Dark Counts in SNSPD Studied With Spatial Resolution

We investigate dark counts in SNSPD with spatial resolution, using nanostrip-patterned devices with 50 Ohm matched coplanar waveguide - tapers. We found that the dark counts originate from one or several points along the strip, which we call ‘weak points’. We found that the dark counts originating from different points can have different dependence of the rate on the temperature, such as the point which makes the main contribution to the dark counts rate at low temperature, makes just a minor contribution or even disappears at higher temperature. We explain this in terms of different realizations of random local environments of different ‘weak points’. Then we tilt the distribution of supercurrent density with a weak perpendicular magnetic field and find that the evolution of count rates generated at different ‘weak points’ is consistent with the suggestion that these ‘weak points’ are located at the edges of the strip. All our findings agree with the picture of fluctuating vortex entry through the minima of the order parameter at the edge of the strip as the origin of dark counts in SNSPD.