Photon-number dependent afterpulsing in superconducting nanostrip single-photon detectors

TL;DR

This study reveals that afterpulsing in superconducting nanostrip single-photon detectors depends on photon number, likely caused by two-level systems in amorphous materials, challenging previous circuit-based explanations.

Contribution

It introduces a photon-number dependent model for afterpulsing in SNSPDs, highlighting the role of slowly relaxing centers like two-level systems in amorphous materials.

Findings

Afterpulsing probability varies with photon number.

Electrical circuit is not the primary cause of afterpulsing.

Two-level systems are plausible physical candidates for afterpulsing centers.

Abstract

Superconducting nanostrip single-photon detectors (SNSPD) are wide-spread tools in photonic quantum technologies. Here, we study afterpulsing in commercial SNSPD made of amorphous superconducting material. We find that 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 one. Our observations exclude the electrical circuit as the primary cause of afterpulsing. We propose a phenomenological model which qualitatively explains our findings via the introduction of slowly relaxing "afterpulsing centers". We argue that two-level systems in amorphous materials are the most plausible physical candidates for the role of such afterpulsing centers.