Photon number dependent afterpulsing in superconducting nanostrip single-photon detectors

Abstract

Superconducting nanostrip single-photon detectors (SNSPDs) are widespread tools in photonic quantum technologies. Here, we study the afterpulsing phenomenon in commercial SNSPDs exhibiting significant levels of afterpulses even at low counting rates. We find different contributions, where the probability of an afterpulse is not a constant but depends on the mean number of photons per light pulse including mean numbers much less than 1. Our observations exclude the electrical circuit as the primary cause of the main contribution to the observed afterpulsing probability, which exhibits a strong dependence on mean photon number. We propose a phenomenological model that qualitatively explains our findings via the introduction of slowly relaxing “afterpulsing centers,” storing the absorbed photons’ energy for several tens of nanoseconds. We conjecture that two-level systems in amorphous materials are physical candidates for the role of such afterpulsing centers.