We report a systematic comparison of niobium nitride thin films deposited on oxidized silicon substrates by reactive DC magnetron sputtering and reactive high-power impulse magnetron sputtering (HiPIMS). After determining the nitrogen gas concentration that produces the highest superconducting critical temperature for each process, we characterize the dependence of the critical temperature on film thickness. The optimal nitrogen concentration is higher for HiPIMS than for DC sputtering, and HiPIMS produces higher critical temperatures for all thicknesses studied. We attribute this to the HiPIMS process enabling the films to get closer to optimal stoichiometry before beginning to form a hexagonal crystal phase that reduces the critical temperature, along with the extra kinetic energy in the HiPIMS process improving crystallinity. We also study the ability to increase the critical temperature of the HiPIMS films through the use of an aluminum nitride buffer layer and substrate heating.

Electronics for Scientists provides a practical and concise introduction to electrical circuits, signals, and instrumentation for undergraduate students in the physical sciences. No previous familiarity with electronics is required and concepts are grounded in the relevant physics. The book aims to give students the electronics background needed to be successful in experimental science. The book begins with the fundamentals of DC circuits. This is followed by AC circuits and their analysis using the concept of impedance. The transfer function is introduced and used to analyze different types of filter circuits. The conversion between time-domain and frequency-domain signal representations is reviewed. Transmission lines are introduced and used to motivate the different approach to designing microwave-frequency circuits as compared to lower-frequency circuits. The physics of semiconductors is reviewed and used to understand the behavior of diodes and transistors, and a number of diode and transistor circuits are analyzed. The operational amplifier (op-amp) is introduced and several op-amp circuits are analyzed. Techniques for quantifying noise in electrical measurements are described and common sources of noise are discussed. The last major topic is digital circuits, which include analog-to-digital conversion, logic gates, and digital memory circuits. The book concludes with a brief introduction to quantum computing. Designed for a one-semester course, this book brings together a range of topics relevant to experimental science that are not commonly found in a single text. Worked examples are provided throughout the book, and each chapter concludes with a set of problems to reinforce the material covered. The subject of electronics is indispensable to a wide array of scientific and technical fields, and this book seeks to provide an approachable point of access to this rich and important subject.

Superconducting nanowire single-photon detectors offer outstanding performance, but the development of large-format imaging arrays is challenging. A new approach based on sectioning a single nanowire enables an eightfold improvement of the spatial resolution and the realization of a 1,024-pixel imager.

We demonstrate impedance-matched low-loss transmission lines with a signal wavelength more than 150 times smaller than the free space wavelength using superconducting nanowires on high permittivity substrates. A niobium nitride thin film is patterned in a coplanar waveguide (CPW) transmission line geometry on a bilayer substrate consisting of 100 nm of epitaxial strontium titanate on high-resitivity silicon. The use of strontium titanate on silicon enables wafer-scale fabrication and maximizes process compatibility. It also makes it possible to realize a 50 Ω characteristic impedance across a wide range of CPW widths, from the nanoscale to the macroscale. We fabricated and characterized an approximately 50 Ω CPW device with two half-wave stub resonators. Comparing the measured transmission coefficient to numerical simulations, we determine that the strontium titanate film has a dielectric constant of 1.1× 103 and a loss tangent of not more than 0.009. To facilitate the design of distributed microwave devices based on this type of material system, we describe an analytical model of the CPW properties that gives good agreement with both measurements and simulations.

Developing compact, low-dissipation, cryogenic-compatible microwave electronics is essential for scaling up low-temperature quantum computing systems. In this paper, we demonstrate an ultra-compact microwave directional forward coupler based on high-impedance slow-wave superconducting-nanowire transmission lines. The coupling section of the fabricated device has a footprint of $416\,\mathrm{\mu m^2}$. At 4.753 GHz, the input signal couples equally to the through port and forward-coupling port (50:50) at $-6.7\,\mathrm{dB}$ with $-13.5\,\mathrm{dB}$ isolation. The coupling ratio can be controlled with DC bias current or temperature by exploiting the dependence of the kinetic inductance on these quantities. The material and fabrication-process are suitable for direct integration with superconducting circuits, providing a practical solution to the signal distribution bottlenecks in developing large-scale quantum computers.

