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This paper presents a full characterization of a Dual Josephson Impedance Bridge (DJIB) at frequencies up to 80 kHz by using the DJIB to compare the best available impedance standards that are (a) directly traceable to the quantum Hall effect, (b) used as part of international impedance comparisons, or (c) believed to have calculable frequency dependence. The heart of the system is a dual Josephson Arbitrary Waveform Synthesizer (JAWS) source that offers unprecedented flexibility in high-precision impedance measurements. The JAWS sources allow a single bridge to compare impedances with arbitrary ratios and phase angles in the complex plane. The uncertainty budget shows that both the traditional METAS bridges and the DJIB have comparable uncertainties in the kilohertz range. This shows that the advantages of the DJIB, including the flexibility which allows the comparison of arbitrary impedances, the wide frequency range, and the automated balancing procedure, are obtained without compromising the measurement uncertainties. These results demonstrate that this type of instrument can considerably simplify the realization and maintenance of the various impedance scales. In addition, the DJIB is a very sensitive tool for investigating the frequency-dependent systematic-errors that can occur in impedance construction and in the voltage provided by the JAWS source at frequencies greater than 10 kHz.

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We present time-domain electrical measurements and simulations of the quantized voltage pulses that are generated from series-connected Josephson junction (JJ) arrays. The transmission delay of the JJ array can lead to a broadening of the net output pulse, depending on the direction of the output pulse propagation relative to the input bias pulse. To demonstrate this, we compare time-domain measurements of output pulses from radio-frequency Josephson Arbitrary Waveform Synthesizer (RF-JAWS) circuits fabricated with two different output measurement configurations, so that the backward-propagating and forward-propagating pulses can be measured. Measurements were made on arrays with 1200 and 3600 JJs and show that the net backward-propagating output pulse is broadened by timing delays in the JJ array while the net forward-propagating output pulse is insensitive to delay effects and can theoretically be further scaled to longer JJ array lengths without significant output pulse broadening. These measurements match well with simulations and confirm the expectation that the net output pulses arise from the time-delayed superposition of individual JJ output pulses from the series array of JJs. The measurements and analysis shown here have important implications for the realization of RF-JAWS circuits to be used as quantum-based reference sources for communications metrology.

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The uniformity of the barriers in Josephson junctions (JJs) is a critical parameter in determining performance and operating margins for a wide variety of superconducting electronic circuits. We present an automated measurement system capable of measuring individual JJs across a 1 × 1 cm die at both ambient temperature and 4 K. This technique allows visualization of the spatial variation over a large area of the critical electrical properties of the junctions and allows for the direct correlation between room-temperature (RT) resistance and low temperature properties. The critical current variation of NbxSi1-x (x = 15%) barriers is found to be about 2.6% (one standard deviation) for 1024 junctions across an individual die and only weakly correlates with RT resistance measurements.

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We describe the implementation of new commercial pulse-bias electronics that have enabled an improvement in the generation of quantum-accurate waveforms both with and without low-frequency compensation biases. We have used these electronics to apply a multilevel pulse bias to the Josephson arbitrary waveform synthesizer and have generated, for the first time, a quantum-accurate bipolar sinusoidal waveform without the use of a low-frequency compensation bias current. This uncompensated 1 kHz waveform was synthesized with an rms amplitude of 325 mV and maintained its quantum accuracy over a1.5 mA operating current range. The same technique and equipment was also used to synthesize a quantum-accurate 1 MHz sinusoid with a 1.2 mA operating margin. In addition, we have synthesized a compensated 1 kHz sinusoid with an rms amplitude of 1 V and a 2.7 mA operating margin.

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In preparation for the redefinition of the International System of Units (SI), five different electronic measurements of the Boltzmann constant have been performed using different Johnson noise thermometry (JNT) systems over the past seven years. In this paper, we describe in detail the JNT system and uncertainty components associated with the most recent National Institute of Standards and Technology (NIST) determination of the Boltzmann constant: k = 1.380642 9(69) × 10-23 J/K, with a relative standard uncertainty of 5.0 × 10-6 and relative offset of -4.05 × 10-6 from the Committee on Data for Science and Technology (CODATA) 2014 recommended value. We discuss the input circuits and the approach we used to match the frequency response of two noise sources. We present new measurements of the correlated noise of the 4 K on-chip resistors in the quantum-accurate, pseudorandom, voltage-noise source, which we used to estimate the correlated, frequency-dependent, nonthermal noise in our system. Finally, we contrast our system with those used in other measurements and speculate on future improvements.

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The root-mean-square (rms) output voltage of the NIST Josephson arbitrary waveform synthesizer (JAWS) has been doubled from 1 V to a record 2 V by combining two new 1 V chips on a cryocooler. This higher voltage will improve calibrations of ac thermal voltage converters and precision voltage measurements that require state-of-the-art quantum accuracy, stability, and signal-to-noise ratio. We achieved this increase in output voltage by using four on-chip Wilkinson dividers and eight inner-outer dc blocks, which enable biasing of eight Josephson junction (JJ) arrays with high-speed inputs from only four high-speed pulse generator channels. This approach halves the number of pulse generator channels required in future JAWS systems. We also implemented on-chip superconducting interconnects between JJ arrays, which reduces systematic errors and enables a new modular chip package. Finally, we demonstrate a new technique for measuring and visualizing the operating current range that reduces the measurement time by almost two orders of magnitude and reveals the relationship between distortion in the output spectrum and output pulse sequence errors.

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This study looks at the changing incidence and aetiology of congenital talipes equinovarus due to the recent population changes within the area. Between 1st June 1992 and the 31st May 2006, 83 consecutive children (121 feet) born with fixed talipes equinovarus (TEV) were assessed and treated (an incidence of 1.6 per 1000 live births) in an observational longitudinal cohort study assessing associated factors. There were 17 syndromal cases in the fixed group (20.8%), 6 cases of non-syndromal distal arthrogryposis (7.2%), and a strong family history in 12 cases (14.5%). This study would suggest that genetic and primary causes of fixed TEV are more common than previously considered. Many of the primary aetiologies were diagnosed months or years after birth.

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We show proof of concept of a transmission-mode wavelength filtering device consisting of layered holographically formed polymer dispersed liquid crystal (H-PDLC) cells. H-PDLC cells were fabricated from a thiolene based polymer composite to have transmission notches in the near-IR wavelength range. Wavelength filtering was achieved by stacking four H-PDLC cells with transmission notches spaced at 10 nm intervals. Results show a broad transmission notch spanning the spectral width of the constituent cells. With bias applied to an individual cell within the stack, the transmission notch of the cell inverts and the overall transmission envelope changes shape. Using a transmitted energy sensing device and a lineshape mapping algorithm, spectral content can be determined to a resolution of 0.1 nm for narrow banded signals. Applications for this switchable wavelength filtering device include serial detection of spectral content for telecom data signals or chemical and biological sample identification through absorption or emission spectroscopy.

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We observe non-exponential absorption as a function of path length for pulses propagating in pure water. Two types of pulses with differing time duration, bandwidth, and repetition rate were compared with simulated absorption predictions. Deviations from exponential behaviour occurred when the launched pulse had a temporal width of 60fs and a repetition rate of 1 kHz. Under these conditions we observe more than 2 orders of magnitude less absorption after propagation through 6 m of water compared to Beer's law prediction. No significant deviation was observed for launched pulses of varying bandwidth with temporal widths of 900fs and repetition rates of 80MHz.

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