An energy-resolving superconducting nanowire photon counter

We report on the energy-resolving capability of a superconducting NbN nanowire photon counter, which is read out by a superconducting quantum interference device. For counters operated at 6.5 K, a resolution of 0.55 eV was measured in the wavelength range from 1000 to 1500 nm ( photon energies 1.2-0.8 eV) along with a counting rate of 2 MHz. The best energy resolution occurred in the spectral range where the quantum efficiency of the counter began to decrease with the wavelength. The results are explained by the change of the detection scenario from the hot-spot formation to unbinding of vortex-antivortex pairs.