  Here, we report on successful operation of a NbN thin film superconducting
nanowire single-photon detector (SNSPD) in a coherent mode (as a mixer) at the
telecommunication wavelength of 1550 nm. Providing the Local Oscillator power
of the order of a few picowatts, we were practically able to reach the quantum
noise limited sensitivity. The intermediate frequency gain bandwidth was
limited by the spectral band of single-photon response pulse of the detector,
which is proportional to the detector size. We observed gain bandwidth of 65
MHz and 140 MHz for 7x7 um^2 and 3x3 um^2 devices respectively. Tiny amount of
the required Local Oscillator power and wide gain and noise bandwidths along
with the needless of any Low Noise Amplification opens possibility for a photon
counting heterodyne-born megapixel array development.
