Probing the hotspot interaction length in NbN nanowire superconducting single photon detectors

Abstract

We measure the maximal distance at which two absorbed photons can jointly trigger a detection event in NbN nanowire superconducting single photon detector (SSPD) microbridges by comparing the one-photon and two-photon efficiency of bridges of different overall lengths, from 0 to 400 nm. We find a length of 23 ± 2 nm. This value is in good agreement with to size of the quasiparticle cloud at the time of the detection event.Nanowire superconducting single photon detectors (SSPDs) [1] are a crucial technology for a variety of applications [2]. These devices consist of a thin superconducting film which detects photons when biased to a significant fraction of its critical current. Although details of the microscopic mechanism are still in dispute [3], the present understanding of this process in NbN SSPDs is as follows [4][5][6][7][8][9][10][11][12][13][14]: after the absorption of a photon, a cloud of quasiparticles is created, which is known as a hotspot. This cloud diffuses, spreading out over some area of the wire. This causes the redistribution of bias current, which triggers a vortex unbinding from the edge of the wire, if the applied bias current is such that the current for vortex entry is exceeded. The transition of a vortex across the wire creates a normal-state region, which grows under the influence of Joule heating from the bias current, leading to a measureable voltage pulse and a detection event [15].Recently, applications of these detectors have been demonstrated or proposed which rely the ability of such devices to operate as multiphoton detectors, such as multiphoton subwavelength imaging [16], ultrasensitive higher order autocorrelation [17] and near-field multiphoton sensing [18]. These applications make use of the fact that when biased at lower currents than required for single-photon detection, the detector responds only when several photons are absorbed simultaneously. This multiphoton response has moreover proven to be of great significance in investigating the question of the working mechanism of such devices.For these multiphoton applications to work, the two photons must be absorbed within some given distance of each other, which we will refer to as the hotspot interaction length s. This length determines the efficiency of an SSPD in the multiphoton regime: photons which are absorbed far away from each other along the wire will not be able to jointly cause a detection event, resulting in a reduction of the two-photon detection probability.In this work, use this effect to measure the hotspot interaction length. Our experiment is based on comparing Figure 1. a) Sketch of the experiment. Top pannel: a nanowire of length L is illuminated uniformly, and the current the nanowire is set to be in the single-photon regime. Photon absorption at any point in the wire is sufficient to cause a detection event. In the bottom pannel, the detector is in the two-photon regime, and a detection is observed only if the second photon is absorbed in the region (red spot) where an excess quasiparticle concentrati...

Photonic quantum information processing: A concise review

“…Research on superconducting detectors is still ongoing, aimed at understanding detection mechanisms in different types of nanowire materials [54][55][56][57][58] , improving its performance in terms of reset times 59 and time jitter 60,61 , and developing new methods of accurate detection efficiency measurements 62 . Although intrinsic dark counts are low, SNSPDs are susceptible to picking up background thermal radiation from the input fiber's roomtemperature environment-this can be overcome by spectral filtering.…”

Hot-spot relaxation time current dependence in niobium nitride waveguide-integrated superconducting nanowire single-photon detectors

“…On the other hand, WSi detectors have been shown to work in two-photon detection regime over a broader current range and exhibit superior performance in terms of efficiency saturation also in the two-photon regime. It is unknown whether this is related to the detectors' geometry or due to an intrinsic advantage of WSi, considering also its longer hot-spot relaxation time and bigger hot-spot interaction length [24] compared to NbN [25]. In this context, a careful examination of the detector geometry should allow to clarify this aspect.…”

“…This is particularly true for low energy photons, such as telecom wavelength light around 1550 nm. In order that the multi-photon absorption generates a detection event, the consecutive absorption of individual photons has to occur not only in a limited region of the wire [24,25], but also in a timespan smaller than the original hot-spot lifetime (relaxation time) [19,24]. This peculiarity of SNSPDs represents a limit for the single-photon operation but, on the other hand, it allows us to exploit this characteristic as a multi-photon correlator [26,27], in analogy with successful application of Transition Edge Sensors (TES) [28].…”

Photonic Quantum Metrology

“…Depending on the photon wavelength, different platforms are employed for detector technology. In the visible range, Si-based avalanche photodiode detectors (APDs) are commonly used 380 , while for the telecom range (around 1310 nm and 1550 nm) superconductive nanowire single photon detectors (SNSPD) reach efficiencies above 0.95 [396][397][398] . Transition-edge sensors (TES) detectors are suitable for high efficiency photon number resolving detection, in both visible and telecom ranges 399,400 .…”