the slight increase of the PM’s temperature \({T}_{{\rm{PM}}} \sim 17{\rm{mK}}\), due to the finite \({\kappa }_{{\rm{xx}}}\). (c) Power-metre calibration; raw data. Noise measured at \({A}_{{\rm{PM}}}\) as a function of the direct heating of the PM, by current \({I}_{{\rm{cal}}}\) sourced from \({S}_{1}^{{\rm{cal}}}\) (while current \(-{I}_{{\rm{cal}}}\) is simultaneously sourced from \({S}_{2}^{{\rm{cal}}}\)). (d) Dissipated power (derived from Eq. M9) as a function of \({T}_{{\rm{PM}}}\). (e) By combining the main measurement (b) with the calibration (d), we can plot the power arriving to the PM as a function of the source temperature, and produce the plot presented in the main text Fig. 2a. A linear fit to the power vs. \({T}_{{\rm{S}}}^{2}\) gives \({\kappa }_{{\rm{xx}}}\).
