Magnetic-field dependence of count rates in superconducting thin-film TaN single-photon detectors

TL;DR

This study investigates how magnetic fields affect dark and photon count rates in superconducting TaN nanowire single-photon detectors at 4 K, revealing asymmetric dark-count behavior and unexpected field insensitivity of photon counts.

Contribution

It provides the first detailed analysis of magnetic-field effects on TaN superconducting single-photon detectors, highlighting asymmetries and challenging existing vortex-crossing detection models.

Findings

Dark-count rates depend asymmetrically on magnetic field direction.

Photon count rates are largely unaffected by magnetic fields.

Critical current reduction at meander turns explains dark-count behavior.

Abstract

We have studied the magnetic-field dependence of both dark-count rates and photon-count rates in a superconducting nanowire single-photon detector made of TaN in external magnetic fields $|\mu_0H| < 10$ mT perpendicular to the plane of the underlying meander structure and at $T = 4$ K. The dark-count rates show a characteristic field-dependence, which is asymmetric with respect to magnetic field direction. The field- and the current dependence of the dark counts can be quantitatively well explained if one assumes that the critical current is reduced to $\approx50%$ at the $180^\circ$ meander turns when compared to a straight strip, and the observed asymmetry can be modeled assuming that the turnarounds are not all strictly equal. Surprisingly, the photon count rates do not show any significant field dependence, which seems to be at odds with existing detection models invoking vortex crossings.