A superconducting nanowire spiking element for neural networks

As the limits of traditional von Neumann computing come into view, the brain's ability to communicate vast quantities of information using low-power spikes has become an increasing source of inspiration for alternative architectures. Key to the success of these largescale neural networks is a power-efficient spiking element that is scalable and easily interfaced with traditional control electronics. In this work, we present a spiking element fabricated from superconducting nanowires that has pulse energies on the order of ~10 aJ. We demonstrate that the device reproduces essential characteristics of biological neurons, such as a refractory period and a firing threshold. Through simulations using experimentally measured device parameters, we show how nanowire-based networks may be used for inference in image recognition, and that the probabilistic nature of nanowire switching may be exploited for modeling biological processes and for applications that rely on stochasticity.Comment: 5 main figures; 7 supplemental figure

A Nanocryotron Memory and Logic Family

The development of superconducting electronics based on nanocryotrons has been limited so far to few-device circuits, in part due to the lack of standard and robust logic cells. Here, we introduce and experimentally demonstrate designs for a set of nanocryotron-based building blocks that can be configured and combined to implement memory and logic functions. The devices were fabricated by patterning a single superconducting layer of niobium nitride and measured in liquid helium on a wide range of operating points. The tests show $10^{-4}$ bit error rates with above $20\,\%$ margins up to $50\,$MHz and the possibility of operating under the effect of a perpendicular $36\,$mT magnetic field, with $30\,\%$ margins at $10\,$MHz. Additionally, we designed and measured an equivalent delay flip-flop made of two memory cells to show the possibility of combining multiple building blocks to make larger circuits. These blocks may constitute a solid foundation for the development of nanocryotron logic circuits and finite-state machines with potential applications in the integrated processing and control of superconducting nanowire single-photon detectors.Comment: Submitted for publication in the Applied Physics Letters special issue "Advances in Superconducting Logic", 8 pages, 5 figure

A Superconducting Nanowire Binary Shift Register

We present a design for a superconducting nanowire binary shift register, which stores digital states in the form of circulating supercurrents in high-kinetic-inductance loops. Adjacent superconducting loops are connected with nanocryotrons, three terminal electrothermal switches, and fed with an alternating two-phase clock to synchronously transfer the digital state between the loops. A two-loop serial-input shift register was fabricated with thin-film NbN and achieved a bit error rate less than $10^{-4}$, operating at a maximum clock frequency of $83\,\mathrm{MHz}$ and in an out-of-plane magnetic field up to $6\,\mathrm{mT}$. A shift register based on this technology offers an integrated solution for low-power readout of superconducting nanowire single photon detector arrays, and is capable of interfacing directly with room-temperature electronics and operating unshielded in high magnetic field environments.Comment: The following article has been published in Applied Physics Letters issue 122. 10 pages, 3 figure

A Superconducting Nanowire-based Architecture for Neuromorphic Computing

Neuromorphic computing is poised to further the success of software-based neural networks by utilizing improved customized hardware. However, the translation of neuromorphic algorithms to hardware specifications is a problem that has been seldom explored. Building superconducting neuromorphic systems requires extensive expertise in both superconducting physics and theoretical neuroscience. In this work, we aim to bridge this gap by presenting a tool and methodology to translate algorithmic parameters into circuit specifications. We first show the correspondence between theoretical neuroscience models and the dynamics of our circuit topologies. We then apply this tool to solve linear systems by implementing a spiking neural network with our superconducting nanowire-based hardware.Comment: 29 pages, 10 figure

Perfluorinated compounds (pfcs) in river waters of central Italy. Monthly variation and ecological risk assessment (era)

Perfluorinated compounds (PFCs) are a wide class of emerging pollutants. In this study, we applied the US EPA method 533 for the determination of 21 PFCs in river water samples. In particular, this method was used to investigate the presence of the target PFCs in six rivers in central Italy during a 4-month-long monitoring campaign. In 73% of the analyzed samples, at least some of the target PFCs were detected at concentrations higher than the limit of detection (LOD). The sum of the 21 target analytes ( n-ary sumation 21PFCs) ranged from 4.3 to 68.5 ng L-1, with the highest concentrations measured in the month of June, probably due to a minor river streamflow occurring in the warmer summer months. Considering the individual congeners, PFBA and PFPeA, followed by PFHxA and PFOA, were the predominantly detected compounds. Short- and medium-chain PFCs (C4-C9) prevail over the long-chain PFCs (C10-C18), likely due to the increased industrial use and the higher solubility of short-chain PFCs compared to long-chain PFCs. The ecological risk assessment, conducted by using the risk quotient method, highlighted that the risk for aquatic environments associated with PFBA, PFPeA, PFBS, PFHxA and PFOA was low or negligible. Only for PFOA, there was a medium level of risk in two rivers in the month of June. With regard to PFOS, 54% of the river water samples were classified as "high risk" for the aquatic environment. The remaining 46% of the samples were classified as "medium risk.

