Effect of magnetic field on the photon detection in thin superconducting meander structures

Abstract

We have studied the influence of an externally applied magnetic field on the photon and dark count rates of meander-type niobium nitride superconducting nanowire single-photon detectors. Measurements have been performed at a temperature of 4.2 K, and magnetic fields up to 250 mT have been applied perpendicularly to the meander plane. While photon count rates are field independent at weak applied fields, they show a strong dependence at fields starting from approximately ±25 mT. This behavior, as well as the magnetic field dependence of the dark count rates, is in good agreement with the recent theoretical model of vortex-assisted photon detection and spontaneous vortex crossing in narrow superconducting lines. However, the local reduction of the superconducting free energy due to photon absorption, which is the fitting parameter in the model, increases much slower with the photon energy than the model predicts. Furthermore, changes in the free-energy during photon counts and dark counts depend differently on the current that flows through the meander. This indicates that photon counts and dark counts occur in different parts of the meander.

Detection mechanism of superconducting nanowire single-photon detectors

“…This functional dependence of the dark-count rate R DC has been confirmed in TaN and NbN SNSPD up to ∼ 100 mT [61,67], as shown in figure 4(b). Furthermore, these studies have found evidence that the dark counts originate in the turnarounds of the meander structure.…”

The effect of magnetic field on the intrinsic detection efficiency of superconducting single-photon detectors

“…This theory predicts an exponential increase of count rate as a function of applied field, at constant bias current. As in the previous experiments, 13,15 we find that the rate of exponential increase predicted by this theory is an order of magnitude away from the experimental value.…”

“…13,39,40 We conclude that there is a difference in the nature of photon and dark counts in SSPDs: photon counts scale with a field scale B C inherent to the material, whereas dark counts scale with the change in critical current under the influence of magnetic field, which depends on geometry. This difference between photon and dark counts is as of yet unexplained and carries implications for the design of SSPDs: it means that the only way in which an SSPD can be made more efficient by an applied magnetic field is by choosing a geometry where the critical current is not adversely affected by the applied field such as a spiral.…”

“…We interpret this feature as a single vortex which is pinned in our material. 13 All our measurements were done in a geometry that is intrinsically photodetecting, and a photodetection event entails a transition of the wire the normal state and Joule heating. Therefore, in-field cooling occurs each time there is a detection event.…”

“…By using a single narrow active area in a bridge-like configuration, we avoid the question of current flow around curved sections of the device, which complicated the interpretation of previous experiments. 13 -15 We find that it is the direct modification of the quasiparticle density of states in the superconductor that governs the magnetic field behaviour of SSPDs. In dirty-limit superconductors (such as thin-film NbN), this density of states is modified by a bias current or a magnetic field.…”