Jonathan M. Williams

Synthesis of precision AC waveforms using a SINIS Josephson junction array

A synthesizer of precision AC waveforms based on a SINIS Josephson junction array has been developed. The array is divided into 15 segments in a binary series and the segments are individually biased using a high speed bias source with a 100 ns rise time. The design considerations and accuracy limitations of the system are discussed and comparisons between the synthesizer and thermal converters are presented.

An automated cryogenic current comparator resistance ratio bridge

The method of measurement of two-terminal cryogenic and four-terminal room-temperature resistors using a DC cryogenic current comparator bridge is well established. A bridge which is based on this technique and is fully automated with a computer controlling both the measurement current and the readings of the bridge imbalance current is described. The bridge is designed to measure integer ratios of 1:1 to 1:25 of four-terminal room-temperature resistors in the range 1 Omega to 10 k Omega . An overall accuracy of 1 part in 10/sup 9/ for differences in the ratio from nominal of up to 100 p.p.m. is desired. Some initial determinations of the resolution have been made for a measurement of 10 Omega :100 Omega . These determinations were carried out without the balance servo, and the output from the null detector was measured with a digital voltmeter connected to the computer. An analysis of a set of 11 reversals gave a standard deviation for a single reversal of 1 part in 10/sup 8/ and therefore a standard error of the mean of 3 parts in 10/sup 9/. >

Development of a 60 Hz Power Standard Using SNS Programmable Josephson Voltage Standards

We are implementing a new standard for 60 Hz power measurements based on precision sinusoidal reference voltages from two independent programmable Josephson voltage standards (PJVS): one for voltage and one for current. The National Institute of Standards and Technology PJVS systems use series arrays of Josephson junctions to produce accurate quantum-based DC voltages. Using stepwise-approximation synthesis, the PJVS systems produce sinewaves with precisely calculable RMS voltage and spectral content. We present measurements and calculations that elucidate the sources of error in the RMS voltage that are intrinsic to the digital-synthesis technique and that are due to the finite rise times and transients that occur when switching between the discrete voltages. Our goal is to reduce all error sources and uncertainty contributions from the PJVS synthesized waveforms to a few parts in 10 7 so that the overall uncertainty in the AC-power standard is a few parts in 106

The simulation and measurement of the response of Josephson junctions to optoelectronically generated short pulses

A new method of driving series arrays of non-hysteretic Josephson junctions using optoelectronically generated pulses to synthesize a variable output voltage is described. Electrical pulses from a photodiode are coupled to the array, and the mean output voltage after filtering is proportional to the pulse repetition frequency with fundamental accuracy. Numerical simulations of the behaviour of the Josephson junctions have been carried out, and these results have been verified by experiment. This technique can be easily extended by using two photodiodes with opposite bias polarities in order to synthesize bipolar alternating or direct voltages.

Precision comparison of the quantum Hall effect in graphene and gallium arsenide

The half-integer quantum Hall effect in epitaxial graphene is compared with high precision to the well-known integer effect in a GaAs/AlGaAs heterostructure. We find no difference between the quantized resistance values within the relative standard uncertainty of our measurement of 8.7 x 10(-11). The result places new tighter limits on any possible correction terms to the simple relation R-K = h/e(2), and also demonstrates that epitaxial graphene samples are suitable for application as electrical resistance standards of the highest metrological quality. We discuss the characterization of the graphene sample used in this experiment and present the details of the cryogenic current comparator bridge and associated uncertainty budget.

Characterization of a High-Resolution Analog-to-Digital Converter with an Ac Josephson Voltage Source

An alternating current (ac) Josephson voltage source (Williams, et. al., 2002) was employed to characterize the dynamic behavior of a high-resolution 28-bit (8 frac12-digit) integrating analog-to-digital converter (ADC) of a digital sampling voltmeter (DSV), which is widely used in ac metrology at the Physikalisch-Technische Bundesanstalt - PTB. Extensive measurements were carried out to validate previous mathematical models of the DSV (Ramm, et. al., 1999) when sampling ac signals. Some measurement methods and results thereof are presented

Operation of graphene quantum Hall resistance standard in a cryogen-free table-top system

We demonstrate quantum Hall resistance measurements with metrological accuracy in a small cryogen-free system operating at a temperature of around 3.8K and magnetic fields below 5T. Operating this system requires little experimental knowledge or laboratory infrastructure, thereby greatly advancing the proliferation of primary quantum standards for precision electrical metrology. This significant advance in technology has come about as a result of the unique properties of epitaxial graphene on SiC.

Modernizing the SI: towards an improved, accessible and enduring system

Recent proposals to redefine four of the base units of the International System (SI) have stimulated some debate as to how the SI might best be developed to meet changes in requirements. In this paper we consider whether the proposals are a timely and proportionate response to those changes in requirements that can be identified today. We conclude that, whilst the proposed changes to the definitions of the mole and kelvin appear to have some advantages, the case for changes in the definitions of the ampere and the kilogram is less well made at the present stage of development of the key experiments involved.

Achieving Sub-100-ns Switching of Programmable Josephson Arrays

The accuracy of voltage waveforms, generated using programmable Josephson junction arrays, is limited by the time taken to switch between quantized levels. Relevant parameters are the time constants within the array chip, the characteristics of the cables, and the bandwidth of the bias electronics. A drive system for a 1-V SINIS array with 8192 junctions has been optimized, and a cable scheme has been devised which compensates for cable reflections. The measurements of the arrays temporal response, made with this cable system, are compared to an RC model, both with 3- and 7-ns single time constants

Realisation of a Quantum Standard for AC Voltage: Overview of a European Research Project

The European JAWS project addresses the lack of instrumentation and knowledge for accurate and traceable measurement and generation of alternating (AC) voltage with arbitrary waveforms. The objective is to develop a standard based on a Josephson Arbitrary Waveform Synthesizer (JAWS), a precise instrument that links the generated voltage to the Josephson effect, which in turn relates the voltage to fundamental constants and a frequency. The standard aims at synthesized voltages up to 10 mV, frequencies up to 10 kHz and uncertainty smaller than 1 part in 104. This paper reports on the progress in developing the key components for JAWS