Mechanisms of nerve damage in neuropathies associated with hematological diseases: lesson from nerve biopsies

Despite the introduction of non-invasive techniques in the study of peripheral neuropathies, sural nerve biopsy remains the gold standard for the diagnosis of several neuropathies, including vasculitic neuropathy and neurolymphomatosis. Besides its diagnostic role, sural nerve biopsy has helped to shed light on the pathogenic mechanisms of different neuropathies. In the present review, we discuss how pathological findings helped understand the mechanisms of polyneuropathies complicating hematological diseases

A Nanocryotron Ripple Counter Integrated with a Superconducting Nanowire Single-Photon Detector for Megapixel Arrays

Decreasing the number of cables that bring heat into the cryocooler is a critical issue for all cryoelectronic devices. Especially, arrays of superconducting nanowire single-photon detectors (SNSPDs) could require more than $10^6$ readout lines. Performing signal processing operations at low temperatures could be a solution. Nanocryotrons, superconducting nanowire three-terminal devices, are good candidates for integrating sensing and electronics on the same technological platform as SNSPDs in photon-counting applications. In this work, we demonstrated that it is possible to read out, process, encode, and store the output of SNSPDs using exclusively superconducting nanowires. In particular, we present the design and development of a nanocryotron ripple counter that detects input voltage spikes and converts the number of pulses to an $N$-digit value. The counting base can be tuned from 2 to higher values, enabling higher maximum counts without enlarging the circuit. As a proof-of-principle, we first experimentally demonstrated the building block of the counter, an integer-$N$ frequency divider with $N$ ranging from 2 to 5. Then, we demonstrated photon-counting operations at 405\,nm and 1550\,nm by coupling an SNSPD with a 2-digit nanocryotron counter partially integrated on-chip. The 2-digit counter operated in either base 2 or base 3 with a bit error rate lower than $2 \times 10^{-4}$ and a maximum count rate of $45 \times 10^6\,$s$^{-1}$. We simulated circuit architectures for integrated readout of the counter state, and we evaluated the capabilities of reading out an SNSPD megapixel array that would collect up to $10^{12}$ counts per second. The results of this work, combined with our recent publications on a nanocryotron shift register and logic gates, pave the way for the development of nanocryotron processors, from which multiple superconducting platforms may benefit

From antiferromagnetism to d-wave superconductivity in the 2D t-J model

We have found that the two dimensional t-J model, for the physical parameter range J/t = 0.4 reproduces the main experimental qualitative features of High-Tc copper oxide superconductors: d-wave superconducting correlations are strongly enhanced upon small doping and clear evidence of off diagonal long range order is found at the optimal doping \delta ~ 0.15. On the other hand antiferromagnetic long range order, clearly present at zero hole doping, is suppressed at small hole density with clear absence of antiferromagnetism at \delta >~ 0.1.Comment: 4 pages, 5 figure

Charge fluctuations close to phase separation in the two dimensional t-J model

We have studied the t-J model using the Green Function Monte Carlo technique. We have obtained accurate energies well converged in the thermodynamic limit, by performing simulations up to 242 lattice sites. By studying the energy as a function of hole doping we conclude that there is no phase separation in the physical region, relevant for HTc superconductors. This finding is further supported by the hole-hole correlation function calculation. Remarkably, by approaching the phase separation instability, for $J_c/t\sim 0.5$,this function displays enhanced fluctuations at incommensurate wavevectors, scaling linearly with the doping, in agreement with experimental findings.Comment: To appear on Phys. Rev. Let

The ACS LCID Project. I. Short-Period Variables in the Isolated Dwarf Spheroidal Galaxies Cetus & Tucana

(abridged) We present the first study of the variable star populations in the isolated dwarf spheroidal galaxies (dSph) Cetus and Tucana. Based on Hubble Space Telescope images obtained with the Advanced Camera for Surveys in the F475W and F814W bands, we identified 180 and 371 variables in Cetus and Tucana, respectively. The vast majority are RR Lyrae stars. In Cetus we also found three anomalous Cepheids, four candidate binaries and one candidate long-period variable (LPV), while six anomalous Cepheids and seven LPV candidates were found in Tucana. Of the RR Lyrae stars, 147 were identified as fundamental mode (RRab) and only eight as first-overtone mode (RRc) in Cetus, with mean periods of 0.614 and 0.363 day, respectively. In Tucana we found 216 RRab and 82 RRc giving mean periods of 0.604 and 0.353 day. These values place both galaxies in the so-called Oosterhoff Gap, as is generally the case for dSph. We calculated the distance modulus to both galaxies using different approaches based on the properties of RRab and RRc, namely the luminosity-metallicity and period-luminosity-metallicity relations, and found values in excellent agreement with previous estimates using independent methods: (m-M)_{0,Cet}=24.46+-0.12 and (m-M)_{0,Tuc}=24.74+-0.12, corresponding to 780+-40 kpc and 890+-50 kpc. We also found numerous RR Lyrae variables pulsating in both modes simultaneously (RRd): 17 in Cetus and 60 in Tucana. Tucana is, after Fornax, the second dSph in which such a large fraction of RRd (~17%) has been observed. We provide the photometry and pulsation parameters for all the variables, and compare the latter with values from the literature for well-studied dSph of the Local Group and Galactic globular clusters.Comment: 26 pages, 24 figures, in emulateapj format. To be published in ApJ. Some figures heavily degraded; See http://www.iac.es/project/LCID/?p=publications for a version with full resolution figure