To better understand the origins of the timing resolution, also known as jitter, of superconducting nanowire single-photon detectors (SNSPDs), we have performed timing characterizations of a niobium nitride SNSPD with a dual-ended readout. By simultaneously measuring both readout pulses along with an optical timing reference signal, we are able to quantify each independent contribution to the total measured jitter. In particular, we are able to determine values for the jitter due to the stochastic nature of hotspot formation and the jitter due to the variation of the photon detection location along the length of the nanowire. We compare the results of this analysis for measurements at temperatures of 1.5 and 4.5 K.

Chemical self-assembly has garnered tremendous interest as a tool for generating nanometer-scale structures and devices. Organosilane self-assembled monolayers (SAMs) are of particular interest because of their ability to assemble on a wide range of substrates with varied chemical functionalities. Nanoshaving, an atomic force lithographic technique, has been demonstrated as a method to generate nanopatterns of organosilane SAMs. However, this method requires extremely high force setpoints, which rapidly dulls atomic force microscopy tips and degrades the resolution of the resulting nanopattern. In this work, we utilize Cu-ligated mercaptohexadecanoic acid (MHDA) multilayers to circumvent this limitation. Initially, a 10-undecenyltrichlorosilane (UTS) SAM is assembled onto a Si substrate, and the terminal olefin groups of the UTS SAM are oxidized to carboxyl groups. Subsequently, a Cu-ligated MHDA multilayer is assembled via the sequential deposition of Cu2+ ions and MHDA molecules. The interface between the oxidized UTS SAM and Cu-ligated MHDA multilayer serves as a natural low force breakpoint for nanoshaving. We demonstrate that the resulting nanopatterns can function as a chemical resist to fabricate metal nanostructures.

To analyze the switching dynamics and output performance of a superconducting nanowire single photon detector (SNSPD), the nanowire is usually modelled as an inductor in series with a time-varying resistor induced by absorption of a photon. Our recent experimental results show that, due to the effect of kinetic inductance, for a SNSPD made of a nanowire of sufficient length, its geometry length can be comparable to or even longer than the effective wavelength of frequencies contained in the output pulse. In other words, a superconducting nanowire can behave as a distributed transmission line so that the readout pulse depends on the photon detection location and the transmission line properties of the nanowire. Here, we develop a distributed model for a superconducting nanowire and apply it to simulate the output performance of a long nanowire designed into a coplanar waveguide. We compare this coplanar waveguide geometry to a conventional meander nanowire geometry. The simulation results agree well with our experimental observations. With this distributed model, we discussed the importance of microwave design of a nanowire and how impedance matching can affect the output pulse shape. We also discuss how the distributed model affects the growth and decay of the photon-triggered resistive hotspot.

Superconducting nanowire single-photon detectors ( SNPSDs ) have demonstrated single-photon detection in the visible and near-infrared with near unity quantum ef fi ciency, ∼ ns reset times, very low dark count rates, and ∼ 10 ps timing resolution. As a result, they have become attractive for some of the most demanding single-photon applications such as quantum cryptography, deep space communication, and single-molecule fl uorescence [ 1 ] . The fi rst SNSPDs were made from niobium nitride ( NbN ) [ 2, 3 . Subsequent work has explored a variety of other low- T c materials, including NbTiN [ 4 ] , NbSi [ 5 ] , WSi 6 ] , MoGe 7 ] and MoSi High- T c materials have also been explored, with limited success to date. A higher critical temperature offers two important advantages: a higher operating temperature, which makes the devices more attractive for many applications; and the possibility of a signi fi cantly faster count rate. In SNSPDs, the reset time is set by the inductive time constant, where the inductance is typically dominated by the kinetic inductance of the nanowire. A faster device can be achieved by reducing this inductance ( e.g. by reducing the nanowire length ) , but if the inductive time constant is no longer slower than the time it takes the photon-induced resistive hotspot to cool, then the device will no longer self-reset following photon detection, instead remaining in a resistive state — a phenomenon known as latching. The hotspot

Nature Photonics 11, 247–251 (2017); published online 27 March 2017; corrected after print 14 August 2017. In the version of this Article originally published, in Fig. 3c, in the y axis label, the units '(c.p.s.)' should not have been included; the label should have read “Dark counts”. This has now been corrected in the online versions of the Article.

