Vortex-assisted photon counts and their magnetic field dependence in single-photon detectors

摘要

We argue that photon counts in a superconducting nanowire single-photondetector (SNSPD) are caused by the transition from a current-biased metastablesuperconducting state to the normal state. Such a transition is triggered byvortices crossing the thin film superconducting strip from one edge to anotherdue to the Lorentz force. Detector counts in SNSPDs may be caused by threeprocesses: (a) a single incident photon with energy sufficient to break enoughCooper pairs to create a normal-state belt across the entire width of the strip(direct photon count), (b) thermally induced single-vortex crossing in theabsence of photons (dark count), which at high bias currents releases theenergy sufficient to trigger the transition to the normal state in a beltacross the whole width of the strip, and (c) a single incident photon withinsufficient energy to create a normal-state belt but initiating a subsequentsingle-vortex crossing, which provides the rest of the energy needed to createthe normal-state belt (vortex-assisted single photon count). We derive thecurrent dependence of the rate of vortex-assisted photon counts. The resultingphoton count rate has a plateau at high currents close to the critical currentand drops as a power-law with high exponent at lower currents. While themagnetic field perpendicular to the film plane does not affect the formation ofhot spots by photons, it causes the rate of vortex crossings (with or withoutphotons) to increase. We show that by applying a magnetic field one maycharacterize the energy barrier for vortex crossings and identify the origin ofdark counts and vortex-assisted photon counts.