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Chinese Physics B

Development of 0.5-V Josephson junction array devices for quantum voltage standards*

Lanruo Wang, Jinjin Li, Wenhui Cao, Yuan Zhong, Zhonghua Zhang

Published Jun 1, 2019• 5 citations
Publisher

Abstract

The design, fabrication, and the characterization of a 0.5-V Josephson junction array device are presented for the quantum voltage standards in the National Institute of Metrology (NIM) of China. The device consists of four junction arrays, each of which has 1200 3-stacked Nb/Nb x Si1−x /Nb junctions and an on-chip superconducting microwave circuit which is mainly a power divider enabling each Josephson array being loaded with an equal amount of microwave power. A direct current (dc) quantum voltage of about 0.5 V with a ∼1-mA current margin of the 1st quantum voltage step is obtained. To further prove the quality of NIM device, a comparison between the NIM device with the National Institute of Standards and Technology (NIST) programmable Josephson voltage standard (PJVS) system device is conducted. The difference of the reproduced 0.5-V quantum voltage between the two devices is about 0.55 nV, which indicates good agreement between the two devices. With the homemade device, we have realized a precise and applicable 0.5-V applicable-level quantum voltage.

DOI
10.1088/1674-1056/28/6/068501

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The design, fabrication, and the characterization of a 0.5-V Josephson junction array device are presented for the quantum voltage standards in the National Institute of Metrology (NIM) of China. The device consists of four junction arrays, each of which has 1200 3-stacked Nb/Nb x Si1−x /Nb junctions and an on-chip superconducting microwave circuit which is mainly a power divider enabling each Josephson array being loaded with an equal amount of microwave power. A direct current (dc) quantum voltage of about 0.5 V with a ∼1-mA current margin of the 1st quantum voltage step is obtained. To further prove the quality of NIM device, a comparison between the NIM device with the National Institute of Standards and Technology (NIST) programmable Josephson voltage standard (PJVS) system device is conducted. The difference of the reproduced 0.5-V quantum voltage between the two devices is about 0.55 nV, which indicates good agreement between the two devices. With the homemade device, we have realized a precise and applicable 0.5-V applicable-level quantum voltage.
The efforts of National Institute of Metrology (NIM) towards large scale Nb/ NbxSi1-x/Nb Josephson junction arrays for the quantum voltage standards are presented in this paper. Physically-linked arrays with 10,000 and 20,000 junctions (corresponding to 1 and 2 junctions vertically) are fabricated. DC I-V characteristics of different stacked arrays are measured and experimental results are discussed. Focusing ion beam (FIB) together with scanning electron microscope (SEM) are used to examine the cross-section of three types of junction stacks which vertically contains 2, 3, and 4 junctions respectively. It is shown that grain boundary defects in the niobium wiring layer exist for all of these junction stacks. A method which potentially could solve this problem is proposed.
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.
Programmable Josephson voltage standard (PJVS) circuits were developed that operate at 16 GHz to 20 GHz with operating margins larger than 1 mA. Two circuit designs were demonstrated, each having a total of ~ 300,000 junctions, which were divided into either 16 or 32 sub-arrays. Triple-stacked junctions were used in order to fit the ~300,000 junctions on each circuit. The amorphous NbxSi1-x-barrier junction technology provided a high degree of uniformity of the barrier and junction electrical properties, which was necessary to achieve the 1 mA operating margin. Although the margins on both the 16-array and 32-array circuits were much greater than 1 mA, the circuit yield for the 16-array design was lower because the longer arrays are more sensitive to defects. The use of lumped-element microwave splitters and tapered arrays significantly reduced the microwave input power and increased the operating margins of these designs. In addition, the broadband microwave response of the designs allowed the PJVS output voltage to be continuously adjusted by using the microwave frequency while remaining on margins.
Analysis of the Josephson junction yield in the National Institute of Standards and Technology 10 V Programmable Josephson Voltage Standard (PJVS) has been performed by fabricating and measuring over 25 million Nb/NbxSi1-x/Nb junctions. Using the 265 116 junctions per PJVS device, it was possible to measure Shapiro steps on current voltage curves and then estimate the number of single-junction defects within these long arrays. By comparing the results of a quantum-based measurement to a model simulating junction defects, the quantity and magnitude of a small number of junction defects can be estimated from an array of many thousands of junctions. Understanding the source of junction defects in the fabrication process is important for maximizing yield and uniformity, both of which directly relate to the current margin of the quantized voltage step. We have recently fabricated 18 imperfect but usable PJVS devices and 16 that were free from defects, many with n = 1 Shapiro step margins approximately equal to 2 mA. The best of these wafers had a junction defect density of around 3 in a million.
NIST is designing a 10 V Programmable Josephson Voltage Standard (PJVS) system with an improved microwave design and arrays of stacked NbxSi1-x-barrier Josephson junctions. For this new design a "ground-up" approach was used that takes into account all system issues in order to produce a robust 10 V system. By improving the uniformity of the microwave drive along the length of each array, constant-voltage steps with a larger current range are generated allowing the use of smaller critical current junctions. Smaller critical currents are important for lowering the total overall power dissipated on chip. Reducing power dissipation also increases the operating margins. Thus, all aspects of the design are interrelated and important for an optimized system.
The advancement of quantum metrology is intrinsically linked to the development of nanoelectronic and quantum devices. The Programmable Josephson Voltage Standard (PJVS), a system comprising thousands of superconducting Josephson junctions with tunnel barriers on the nanoscale, provides a fundamental quantum-based AC voltage reference. However, extending this quantum traceability to harmonic power measurements, essential for modern power systems with extensive power electronics, remains a significant challenge. This work demonstrates a high-accuracy harmonic power standard by leveraging the unique quantum properties of a nano-fabricated PJVS array. We introduce a time-division calibration scheme where the precise voltage steps of the PJVS are used to calibrate a high-performance dual-channel analog-to-digital converter (ADC). This method bypasses the need for complex synchronous sampling and heavy computational algorithms, relying instead on the intrinsic accuracy of the Josephson quantum voltage. The system achieves a measurement uncertainty of 14 × 10 ×6 ( k = 2) for fundamental power and 50 × 10 −6 ( k = 2) for harmonic power up to the 60th order. Comparative tests with national primary standards show a maximum deviation of only 6 × 10 −6 for harmonic power metrology but also serves as a compelling demonstration of a practical, high-precision application for superconducting nanoelectronic devices, bridging the gap between quantum standards and industrial measurement needs.