Fast tunable high Q-factor superconducting microwave resonators

We present fast tunable superconducting microwave resonators fabricated from planar NbN on a sapphire substrate. The $3\lambda/4$ wavelength resonators are tuning fork shaped and tuned by passing a dc current which controls the kinetic inductance of the tuning fork prongs. The $\lambda/4$ section from the open end operates as an integrated impedance converter which creates a nearly perfect short for microwave currents at the dc terminal coupling points, thus preventing microwave energy leakage through the dc lines. We measure an internal quality factor $Q_{\rm int}>10{^{5}}$ over the entire tuning range. We demonstrate a tuning range of $> 3\%$ and tuning response times as short as 20 ns for the maximum achievable detuning. Due to the quasi-fractal design, the resonators are resilient to magnetic fields of up to 0.5 T

Two-level systems in superconducting quantum devices due to trapped quasiparticles

A major issue for the implementation of large scale superconducting quantum circuits is the interaction with interfacial two-level system defects (TLS) that leads to qubit relaxation and impedes qubit operation in certain frequency ranges that also drift in time. Another major challenge comes from non-equilibrium quasiparticles (QPs) that result in qubit dephasing and relaxation. In this work we show that such QPs can also serve as a source of TLS. Using spectral and temporal mapping of TLS-induced fluctuations in frequency tunable resonators, we identify a subset of the general TLS population that are highly coherent TLS with a low reconfiguration temperature $\sim$ 300 mK, and a non-uniform density of states. These properties can be understood if these TLS are formed by QPs trapped in shallow subgap states formed by spatial fluctutations of the superconducting order parameter $\Delta$. Magnetic field measurements of one such TLS reveals a link to superconductivity. Our results imply that trapped QPs can induce qubit relaxation.Comment: 7 pages, 4 figures, plus 18 pages, 13 figures supplementa

Reflection-enhanced gain in traveling-wave parametric amplifiers

The operating principle of traveling-wave parametric amplifiers is typically understood in terms of the standard coupled mode theory, which describes the evolution of forward propagating waves without any reflections, i.e., for perfect impedance matching. However, in practice, superconducting microwave amplifiers are unmatched nonlinear finite-length devices, where the reflecting waves undergo complex parametric processes, not described by the standard coupled mode theory. Here, we present an analytical solution for the TWPA gain, which includes the interaction of reflected waves. These reflections result in corrections to the well-known results of the standard coupled mode theory, which are obtained for both three-wave and four-wave mixing processes. Due to these reflections, the gain is enhanced and unwanted nonlinear phase modulations are suppressed. Predictions of the model are experimentally demonstrated on two types of unmatched TWPA, based on coplanar waveguides with a central wire consisting of (i) a high kinetic inductance superconductor, and (ii) an array of 2000 Josephson junctions

Greater fuel efficiency is potentially preferable to reducing NOx emissions for aviation’s climate impacts

Aviation emissions of nitrogen oxides (NOx) alter the composition of the atmosphere, perturbing the greenhouse gases ozone and methane, resulting in positive and negative radiative forcing effects, respectively. In 1981, the International Civil Aviation Organization adopted a first certification standard for the regulation of aircraft engine NOx emissions with subsequent increases in stringency in 1992, 1998, 2004 and 2010 to offset the growth of the environmental impact of air transport, the main motivation being to improve local air quality with the assumed co-benefit of reducing NOx emissions at altitude and therefore their climate impacts. Increased stringency is an ongoing topic of discussion and more stringent standards are usually associated with their beneficial environmental impact. Here we show that this is not necessarily the right direction with respect to reducing the climate impacts of aviation (as opposed to local air quality impacts) because of the tradeoff effects between reducing NOx emissions and increased fuel usage, along with a revised understanding of the radiative forcing effects of methane. Moreover, the predicted lower surface air pollution levels in the future will be beneficial for reducing the climate impact of aviation NOx emissions. Thus, further efforts leading to greater fuel efficiency, and therefore lower CO2 emissions, may be preferable to reducing NOx emissions in terms of aviation’s climate impacts

Constitutional Segregation of Al2O3 in Mold Slag and Its Impact on Steel Cleanliness During Continuous Casting

