Recent research trends for superconducting detectors: introduction for the special issue ‘Focus on Superconducting Dectectors’

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Superconducting Nb nanobridges for reduced footprint and efficient next generation electronics

We optimised a process to reliably fabricate thin Nb nanobridge weak links having physical size comparable with Nb coherence length ξ(4.2K) ∼ 16 nm, controlled degraded superconductivity with respect to the electrodes and excellent edge roughness. We then investigated the feasibility to use these nanobridges as Josephson element for reduced footprint and efficient next generation single flux quantum (SFQ) logic electronics. First of all, we demonstrated that in such thin Nb nanobridges there is no thermal hysteresis in the current-voltage characteristics (IVC) that instead is usually observed in other weak links and prevents their use in SFQ electronics. We fitted the experimental IVCs of nanobridges with the resistively shunted junction (RSJ) model implemented with piecewise linear current-phase relation (CPR) finding a very good agreement with data. This allowed us to infer the CPR parameters and evaluate the product of critical current and normal resistance, $I_{c}R_{n}\sim mV$ IcRn, at varying of the temperature. Using this data, we simulated the generation of voltage pulses at varying of CPR and verified that they still have a quantized area equal to the magnetic flux quantum $\Phi_{0}$ and the product $I_{c}R_{n}$ allows for speed of operation $I_{c}R_{N} \,/\,\Phi_{0}\gg\,100$ GHz. Moreover, their critical current $I_{c}\approx \,100$ μA, comparable with that of tunnel Josephson junctions (JJs) used in SFQ electronics, is orders of magnitude larger than thermal current noise $I_{TN}=(2\pi\,/\,\Phi_{0})\,k_{B}\,T\,\approx\,0.2$ μA at temperature T = 4.2 K, for stable and, at the same time, efficient operation with energy per switch of only $E_{J}\approx I_{c}\Phi_{0}\lesssim 1$ aJ. To assess potential use of these nanobridges in SFQ logic electronics with large number of elements, we used an open-source simulation software (JSim) to simulate the behaviour of a standard DC-to-SFQ converter circuit. From the simulation made by implementing the CPR inferred from experimental data, we observed that the circuit behaves exactly as intended. Our results strongly suggests that these nanobridges can be used to develop large scale SFQ electronics with several advantages over tunnel JJs. The reduced footprint, just 1/3 or less than standard tunnel JJs, and simplified fabrication process, with only 2 steps involved against typically ∼20 for tunnel JJs, could allow for a better fabrication tolerance, higher control on operation parameters, higher circuit density and easier integration with other technology platforms. These characteristics could be very appealing also to replace tunnel JJs in quantum technology devices like transmon qubits and superconducting parametric amplifiers

Nano-optical observation of cascade switching in a parallel superconducting nanowire single photon detector

The device physics of parallel-wire superconducting nanowire single photon detectors is based on a cascade process. Using nano-optical techniques and a parallel wire device with spatially-separate pixels we explicitly demonstrate the single- and multi-photon triggering regimes. We develop a model for describing efficiency of a detector operating in the arm-trigger regime. We investigate the timing response of the detector when illuminating a single pixel and two pixels. We see a change in the active area of the detector between the two regimes and find the two-pixel trigger regime to have a faster timing response than the one-pixel regime.Comment: 11 pages, 2 figure

A common microbial signature is present in the lower airways of interstitial lung diseases including sarcoidosis

