Measuring Hot-Spot Interaction Length in Single-Strip SNSPD

Superconducting nanowire single photon detectors (SNSPD) functionality is based on the local suppression of the superconducting order parameter upon photon absorption, causing a change in resistance which can be detected as a DC voltage pulse. Finding the size and the profile of the superconducting order parameter suppression area (the so-called “hot-spot”) experimentally is a challenging task. Here, we report the results of a quantum detector tomography on micron-length SNSPDs. The high internal detection efficiency of our SNSPDs allowed us to extract single and double photon efficiencies from the count rate vs radiation power dependence and extract hot-spot interaction length. We investigated a series of SNSPD samples made of NbN films with different sheet resistance and a MoSi film, with various widths of the stripe. The experimental results confirmed that regardless of the material or the film resistance, the hot-spot interaction length coincides with the strip width which is a promising feature for making a 2-photon counter with high fidelity.