The alumina pickup in a range of mold fluxes used for continuous casting of aluminum (Al)-killed ultralow carbon, low carbon, and peritectic steel have been measured. The Al2O3 pickup in slag varies approximately from 7 to 12 pct and depends on the slag basicity. Significantly higher Al2O3 pickup reported in basic slags and polynomial relationship exists between them. The effect of chemical composition on microstructure evolution and Al2O3 partitioning during crystallization was identified in all three types of mold slags. The microsegregation of Al2O3 inclusions in the constituent phase of CaO-SiO2-Al2O3 based slag film is presented. Constitutional segregation of Al2O3 inclusion in slag was found to affect the Al2O3 pickup phenomena during continuous casting. Segregation of alkalis like Na and K was also observed in an Si-rich interdendritic matrix, whereas F was retained in the dendrites of all the slags studied. The Al2O3 inclusion partitioning and interdendritic segregation in the mold slag is studied with metallographic evidence. (C) The Minerals, Metals & Materials Society and ASM International 201

Assessing the impact of aviation on climate

We present an assessment of the marginal climate impacts of new aviation activities. We use impulse response functions derived from carbon-cycle and atmospheric models to estimate changes in surface temperature for various aviation impacts (CO2, NOx on methane, NOx on ozone, sulfates, soot, and contrails/induced cirrus). We use different damage functions and discount rates to explore health, welfare and ecological costs for a range of assumptions and scenarios. Since uncertainty is high regarding many aviation effects, we explicitly capture some uncertainty by representing several model parameters as probabilistic distributions. The uncertainties are then propagated using Monte Carlo analysis to derive estimates for the impact of these uncertainties on the marginal future climate impacts. Our goal is to provide a framework that will communicate the potential impacts of aviation on climate change under different scenarios and assumptions, and that will allow decision-makers to compare these potential impacts to other aviation environmental impacts. We present results to describe the influence of parametric uncertainties, scenarios, and assumptions for valuation on the expected marginal future costs of aviation impacts. Estimates of the change in global average surface temperature due to aviation are most sensitive to changes in climate sensitivity, the radiative forcing attributed to short-lived effects (in particular those related to contrails and aviation-induced cirrus), and the choice of emissions scenario. Estimates of marginal future costs of aviation are most sensitive to assumptions regarding the discount rate, followed by assumptions regarding climate sensitivity, and the choice of emissions scenario

Fast Tunable High- Q -Factor Superconducting Microwave Resonators

We present fast tunable superconducting microwave resonators fabricated from planar NbN on a sapphire substrate. The 3λ/4 wavelength resonators are tuning fork shaped and tuned by passing a dc current that controls the kinetic inductance of the tuning fork prongs. The λ/4 section from the open end operates as an integrated impedance converter that creates a nearly perfect short for microwave currents at the dc terminal coupling points, thus preventing microwave energy leakage through the dc lines. We measure an internal quality factor Qint>105 over the entire tuning range. We demonstrate a tuning range of greater than 3% and tuning response times as short as 20 ns for the maximum achievable detuning. Because of the quasifractal design, the resonators are resilient to magnetic fields of up to 0.5 T

Optical response of a cold-electron bolometer array integrated in a 345-GHz cross-slot antenna

Two series/parallel arrays of ten cold-electron bolometers with superconductor–insulator–normal tunnel junc- tions were integrated in orthogonal ports of a cross-slot antenna. To increase the dynamic range of the receiver, all single bolome- ters in an array are connected in parallel for the microwave signal by capacitive coupling. To increase the output response, bolometers are connected in series for dc bias. With the mea- sured voltage-to-temperature response of 8.8 μV/mK, absorber volume of 0.08 μm3, and output noise of about 10 nV/Hz1/2, we estimated the dark electrical noise equivalent power (NEP) as NEP=6∗10−18W/Hz1/2. The optical response down to NEP =2∗10−17W/Hz1/2 was measured using a hot/cold load as a radiation source and a sample temperature down to 100 mK. The fluctuation sensitivity to the radiation source temperature is 1.3∗10−4 K/Hz1/2 . A dynamic range over 43 dB was measured using a backward-wave oscillator, a variable polarization grid attenuator, and cold filters/attenuators