Background: The etiology of pulmonary sarcoidosis is not well established. Although the mechanism triggering pulmonary sarcoidosis remains to be established, inflammatory reactions seem to play an important role in this process. Objectives: The aim of this study was to define the composition of the lower airway microbiota in the bronchoalveolar lavage (BAL) of patients affected by interstitial lung diseases, including sarcoidosis, to determine whether the bacterial signature differs among these diseases. Methods: Ten patients affected by pulmonary sarcoidosis and 9 patients affected by other interstitial lung diseases were enrolled. 16S rRNA next-generation sequencing was used to study BAL microbial composition of these patients, and were also compared with already published microbial content in higher airways of such diseases. Results: Four phyla dominated the lower airway microbiota, Bacteroidetes being the most abundant phylum in both groups (56.9%). Diversity analysis showed no significant differences between the various diseases, particularly between pulmonary sarcoidosis and other interstitial lung diseases affecting lower airways. Conclusions: Our data indicate that the bacterial lower airways microbiota share the same signature and, therefore, cannot be used as a diagnostic tool to discriminate among different interstitial lung diseases, including sarcoidosis, while microbial diversity is present when considering lower or higher respiratory airways

Reset dynamics and latching in niobium superconducting nanowire single-photon detectors

We study the reset dynamics of niobium (Nb) superconducting nanowire single-photon detectors (SNSPDs) using experimental measurements and numerical simulations. The numerical simulations of the detection dynamics agree well with experimental measurements, using independently determined parameters in the simulations. We find that if the photon-induced hotspot cools too slowly, the device will latch into a dc resistive state. To avoid latching, the time for the hotspot to cool must be short compared to the inductive time constant that governs the resetting of the current in the device after hotspot formation. From simulations of the energy relaxation process, we find that the hotspot cooling time is determined primarily by the temperature-dependent electron-phonon inelastic time. Latching prevents reset and precludes subsequent photon detection. Fast resetting to the superconducting state is therefore essential, and we demonstrate experimentally how this is achieved

Nano-optical photoresponse mapping of superconducting nanowires with enhanced near infrared absorption

Superconducting nanowire single-photon detectors (SNSPDs) play an important role in emerging optical quantum technologies. We report on advanced nanometric characterization of a high efficiency near infrared SNSPD design based on a low roughness Tantalum pentoxide (Ta2O5)/ silicon dioxide (SiO2) distributed Bragg reflector (DBR) cavity structure. We have performed high resolution transmission electron microscopy (TEM) analysis to verify the smoothness of the DBR. Optical reflectance measurements show excellent correspondence with DBR simulations. We have carried out precision nano-optical photoresponse mapping studies at 940 nm wavelength at T = 3.5 K, indicating excellent large area device uniformity (peak efficiency 55 % at 100 Hz dark count rate [DCR]) with a full width half maximum (FWHM) timing jitter of 60 ps. With manual fibre coupling with single mode fibre, we achieve a system detection efficiency (SDE) of 57.5% at 940 nm wavelength (100 Hz DCR) at T = 2.3 K and a low polarization dependence of 1.20 ± 0.03. For coupling with multimode fibre, we achieve SDE of 90% at 940 nm (200 Hz DCR) at T= 2.3 K. These SNSPD devices are promising candidates for use in quantum dot photoluminescence studies and optical quantum technology applications

Superconducting circuits that mimic the brain

A hybrid superconducting optoelectronic circuit could be used to develop spiking neuromorphic networks that operate at the single-quantum level

Large Area Superconducting Nanowire Single Photon Detector Arrays

Superconducting Nanowire Single Photon Detectors (SNSPDs) are a promising emerging technology for high efficiency single infrared photon detection. Excellent signal-to-noise ratio, high efficiency and good timing resolution can currently be achieved in small single pixel SNSPDs (area smaller than 14×14 μm2). This small area is a severe practical limitation for emerging infrared photon counting applications requiring multimode fibre or free space optical coupling. In this paper we demonstrate fabrication and full optical testing of an array of 2×2 SNSPDs, using nanowires in parallel configuration, covering an active area of 30 × 30 μm2. We report a system detection efficiency (~ 2.6 % @ λ=1550 nm) and low timing jitter (FWHM ~ 70 ps). Moreover, advanced nano-optical characterization, using an innovative cryogenic miniature confocal microscope, enabled us to confirm the high uniformity of patterned nanowires across the entire sensitive area of the SNSPD array