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Superconductor Science and Technology

A dry-cooled AC quantum voltmeter

M. Schubert, M. Starkloff, Katja Peiselt, Solveig Anders, Richard Knipper, J. Lee, Ralf Behr, L. Palafox, A. C. Bock, L. Schaidhammer, P. M. Fleischmann, H.‐G. Meyer

Published Sep 15, 2016• 14 citations
Publisher

Abstract

Abstract The paper describes a dry-cooled AC quantum voltmeter system operated up to kilohertz frequencies and 7 V rms. A 10 V programmable Josephson voltage standard (PJVS) array was installed on a pulse tube cooler (PTC) driven with a 4 kW air-cooled compressor. The operating margins at 70 GHz frequencies were investigated in detail and found to exceed 1 mA Shapiro step width. A key factor for the successful chip operation was the low on-chip power consumption of 65 mW in total. A thermal interface between PJVS chip and PTC cold stage was used to avoid a significant chip overheating. By installing the cryocooled PJVS array into an AC quantum voltmeter setup, several calibration measurements of dc standards and calibrator ac voltages up to 2 kHz frequencies were carried out to demonstrate the full functionality. The results are discussed and compared to systems with standard liquid helium cooling. For dc voltages, a direct comparison measurement between the dry-cooled AC quantum voltmeter and a liquid-helium based 10 V PJVS shows an agreement better than 1 part in 10 10 .

DOI
10.1088/0953-2048/29/10/105014

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Abstract The paper describes a dry-cooled AC quantum voltmeter system operated up to kilohertz frequencies and 7 V rms. A 10 V programmable Josephson voltage standard (PJVS) array was installed on a pulse tube cooler (PTC) driven with a 4 kW air-cooled compressor. The operating margins at 70 GHz frequencies were investigated in detail and found to exceed 1 mA Shapiro step width. A key factor for the successful chip operation was the low on-chip power consumption of 65 mW in total. A thermal interface between PJVS chip and PTC cold stage was used to avoid a significant chip overheating. By installing the cryocooled PJVS array into an AC quantum voltmeter setup, several calibration measurements of dc standards and calibrator ac voltages up to 2 kHz frequencies were carried out to demonstrate the full functionality. The results are discussed and compared to systems with standard liquid helium cooling. For dc voltages, a direct comparison measurement between the dry-cooled AC quantum voltmeter and a liquid-helium based 10 V PJVS shows an agreement better than 1 part in 10 10 .
The two main challenges to operating a programmable Josephson voltage standard (PJVS) on a cryocooler are the available cooling power and the temperature oscillations of the cold head. We overcame these challenges and successfully operated a PJVS circuit on a cryocooler by employing one supercritical helium buffer that damps the temperature oscillations, developing a new cryogenic package that increases the thermal conductivity between the chip and the cold head, and increasing overall device performance with fabrication improvements. A 1.32 mA step width of the quantized voltage produced with all of the subarrays of the PJVS circuit biased was achieved at an operating temperature of 4.3 K. The quantum accuracy of the PJVS is maintained at temperatures up to 4.8 K. This result was obtained with a cryocooler that employs a 3 kW water-cooled compressor to produce at the chip about 270 mW of net cooling power at 4.3 K.
For a more wide-spread use of Josephson voltage standards, cryogen-free operation, by means of an appropriate closed-cycle refrigerator, is highly desirable. In this work we present a low-noise pulse tube cooler (PTC) that is capable to cool the voltage standard circuits to temperatures below 4 K. As a low loss dielectric waveguide with a very small thermal conductivity a Teflon strip was used for the microwave transmission. The matched transition from the WR12 rectangular waveguide to the Teflon strip and vice versa was made using special exponential tapers. Programmable 1 Volt 14 bit SINIS and 10 Volt SIS arrays assembled in the PTC operated well, i.e., demonstrated a similar performance to that measured in liquid helium. The Josephson junction arrays were integrated in coplanar strips instead of the commonly used microstriplines for the microwave transmission lines. The 1 Volt SINIS array contained 8192 Josephson junctions (JJs). It generated an inherently stable Josephson voltage step of 1.19 V at a driving microwave frequency of 70 GHz. The step width was 150 /spl mu/A and the critical current of the junctions was 550 /spl mu/A. Similar results were obtained under liquid helium cooling conditions. In another experiment a 10 Volt voltage standard chip with 19700 SIS JJs was installed. It was possible to generate stable Shapiro steps at the 10 V level.
Improvements in fabrication and packaging of our 10 V Programmable Josephson Voltage Standard circuits have enabled the successful operation of these devices on a cryocooler. Limited cooling power and temperature oscillations at the chip must not compromise the accuracy of the quantized voltage steps. Flat-spot measurements of quantized voltages produced with all of the sub-arrays when the chip is biased at a temperature of 4.35 K produced a step width of 0.78 mA. This result was obtained with a cryostat that uses a modest 1.3 kW air-cooled compressor that generates 180 mW of cooling power at the chip.
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.
We report on the performance of a 1-V dc programmable Josephson voltage standard (PJVS) operating on a commercially available Gifford-McMahon cryocooler at 4 K. The NIST-fabricated niobium superconductor-normal metal-superconductor junction chip has been shown to operate successfully as integrated with this cryopackage, generating 1.229 V at the maximum operating frequency of 19.364 GHz with Shapiro steps at constant voltage that are flat to within less than 100 nV. Because of the large operating margin (greater than 1 mA), these steps were highly stable and immune from noise. Differences in behavior of the PJVS in liquid helium and on the cryocooler are discussed. This demonstration paves the way towards automated warming and cooling of the Josephson device and a fully automated voltage standard system.
The paper deals with recent progresses at INRiM towards the development and characterization of a programmable Josephson voltage standard (PJVS) operating in a small liquid helium dewar as well as with its integration for the realization of a practical quantum sampling electrical power standard. The PJVS is based on a 1 V superconductor-normal metal-superconductor (SNS) binary-divided array of 8192 Josephson junctions. To ensure proper operating conditions of the PJVS chip, a custom short cryoprobe was designed, built and successfully tested. The overall system is being developed in the framework of EMPIR project 19RPT01-QuantumPower. The goal is to establish a new quantum power standard (QPS) based on a single Josephson voltage standard for sampled power measurements and to gain confidence in running PJVS for precise calibration of digital sampling multimeters and arbitrary waveform digitizers used in the ac-voltage and power metrology community.