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IEEE Transactions on Applied Superconductivity

Improved 10 V SINIS Series Arrays for Applications in AC Voltage Metrology

F.. Mueller, Ralf Behr, L. Palafox, J. Kohlmann, Ruediger Wendisch, I. Ya. Krasnopolin

Published Jun 1, 2007• 58 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2007.898736

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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.
Programmable Josephson voltage standards (PJVSs) in combination with fast switchable DC current sources have opened up new applications in the field of low-frequency AC metrology. The growing interest in output voltages of up to plusmn10 V initiated efforts by several National Metrological Institutes to realize 10 V PJVSs. Presently, only 10 V PJVSs from PTB based on SINIS junctions have been successfully incorporated into existing setups for AC metrology. However, the fabrication of 10 V SINIS arrays that are driven at 70 GHz suffers from very low yield. The recent technological progress made at NIST enabled the drop-in replacement of the low-yield SINIS arrays by more robust SNS arrays. The N-material is an amorphous NbxSi1-xalloy near the metal-insulator transition and is deposited by co-sputtering. For the first time, fully operational 1 V and 10 V PJVSs with SNS junctions that are suitable for a 70 GHz drive have been fabricated and tested. This work was done in close cooperation between NIST and PTB.
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.
A programmable dc voltage standard for output voltages of up to 10 V has been realized. The series arrays, consisting of about 69120 overdamped superconductor/insulator/normal metal/insulator/superconductor (SINIS) Josephson junctions, have been fabricated using the reliable Nb-Al/Al/sub 2/O/sub 3/ technology. The arrays can be operated in conventional Josephson voltage standards at microwave frequencies from 70 GHz to 75 GHz. Steps of constant voltage are observed at very low microwave power levels, since the major part of the microwave power is generated by the junctions themselves. The operation of the arrays and the formation of Shapiro steps are discussed.
PTB started using the robust NbxSi1-xbarrier junction technology for the fabrication of large arrays for the Programmable Josephson Voltage Standard (PJVS) and for the Josephson Arbitrary Waveform Synthesizer 3 years ago. We demonstrate how Nb-doping of the amorphous Si barrier causes the transition from an underdamped to a desired overdamped junction behavior. Special dc SQUIDs have been used to evaluate junction capacitance and noise properties. The critical current of small junctions as a function of applied magnetic field has been investigated. On the basis of an existing 70 GHz design previously used for 10 V PJVS chips, we fabricated for the first time 20 V circuits with nearly 140 000 double-stacked Josephson junctions. A direct on-chip comparison between the two 10 V halves of the binary-divided array confirmed the metrological suitability of the 20 V circuits for dc and ac applications.