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Revolutionising voltage standards

The science that deals directly with measurement, namely metrology, targets improving measurement-based decisions in laboratory, calibration, manufacturing, and management processes at all levels of accuracy using standards. In particular, the Josephson array voltage standard is the key voltage standard accepted by and used in the foremost calibration laboratories worldwide. The ProVolt project resulted in setting up the basis for the production of a programmable Josephson voltage standard, aimed to be used as a wide scaled voltmeter, for primary and precise AC and DC metrology.
Revolutionising voltage standards
In the early sixties, Brian Josephson estimated that a superconducting tunnel junction, when subjected to microwave frequency radiation outputs an average voltage that is an integer multiple of a quantised value. This key relation between voltage and frequency, which was later experimentally confirmed, has been used in most countries since the early seventies to link the unit of voltage to that of frequency. The first Josephson voltage standards normally employed a single junction and generated no more than a few millivolts, but during the eighties highly specialised techniques were exploited to fabricate series of arrays of thousands of junctions to generate quantum voltages greater than 10 volts. Recently, the studies have been directed towards pulse-driven arrays of junctions for creating accurate AC waveforms that may revolutionise metrology at audio frequencies.

Focusing on the development of a rapid programmable DC-voltage standard with the possibility to synthesise low frequency AC waveforms, the ProVolt project resulted in the development of new kinds of Josephson voltage standard arrays, with high intrinsic reference voltage stability and the possibility for high-speed selection of certain reference voltages, including the option to synthesise AC voltages. The design of such arrays required an improved understanding of the nonlinear dynamics of Josephson junctions, and optimum circuit parameters for increased reliability and yield of the arrays.

These results can be easily applied in an entirely automated calibration of DC Voltage standards up to 1 V with the highest possible credibility. In addition, they can be used for linear testing and absolute calibration of voltmeters, as well as for high precision DC Voltage calibrators. Moreover, the possibility of producing AC voltages will have a great impact on AC voltage metrology and research in many ways, including calibration of AC voltage standards in the mV range, calibration and linearity testing of high-precision AC voltmeters and calibrators, as well as of high precision wattmeters.

In practice, the Josephson set-up is a quantised voltmeter that can be utilised by research and industrial production processes as an instrument for further improvement of the quality of measurements. Currently, the calibration procedures using the high-performance digital voltmeters are costly, annually-repeated procedures, conducted according to secondary standards and with limited applications. With the introduction of the new instrument, secondary standards will become needless and AC voltage metrology will be included.

With these remotely controllable arrays, the realisation of remote or virtual calibration via internet or a telephone line is expected to ease calibration procedures. Additionally, based on these improved arrays, the fabrication of advanced arrayed circuits, such as Rapid Single Flux Quantum circuits will be feasible. Hence, superconducting circuits that exhibit a high degree of integration and smaller line widths will substantially benefit the high-performance super-conducting electronics industry. Essentially, meeting not only the present requirements of industrial and governmental standard laboratories, but the future as well.

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