Detecting spatial and temporal information of individual photons is critical to applications in spectroscopy, communication, biological imaging, astronomical observation and quantum-information processing. Here we demonstrate a scalable single-photon imager using a single continuous superconducting nanowire that is not only a single-photon detector but also functions as an efficient microwave delay line. In this context, photon-detection pulses are guided in the nanowire and enable the readout of the position and time of photon-absorption events from the arrival times of the detection pulses at the nanowire's two ends. Experimentally, we slowed down the velocity of pulse propagation to ∼2% of the speed of light in free space. In a 19.7 mm long nanowire that meandered across an area of 286 × 193 μm 2 , we were able to resolve ∼590 effective pixels with a temporal resolution of 50 ps (full width at half maximum). The nanowire imager presents a scalable approach for high-resolution photon imaging in space and time.

The development of methods to produce nanoscale features with tailored chemical functionalities is fundamental for applications such as nanoelectronics and sensor fabrication. The molecular-ruler process shows great utility for this purpose as it combines top-down lithography for the creation of complex architectures over large areas in conjunction with molecular self-assembly, which enables precise control over the physical and chemical properties of small local features. The molecular-ruler process, which most commonly uses mercaptoalkanoic acids and metal ions to generate metal-ligated multilayers, can be employed to produce registered nanogaps between metal features. Expansion of this methodology to include molecules with other chemical functionalities could greatly expand the overall versatility, and thus the utility, of this process. Herein, we explore the use of alkanethiol molecules as the terminating layer of metal-ligated multilayers. During this study, it was discovered that the solution deposition of alkanethiol molecules resulted in low overall surface coverage with features that varied in height. Because features with varied heights are not conducive to the production of uniform nanogaps via the molecular-ruler process, the vapor-phase deposition of alkanethiol molecules was explored. Unlike the solution-phase deposition, alkanethiol islands produced by vapor-phase deposition exhibited markedly higher surface coverages of uniform heights. To illustrate the applicability of this method, metal-ligated multilayers, both with and without an alkanethiol capping layer, were utilized to create nanogaps between Au features using the molecular-ruler process.

Nature Photonics 11, 247–251 (2017); published online 27 March 2017; corrected after print 14 August 2017. In the version of this Article originally published, in Fig. 3c, in the y axis label, the units '(c.p.s.)' should not have been included; the label should have read “Dark counts”. This has now been corrected in the online versions of the Article.

Detecting spatial and temporal information of individual photons by using single-photon-detector (SPD) arrays is critical to applications in spectroscopy, communication, biological imaging, astronomical observation, and quantum-information processing. Among the current SPDs1,detectors based on superconducting nanowires have outstanding performance2, but are limited in their ability to be integrated into large scale arrays due to the engineering difficulty of high-bandwidth cryogenic electronic readout3-8. Here, we address this problem by demonstrating a scalable single-photon imager using a single continuous photon-sensitive superconducting nanowire microwave-plasmon transmission line. By appropriately designing the nanowire's local electromagnetic environment so that the nanowire guides microwave plasmons, the propagating voltages signals generated by a photon-detection event were slowed down to 2 arrivals of the signals at the two ends of the nanowire naturally encoded the position and time of absorption of the photon. Thus, with only two readout lines, we demonstrated that a 19.7-mm-long nanowire meandered across an area of 286 μm * 193 μm was capable of resolving 590 effective pixels while simultaneously recording the arrival times of photons with a temporal resolution of 50 ps. The nanowire imager presents a scalable approach to realizing high-resolution photon imaging in time and space.

We study the microwave impedance of extremely high aspect ratio (length/width ~ 5,000) superconducting niobium nitride nanowires. The nanowires are fabricated in a compact meander geometry that is in series with the center conductor of a 50 ohm coplanar waveguide transmission line. The transmission coefficient of the sample is measured up to 20 GHz. At high frequency, a peak in the transmission coefficient is seen. Numerical simulations show that this is a half-wave resonance along the length of the nanowire, where the nanowire acts as a high impedance, slow wave transmission line. This resonance sets the upper frequency limit for these nanowires as inductive elements. Fitting simulations to the measured resonance enables a precise determination of the nanowire's complex sheet impedance at the resonance frequency. The real part is a measure of dissipation, while the imaginary part is dominated by kinetic inductance. We characterize the dependence of the sheet resistance and sheet inductance on both temperature and current and compare the results to recent theoretical predictions for disordered superconductors. These results can aid in the understanding of high frequency devices based on superconducting nanowires. They may also lead to the development of novel superconducting devices such as ultra-compact resonators and slow-wave structures.

