Position-Dependent Local Detection Efficiency in a Nanowire Superconducting Single-Photon Detector

Abstract

We probe the local detection efficiency in a nanowire superconducting single-photon detector along the cross-section of the wire with a spatial resolution of 10 nm. We experimentally find a strong variation in the local detection efficiency of the device. We demonstrate that this effect explains previously observed variations in NbN detector efficiency as function of device geometry.Nanowire superconducting single-photon detectors (SSPDs) consist of a superconducting wire of nanoscale cross-section [1], typically 4 nm by 100 nm. Photon detection occurs when a single quantum of light is absorbed and triggers a transition from the superconducting to the normal state. SSPDs have high efficiency, low jitter, low dark count rate and fast reset time [2], and are therefore a key technology for, among others, quantum key distribution Although progress has been made recently, the underlying physical mechanism responsible for photon detection on the nanoscale is still under active investigation. A combination of theory [6,7], experiments [8][9][10] and simulations [11,12] on NbN SSPDs indicates that the absorption of a photon destroys Cooper pairs in the superconductor and creates a localized cloud of quasiparticles that diverts current across the wire. This makes the wire susceptible to the entry of a superconducting vortex from the edge of the wire. Energy dissipation by this moving vortex drives the system to the normal state.An important and unexpected implication of this detection model is a nanoscale position variation in the photodetection properties of the device. The conditions at the entry point of the vortex determine the energy required for it to cross the wire. This causes photons absorbed close to the edge to have a local detection efficiency (LDE) [13] compared to photons absorbed in the center of the wire [12]. This effect has practical implications for the operation of SSPDs, since it represents a potential limitation of the detection efficiency. In addition, SSPDs have been proposed for nanoscale sensing, either in a near-field optical microscope configuration [14] or as a subwavelength multiphoton probe [15], where this effect would be of major importance for the properties of such a microscope. While this effect has been predicted theoretically, clear experimental evidence is missing. * renema@physics.leidenuniv.nlIn this work, we experimentally explore the nanoscale variations in the intrinsic response of the detector. We spatially resolve the LDE with a resolution of approximately 10 nm, better than λ/50, using far-field illumination only. We find that our results are qualitatively consistent with numerical simulations [11,12]. Our results provide excellent quantitative agreement with experiments that indicate a polarization dependence in the LDE that was hitherto not understood [16].The key technique used in this work is a differential polarization measurement that probes the IDE of the detector (see Figure 1). The technique is based on the fact that polarized light is absorbed preferentially in dif...

Detection mechanism of superconducting nanowire single-photon detectors

“…the threshold current I th for a given photon energy depends on the absorption position along the cross-section of the nanowire. The existence of such a position-dependent detection efficiency was very recently verified by polarization measurements [99]. Light polarized perpendicularly to the wire is absorbed preferentially in the center of the wire, whereas light polarized parallel to the wire is absorbed uniformly.…”

“…In the inset of figure 12(a) a comparison is shown between detection probabilities obtained in Figure 12. (a) Calculated dependence of I th on the photon absorption position in a long, 150 nm-wide NbN strip for several photon wavelengths (data from [99]). The black dash-dotted line indicates the vortex-entry current obtained from the calculations.…”

Optically probing the detection mechanism in a molybdenum silicide superconducting nanowire single-photon detector

“…In this model, different parts of the cross section of the superconducting nanowire become photodetecting at different bias currents, due to an intrinsic position dependence in the fundamental detection mechanism. 27,35 In such a model, different points in the crosssection of the wire have different energy-current relations. Consequently, this gives rise to additional broadening of the transition (in addition to the Fano fluctuations), where the width of the transition is given by DI b ¼ I min (E) À I max (E), where I min and I max are the threshold currents at the most efficient point (edge) and the least efficient point (middle) along the cross-section of the wire, respectively.…”

“…Consequently, this gives rise to additional broadening of the transition (in addition to the Fano fluctuations), where the width of the transition is given by DI b ¼ I min (E) À I max (E), where I min and I max are the threshold currents at the most efficient point (edge) and the least efficient point (middle) along the cross-section of the wire, respectively. For such a model, one expects the width of the transition to increase with higher photon energies, 27,36 which could explain why the error function fit is not as good at higher photon energies. Moreover, due to the sharpening of the error-function transition (Fig.…”

Experimental investigation of the detection mechanism in WSi nanowire superconducting single photon detectors

“…In NbN, in contrast, the evidence points to the fact that quasiparticle multiplication and diffusion set the relevant timescales. Both experimental evidence [19,43] and theoretical calculations [29] point to a hotspot size of about 20-30 nm in diameter, which leads to an estimated detection time of 2-5 ps, much shorter than the QP recombination time. These results demonstrate that there are substantial differences both in the phenomenology and the theoretical modeling of these materials.…”