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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 t...

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 K and 4.5 K.

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 $50$ $\Omega$ characteristic impedance across a wide range of CPW widths, from the nanoscale to the macroscale. We fabricated and characterized an approximately $50$ $\Omega$ 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 $1.1 \times 10^3$ 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 ultracompact 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 $416\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}$. At 4.753 GHz, the input signal couples equally to the through port and forward-coupling port (50:50) at $\ensuremath{-}6.7\phantom{\rule{0.2em}{0ex}}\mathrm{dB}$ with $\ensuremath{-}13.5\phantom{\rule{0.2em}{0ex}}\mathrm{dB}$ 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 processes are suitable for direct integration with superconducting circuits, providing a practical solution to the signal distribution bottlenecks in developing large-scale quantum computers.

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.

A superconducting nanowire acting as a single-photon detector and as a microwave delay line is used to demonstrate an imaging device at the single-photon level with sub-20-µm spatial resolution and 50-ps temporal resolution.

Conventional readout of a superconducting nanowire single-photon detector (SNSPD) sets an upper bound on the output voltage to be the product of the bias current and the load impedance, I B × Z load , where Z load is limited to 50 Ω in standard r.f. electronics. Here, we break this limit by interfacing the 50 Ω load and the SNSPD using an integrated superconducting transmission line taper. The taper is a transformer that effectively loads the SNSPD with high impedance without latching. At the expense of reduced maximum counting rate, it increases the amplitude of the detector output while preserving the fast rising edge. Using a taper with a starting width of 500 nm, we experimentally observed a 3.6× higher pulse amplitude, 3.7× faster slew rate, and 25.1 ps smaller timing jitter. The results match our numerical simulation, which incorporates both the hotspot dynamics in the SNSPD and the distributed nature in the transmission line taper. The taper studied here may become a useful tool to interface high-impedance superconducting nanowire devices to conventional low-impedance circuits., National Science Foundation (U.S.) (Contract ECCS-1509486)

We characterize the evolution of the electrical properties of ultra-thin niobium films stored in ambient conditions over a period of approximately seven months. Patterned films with thicknesses between 8 and 16 nm were fabricated via electron-beam deposition on unheated silicon substrates using a lift-off process. The film quality is similar to previous results obtained with sputter deposition onto unheated silicon substrates. The increase of the resistance and the decrease of the superconducting critical temperature are well described by an exponential function with a time constant of approximately 37 days.

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.

© 2018 Author(s). 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 the 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 geometrical 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 discuss 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.

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.

We study the microwave impedance of extremely high aspect ratio (length/width ≈ 5000) superconducting niobium nitride nanowires. The nanowires are fabricated in a compact meander geometry that is in series with the center conductor of a 50 Ω 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., National Science Foundation (U.S.) (grant CCS‐1509253 (UNF)), National Science Foundation (U.S.) (grant ECCS‐1509486 (MIT)), Research Corporation for Science Advancement (Cottrell College Science Award)