We report on the energy resolution of a recently developed superconductor single-photon quantum detector. In a
superconducting strip quasiparticles created by a single absorbed photon and a bias current jointly cause a normal domain and, subsequently, a voltage transient that manifests absorption of the photon. Given a constant optical coupling, the mechanism suggests a moderate to good energy resolution in the wavelength range from near-infrared to X-rays. We implemented a meander line from a 80-nm wide and 5-nm thick NbN strip to detect single near-infrared photons with the repetition rate 5•107 sec-1 and quantum efficiency of few per cent. Although with this detector operated at 2 K we have indeed observed photon-energy dependent detector response, the energy resolving capability appeared
smaller than the detector model predicted. We suggest that the inconsistency owes to the influence of the bias current.
We analyze the spectral performance of recently developed single-photon quantum detector that consists of a narrow, nanometer sized meander-line made from ultra-thin superconducting film. The detector exploits a combined detection mechanism, in which avalanche multiplication of quasiparticles after absorption of a single photon and the bias current jointly produce a normal domain that results in a voltage pulse developing between the meander ends. With either the wavelength increase or the bias current decrease, the single-photon detection regime exhibits a cut-off. The wavelength, at which the cut-off occurs, varies from infrared waves to visible light depending on the superconducting material and operation conditions. Structural and geometrical non-uniformities of the meander line smooth out the otherwise expected sharp drop of the detection efficiency beyond the cut-off. We refine the early detector model and propose a tentative explanation of how superconducting fluctuations may additionally extend the detection efficiency beyond the cut-off wavelength.