Figure 1

Operation principle of a SNSPD as a coherent detector and layout. (а) Schematic view of the SNSPD in coherent detection mode, in which two EM waves with slightly different frequencies (f _LO , f _S ) are sent to the SNSPD. (b) Time domain beating of the EM-field generated overlapping LO and S fields, with slightly different frequencies. (с) Power oscillation in time at an intermediate frequency f _IF  = |f _LO  − f _S |. (d) Schematic representation of the detection pulses frequency modulation in presence of beating, which reproduces the optical power amplitude modulation. (e) Schematic view of the counts histogram vs time. By measuring the frequency of the counts oscillations f _IF and knowing the f _LO , one can determine the frequency of the unknown signal f _S  = |f _LO  − f _IF |. (f) Representation of the coherent detection in the frequency domain. The intermediate frequency is transferred from the IR range to a lower frequency. In addition to the signal and LO, the image channel contributes noise to P _IF . (g) False color optical micrograph of the nanophotonic devices. Each device consists of two focusing grating couplers, nanophotonic waveguides with a 50:50 Y-splitter, where one branch reaches a NbN nanowire connected to Au contact pads. (h) False color SEM image of a typical NbN nanowire atop a nanophotonic waveguide.