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Conference on Precision Electromagnetic Measurements

Towards quantum AC voltage standard

Jari S Penttilä, P. Helistö, Heikki Seppä

Published Jun 25, 2003• 1 citation
Publisher

Abstract

We have studied the possibility of realizing AC voltages with metrological accuracy. In this paper we propose two experimental setups for generating accurate sinusoidal voltage waveforms from square-wave biased nonhysteretic programmable Josephson array voltage standards.
DOI
10.1109/cpem.2002.1034940

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Arrays of Josephson junctions have been used successfully for many years as the primary standard of voltage. A recent development has been the fabrication of arrays, based on non-hysteretic Josephson junctions, which allows the voltage output to be changed almost instantaneously. When these arrays are divided into binary segments a ‘digital-to-analogue converter’ with quantum accuracy is created. The properties of two types of array, a SINIS array, using intrinsic shunting of the Josephson junctions, and a SIS array with external, on-chip shunting have been investigated. The accuracy of the generated voltages and AC properties of the arrays when driven with a purpose-built, programmable current bias source are discussed. It is anticipated that a Josephson standard having waveforms with calculable rms values of AC voltage will soon be available.
The past few decades have witnessed great progress being made from the first voltage standards based on arrays of Josephson junctions. Continuous improvements in the fabrication process have increased the quality and “user-friendliness” of these superconducting circuits. Combined with the improvements in measuring electronics, the range of applications of Josephson-based measurements with unmatched precision has extended from DC into AC voltage metrology for signal frequencies up to 500 kHz. In this poster we will present several Josephson-based measurement systems developed for metrology applications. Two versions of quantum AC standards will be used as Josephson references: Programmable Josephson Voltage Standards (PJVS), which are based on serial arrays with up to 300,000 Josephson junctions, and pulse-driven arrays with more than 50,000 junctions, typically referred to as Josephson Arbitrary Waveform Synthesizers (JAWS). PJVS can reach amplitudes of up to 80 V, while pulse-driven arrays can reach up to 1 V or even 4 V. The advantage of pulse-driven Josephson voltage standards is that they are ideally suited as quantum-based voltage synthesizers for arbitrary waveforms or sine waves with high spectral purity. Therefore, they can be used directly to characterize or calibrate ADCs. In addition, JAWS are particularly useful in Josephson impedance bridges, which perform automated impedance calibrations at the 10-9 uncertainty level over a wide frequency range from 50 Hz to 80 kHz in a single bridge. We will further show how PJVS are used in commercial quantum voltmeters for DC and AC voltages and are typically used as quantum voltage references in measurement systems. Using differential sampling based on PJVS, uncertainties at the level of a few parts in 108 for frequencies up to the kHz range can be achieved. Such quantum-based AC voltage calibrations are at least 30 times faster than conventional AC-DC transfer and offer even better uncertainties. This idea has been quickly adopted by many National Metrology Institutes to establish such systems for AC voltage calibrations. As an example, the BIPM will now start worldwide onsite comparisons to ensure the uniformity of the new method which we introduced in 2006. But developments continue. Recently, a sub-sampling technique was used to extend the frequency range of the AC quantum voltmeter to up to 100 kHz with uncertainties below 5 parts in 106. In addition, a novel method, also for differential sampling, enables the measurement of arbitrary waveforms and the calibration of specific tones. Our poster will provide an overview of these Josephson-based metrological applications and present the latest developments from this fast-developing area.
The Josephson primary DC voltage standard is very successful quantum standard. Recently it has become possible to divide Josephson arrays up into binary segments, thereby creating the possibility of developing a digital to analogue converter with quantum accuracy. These new arrays are based upon non-hysteretic Josephson junctions, which allow the voltage to be changed almost instantaneously. In this paper we will investigate the properties of two different types of Josephson arrays, one type is an intrinsically shunted array of junctions or so-called SINIS array, while the second type is externally shunted via small on-chip resistors. The arrays are biased using a purpose built computer controlled bias source discussed in a separate paper in this conference. We will discuss the accuracy of the generated voltage and investigate the ac properties.
Recent metrological activities with programmable Josephson voltage standards (PJVS) are focused on extending the quantum accuracy to AC voltages, e.g. for the step-wise approximation of sine waves. Stimulated by the progress in this field, new requirements for the quality and availability of 1 V and 10 V circuits have been established. Based on the experience with programmable 1 V SINIS circuits fabricated at PTB and used at present by 13 National Metrological Institutes worldwide, the fabrication process and the microwave design were improved. The experience gained hereby enabled us to improve the properties of programmable 10 V SINIS arrays which were realized by us for the first time in 2000. The new 10 V circuits have 17 bit resolution and develop under 70 GHz microwave irradiation flat Shapiro steps at the 10 V level which are more than 600 muA wide. A direct comparison of the output voltage with a conventional 10 V SIS Josephson voltage standard confirmed the suitability for metrological applications. Incorporating 10 V-PJVS into existing AC metrology setups is expected to reduce their uncertainty budgets by about one order of magnitude.
Digital to analog converters based on the Josephson effect are promising for voltage standards, because they produce voltage steps with ultimate precision and stability. In this paper, we describe a project to develop a Josephson D/A converter designed for synthesizing a sinusoidal waveform with metrological accuracy. The D/A converter is based on RSFQ (Rapid Single Flux Quantum) logic circuits, and consists of a frequency multiplier (FM), a pulse distributor (PD), and a number of voltage multipliers (VMs). Each VM circuit, corresponding to the n-th bit digital code, multiplies the number of SFQ pulses by a factor of 2/sup n/. By gating the input SFQ pulses from the FM to the VMs using the PD circuits, a programmable output voltage is obtained. Possible sources of uncertainties in the measurement of the rms value of the synthesized sine wave are discussed.