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Thermal properties of NbN single-photon detectors
We investigate thermal properties of a NbN single-photon detector capable of unit internal detec-
tion efficiency. Using an independent calibration of the coupling losses we determine the absolute
optical power absorbed by the NbN film and, via a resistive superconductor thermometry, the ther-
mal resistance Z(T) of the NbN film in dependence of temperature. In principle, this approach
permits a simultaneous measurement of the electron-phonon and phonon-escape contributions to
the energy relaxation, which in our case is ambiguous for their similar temperature dependencies.
We analyze the Z(T) within the two-temperature model and impose an upper bound on the ratio
of electron and phonon heat capacities in NbN, which is surprisingly close to a recent theoretical
lower bound for the same quantity in similar devices.