| because of their universal nature , fano fluctuations are expected to influence the response of superconducting nanowire single - photon detectors ( snspds ) . we predict that photon counting rate ( @xmath0 ) as a function of bias current ( @xmath1 ) in snspds is described by an integral over a transverse coordinate - dependent complementary error function . the latter describes smearing of local responses due to fano fluctuations of the amount of energy deposited into electronic system . the finite width , @xmath2 , of the @xmath0 vs @xmath1 arises from fluctuations in the energy partition between quasiparticles and phonons during the energy down - conversion cascade . in narrow - nanowire snspds the local responses are uniform , and the effect of fano - fluctuations on @xmath2 is dominant . in wide - nanowire snspds with strong coordinate dependence of local responses due to vortex - antivortex unbinding and vortex entry from edges , fano - fluctuations smear singularities imprinted by vorticity on the transition part of @xmath0 curve . we demonstrate good agreement between theory and experiments for a series of bath temperatures and photon energies in narrow - wire wsi snspds . the time - resolved hotspot relaxation curves predicted by fano fluctuations match the lorentzian shapes observed in experiments over the whole range of bias currents investigated except for their tails . |