Detecting spatial and temporal information of individual photons is a crucial technology in todayu0027s quantum information science. Among the existing single-photon detectors, superconducting nanowire single-photon detectors (SNSPDs) have been demonstrated with a sub-50 ps timing jitter, near unity detection efficiency1, wide response spectrum from visible to infrared and ~10 ns reset time. However, to gain spatial sensitivity, multiple SNSPDs have to be integrated into an array, whose spatial and temporal resolutions are limited by the multiplexing circuit. Here, we add spatial sensitivity to a single nanowire while preserving the temporal resolution from an SNSPD, thereby turning an SNSPD into a superconducting nanowire single-photon imager (SNSPI). To achieve an SNSPI, we modify a nanowireu0027s electrical behavior from a lumped inductor to a transmission line, where the signal velocity is slowed down to 0.02c (where c is the speed of light). Consequently, we are able to simultaneously read out the landing locations and arrival times of the photons from the output electrical pulses using only two connections. We have demonstrated single-photon imaging by using a 19.7 mm long SNSPI, which is meandered into an imaging area of 286 {mu}m*193 {mu}m. The nanowire has a temporal resolution of 50 ps for detecting 1.5 {mu}m photons. The 2D spatial resolution is 13.0 {mu}m in the vertical direction and 5.6 {mu}m in the horizontal direction. The maximum number of resolvable locations (i.e., the effective number of pixels) in such a long nanowire is calculated to be 590. Rather than operating an individual detector in a scanning mode or using SPD arrays, this SNSPI gives an alternative approach to taking large-scale single-photon images and measuring temporal and spatial correlation.

Detecting spatial and temporal information of individual photons by using single-photon-detector (SPD) arrays is critical to applications in spectroscopy, communication, biological imaging, astronomical observation, and quantum-information processing. Among the current SPDs1,detectors based on superconducting nanowires have outstanding performance2, but are limited in their ability to be integrated into large scale arrays due to the engineering difficulty of high-bandwidth cryogenic electronic readout3-8. Here, we address this problem by demonstrating a scalable single-photon imager using a single continuous photon-sensitive superconducting nanowire microwave-plasmon transmission line. By appropriately designing the nanowire's local electromagnetic environment so that the nanowire guides microwave plasmons, the propagating voltages signals generated by a photon-detection event were slowed down to ~ 2% of the speed of light. As a result, the time difference between arrivals of the signals at the two ends of the nanowire naturally encoded the position and time of absorption of the photon. Thus, with only two readout lines, we demonstrated that a 19.7-mm-long nanowire meandered across an area of 286 {\mu}m * 193 {\mu}m was capable of resolving ~590 effective pixels while simultaneously recording the arrival times of photons with a temporal resolution of 50 ps. The nanowire imager presents a scalable approach to realizing high-resolution photon imaging in time and space.

Luttinger liquid theory predicts that collective electron excitations due to strong electron-electron interactions in a one-dimensional (1D) system will result in a modification of the collective charge-propagation velocity. By utilizing a circuit model for an individual metallic single-walled carbon nanotube as a nanotransmission line, it has been shown that the frequency-dependent terahertz impedance of a carbon nanotube can probe this expected 1D Luttinger liquid behavior. We excite terahertz standing-wave resonances on individual antenna-coupled metallic single-walled carbon nanotubes. The terahertz signal is rectified using the nanotube contact nonlinearity, allowing for a low-frequency readout of the coupled terahertz current. The charge velocity on the nanotube is determined from the terahertz spectral response. Our measurements show that a carbon nanotube can behave as a Luttinger liquid system with charge-propagation velocities that are faster than the Fermi velocity. Understanding what determines the charge velocity in low-dimensional conductors is important for the development of next generation nanodevices.