Superconducting nanowire single photon detectors (SSPDs/SNSPDs) are a highly promising infrared photon counting technology, offering high efficiency, low noise, high timing resolution photon counting [1]. We are evaluating these devices for a range of imaging and sensing applications, including dose monitoring for laser cancer treatment [2] and atmospheric remote sensing [3]. We are now focussing on development of next generation SNSPD devices tailored to these demanding applications. To date we have developed 2 x2 pixel arrays with an active area of 60 micrometres x 60 micrometres, tailored for multimode fiber coupling and operation at 1550 nm wavelength. We will discuss challenges in the scale-up from single pixel devices (10 micrometer diameter) to large area (>100 micrometre diameter) mutlipixel photon-counting cameras, and the promise of new concepts such as nano-antenna coupled SNSPDs[4].

[1] CM Natarajan, MG Tanner, RH Hadfield Superconducting nanowire single-photon detectors: physics and applications Superconductor Science and Technology 25 063001 (2012)
[2] NR Gemmell, A McCarthy, B Liu, MG Tanner, SN Dorenbos, V Zwiller, MS Patterson, GS Buller, BS Wilson, RH Hadfield Singlet oxygen luminescence detection with a fiber-coupled superconducting nanowire single-photon detector Optics Express 21 (4) 5005 (2013)
[3] A McCarthy, N Krichel, X Ren, NR Gemmell, MG Tanner, SN Dorenbos, V Zwiller, RH Hadfield, GS Buller Kilometer range time-of-flight depth imaging at 1560 nm wavelength with a superconducting nanowire single-photon detector Optics Express 21 7 8904 (2013)
[4] RM Heath, MG Tanner, TD Drysdale, S Miki, V Giannini, SA Maier, RH Hadfield Nanoantenna enhancement for telecom-wavelength superconducting single photon detectors Nano Letters 15 (2) 819 (2015)

RHH acknowledges support from the UK Engineering Physical Sciences Research Council including the QUANTIC quantum technology hub, the Royal Society of London and the European Research Council. AC acknowledges a Marie Curie Intra-European Fellowship.

Superconducting single photon detectors (SSPDs) are among the key technologies for quantum communication. In SSPDs, the absorption of photons creates a cloud of quasiparticles which can cause the formation of a resistive area, which appears in the read-out circuit as a voltage pulse. Recently the multiphoton detection regime was investigated in order to gain insight in the detection process and for possible application in multiphoton measurements . When the bias current is much lower than the critical value, the energy carried by one photon is not enough to trigger a detection event but two or more photons are required. This regime has applications in quantum optics, where multiphoton detection is a typical final step of many experiments. It was also shown to enable ultrasensitive, multiphoton interferometric autocorrelation. The efficicency of the multiphoton regime is limited, however, by the finite dimension of the quasiparticle cloud, to which we refer here as the hot spot. Only if the photons are absorbed close enough to each other they interact and count as single excitation. Here, we report on experiments to measure this interaction distance, which is of interest both for understanding the detection mechanism and for multiphoton applications. The minimum distance required for the interaction of two hot spots is related to the hot spot length scale and therefore provides relevant information about the detection mechanism. We exploited the two photon absorption to measure the hot spot length scale, s. In the case of a detector clicking when two photons are absorbed, the probability of a detection event will depend on L and s, where L is the length of the wire. This is because the first photon can be absorbed anywhere along L but, in order to cause a click, the second needs to be absorbed within s from the first one. To estimate s for NbN detectors, we fabricated wires with different lengths and identical widths and by means of detector tomography we determined, for each wire, the probability of a detection caused by the absorption of one and two photons. The squared ratio of the two detection probabilities, plotted as a function of wire length, reveals, as expected, a linear relation that allows us to estimate s. The experimental results show that the typical hot spot length scale is of the order of 20nm in NbN nanowire.

The authors would like to acknowledge the research programme of the Foundation for Fundamental Research on Matter (FOM), which is financially supported by the Netherlands Organization for Scientific Research (NWO) and by NanoNextNL.

We investigate the photoresponse of nanowire superconducting single photon detectors (SSPDs) in an external magnetic field. We demonstrate experimentally that the change in photodetection probability is caused by direct modification of the electronic state of the wire. By observing the magnetic field scale associated with both this process and the decrease of the critical current, we settle the question of whether it is possible to enhance the detection efficiency of an SSPD by applying a magnetic field: for a straight wire, this is not possible.
SSPDs are a crucial technology for fundamental research and technological applications. These detectors consist of a thin film, patterned into a wire. When biased closed to the critical current, photodetection events occur in the wire. Since superconductivity is generally weakened by the application of a magnetic field, it is natural to ask whether a detector which functions by breaking superconductivity will function better in an external field.
In our work, we demonstrate that the detection mechanism is indeed enhanced in an external magnetic field. We find that for low fields (< 50 mT), curves of constant count rate depend quadratically on the applied field. Our results show excellent quantitative agreement with a prediction based on the Usadel equation as applied to a homogeneous, current-carrying wire. At higher fields, we find deviations from this behaviour, which we interpret as being due to the permanent presence of a vortex at the center of the detector.
Our result has implications for the study of the detection mechanism, a topic which has seen a flurry of activity recently. The direct modification of the electronic state had not been considered in theory as a mechanism for the enhancement of the detection efficiency in an applied magnetic field.
We also measure the critical current of our samples, which are a short (200 nm) wire and a nanodetector bridge. The critical current shows a transition from an induced depairing regime to a flux-flow regime. For our geometry, the reduction of the critical current dominates over the enhancement of the detection efficiency. Since the field-dependence of the critical current is set by the geometry, our results open the way for design of detectors which have a field-enhanced photoresponse.

