Dependence of count rate on magnetic field in superconducting thin-film TaN single-photon detectors

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 $|{\ensuremath{\mu}}_{0}H|<10$ 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 $\ensuremath{\approx}$$50%$ at the ${180}^{\ensuremath{\circ}}$ 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.