A superconducting nanowire single photon detector (SNSPD) with large receiving area of 300 micrometer in diameter was developed for photons detection applications in free space based on tapered fiber and micro lenses. The fiber core of 300 micrometer at receiving port was reduced to 60 micrometer by a long tapered fiber designed with core ratio of 5:1, and further compressed to be about 20 micrometer by dual micro lenses equipped at the end of the fiber. The transmission efficiencies of tapered fiber and micro lenses were measured to be 80% and 95% respectively. The proposed structure was applied to a cavity enhanced SNSPD, whose detection area was 20×20 um² and detection efficiency was 70%, produced a system efficiency of 53%.

This work was financially supported by National Basic Research Program of China (No. 2011CBA00202) and National Natural Science Foundation of China (No. 61471189 and 11227904).

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[4] L. N. Bulaevskii et al., Phys. Rev. B 85, 014505 (2012).
[5] A. Engel et al., J. Appl. Phys. 114, 214501 (2013)

While tungsten silicide (WSi) based superconducting nanowire single photon detectors (SNSPDs) have been used to demonstrate the highest reported system detection efficiency at 1550nm with a single device [1], they lag behind niobium nitride (NbN) based SNSPDs in terms of timing performance, as both the reset time and the timing jitter are each one order of magnitude larger in WSi than in NbN devices. WSi SNSPDs have higher jitter than NbN devices because they have lower switching current and consequently lower signal to noise ratio (SNR) [2]. In order to decrease the jitter of WSi SNSPDs, a nanocryroton (nTron) based readout could be used to increase the output signal after photodetection and thus reduce jitter. The nTron is a three terminal superconducting device that allows a small gate current to switch an orders-of-magnitude larger channel current. The reduction of jitter by readout of NbN SNSPDs with NbN nTrons was previously shown [3].
We designed and fabricated SNSPDs and nTrons on a single layer of 5nm thick WSi on SiOx. Fabrication was performed with standard electron beam lithography techniques using hydrogen silsesquioxane (HSQ) electron beam resist. This process was shown to yield superconducting WSi devices and we have demonstrated that the addition of a DC bias current at the nTron gate can strongly suppress the switching current of the nTron channel. Additionally, an off chip coupling circuit was designed and built to couple the SNSPD output signal into the nTron gate. We used an electro-thermal model of superconducting nanowires contained in SPICE in order to design the coupling circuit, to ensure that both the SNSPD and the nTron could self-reset while being operated at full speed. This work should enable the reduction of noise related jitter in WSi SNSPD systems by readout with integrated nTrons.
References
[1] F. Marsili et al, "Detecting single infrared photons with 93% system efficiency," Nature Photonics 2013
[2] L. You et al, "Jitter analysis of a superconducting nanowire single photon detector," AIP Advances 3, 072135, 2013
[3] A. McCaughan and K. Berggren, "A Superconducting-Nanowire Three-Terminal Electrothermal Device," Nano Lett., 14(10), 2014

Andrew Dane was supported by a NASA Space Technology Fellowship, grant # NNX14AL48H.

Superconducting Nanowire Single-Photon Detectors (SNSPDs) are ultra-sensitive devices for photon detection from visible to near infrared wavelength. Implemented in large scale detector arrays, SNSPDs will play a major role in many future applications, such as imaging systems or optical quantum information processing. To handle the system complexity as well as to obtain the time and spatial resolution of each pixel in the array, a frequency-division multiplexing (FDM) scheme is desired.

We propose a novel approach to bias a SNSPD with a microwave signal. The detector is embedded into a lumped-element resonant circuit, which allows the assignment to a defined resonance frequency. The microwave current inside the resonator is used to bias the SNSPD close to the critical current. We have performed detailed measurements of the microwave behaviour of such a device as well as a detailed analysis of the spectral properties of the detector. The performance at different bias and light power levels will be quantitatively discussed and the achieved detector efficiencies over a wide optical bandwidth will be compared with a conventional SNSPD. Further, the proposed embedded solution allows an ideal implementation of a FDM read-out of SNSPDs. We will demonstrate readout of embedded SNSPD arrays with FDM.