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Low Temperature Physics

Development of josephson arbitrary waveform synthesizer at physikalisch-Technische bundesanstalt (PTB)

Oliver Kieler

Published Nov 1, 2024• 6 citations

Abstract

This paper presents the basic principle of the pulse-driven ac Josephson voltage standard, also called “Josephson arbitrary waveform synthesizer” (JAWS). We will mainly focus on activities and developments at the Physikalisch-Technische Bundesanstalt (PTB). The JAWS is suitable to generate high precision quantized waveforms, which can be arbitrarily programmed in the frequency and time domain with extremely high spectral purity (typical signal-to-noise ratios SNR of 120–140 dBc). A brief overview of the fabrication technology for highly integrated JAWS circuits with more than 50 000 junctions per chip will be described. This includes the stacked junction technology with up to 5-stacked junctions. The experimental setup and the key-features of JAWS will be shown too. New approaches designed to further increase the output voltage, to simplify the experimental setup, and to extend the range of applications are the on-chip power dividers, the optical pulse drive, and the RF-JAWS. A selected application of the JAWS is described to demonstrate the usability of sophisticated JAWS waveforms for the complex characterization of electrical devices. The PTB JAWS circuits already have a significant impact on the quantum ac voltage metrology community. JAWS chips made at PTB have been distributed to more than 10 national metrology and other research institutes within the framework of international research projects.

DOI: 10.1063/10.0030410

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This paper presents the basic principle of the pulse-driven ac Josephson voltage standard, also called “Josephson arbitrary waveform synthesizer” (JAWS). We will mainly focus on activities and developments at the Physikalisch-Technische Bundesanstalt (PTB). The JAWS is suitable to generate high precision quantized waveforms, which can be arbitrarily programmed in the frequency and time domain with extremely high spectral purity (typical signal-to-noise ratios SNR of 120–140 dBc). A brief overview of the fabrication technology for highly integrated JAWS circuits with more than 50 000 junctions per chip will be described. This includes the stacked junction technology with up to 5-stacked junctions. The experimental setup and the key-features of JAWS will be shown too. New approaches designed to further increase the output voltage, to simplify the experimental setup, and to extend the range of applications are the on-chip power dividers, the optical pulse drive, and the RF-JAWS. A selected application of the JAWS is described to demonstrate the usability of sophisticated JAWS waveforms for the complex characterization of electrical devices. The PTB JAWS circuits already have a significant impact on the quantum ac voltage metrology community. JAWS chips made at PTB have been distributed to more than 10 national metrology and other research institutes within the framework of international research projects.

The establishment at PTB of an AC Josephson voltage standard (Josephson Arbitrary Waveform Synthesizer- JAWS) based on pulse-driven Josephson arrays is focused on achieving an output voltage of at least 1 Vrms which is required for many metrology applications. In this paper, we approached this goal by increasing the number of active junctions in two ways. Firstly, we fabricated arrays containing triple-stacked SNS-type Josephson junctions with NbxSi1-xas barrier material. We obtained current operation margins of about 0.2 mA with arrays of up to 9000 Josephson junctions at an rms voltage of 355 mV. Secondly, we used a new 8-channel ternary pulse pattern generator (PPG) to bias up to 8 arrays connected in series. An output voltage of VRMS= 1006 mV(VPP= 2.845 V) was achieved by using 8 arrays (arranged on 4 separate chips) with 63000 junctions in total. Higher harmonics are suppressed by at least -116 dBc. The fabrication process and the experimental setup will be described, as well as experimental results that are leading towards our 1 V goal.

Overdamped Josephson junctions for AC voltage standards are realized with NbxSi1-xbarrier junctions at PTB. The characteristic parameters of the junctions can be tuned over a wide range by adjusting the Nb content and the thickness of the barrier. For the pulse-driven Josephson Arbitrary Waveform Synthesizer (JAWS), series arrays of up to 15,000 Josephson junctions have been fabricated using junction stacks containing five Josephson junctions. In addition, two types of on-chip power dividers have been investigated at PTB, namely Wilkinson and serial-parallel power dividers, respectively. These developments aim for further increasing the number of Josephson junctions on a single chip and thus for increasing the output voltage.

The pulse-driven Josephson Voltage Standard, also called Josephson Arbitrary Waveform Synthesizer (JAWS) is already well established for different applications in AC voltage metrology. To further increase the output voltage towards 10 V and to reduce the complexity of the JAWS systems we investigated two different approaches, which finally can be combined. One approach is to integrate an optimized on-chip power splitter to reduce the number of high-frequency (HF) channels from room temperature down to 4 K. A pulse pattern generator with less HF outputs will directly reduce the complexity and costs of a JAWS system. The second approach is to use an optical pulse-drive implementing cold photodiodes close to the JAWS chip. The use of optical fiber will have two main advantages: the optical fibers will reduce the high frequency noise and will enable an easy splitting into parallel optical channels. We will present first results with both approaches.

The PTB Josephson Arbitrary Waveform Synthesizer (JAWS) enables the generation of arbitrary waveforms up to voltages of 70 mVRMS (199 mVPP) using two Josephson arrays in series containing 4000 Josephson junctions each. The SNS-like double-stacked junctions are based on NbxSi1-x as barrier material. While the JAWS system is typically operated using a Dewar with liquid helium, the operation in a closed-cycle pulse-tube cryocooler at temperatures around 4.2 K range was here investigated and successfully demonstrated. For this purpose a special designed cryoprobe was used to provide high quality pulses to the Josephson arrays.

This paper describes the status of developing Josephson arbitrary waveform synthesizer (JAWS) chips at NIM (National Institute of Metrology, China). To obtain high junction integration density and fewer data input channels, the chip employs an on-chip Wilkinson power divider and inside/outside dc blocks, enabling both arrays to be driven by a single pulse-generator channel. In addition, the tapered coplanar waveguide structure is used to ensure the microwave uniformity of the long-junction array. Each array consisted of 4000 double-stack Nb/NbxSi1−x/Nb junctions, and 16,000 junctions are integrated in the chip in total. The JAWS chip demonstrates good performance, capable of synthesizing a 300 mV root mean square (rms) voltage with exceptionally low harmonic distortion. Dc and ac voltage-current characteristics measurements indicate that the junctions are with a critical current of 2.5 mA, and a normal-state resistance of 4.5 mΩ per junction. Contact aligners are manually operated to fabricate the chips, and process errors in the fabrication are estimated in this paper.

Superconductor/normal conductor/superconductor (SNS) Josephson junction series arrays are the subject of research for the realization of a Josephson arbitrary waveform synthesizer (JAWS) based on pulse-driven Josephson junctions. Lumped arrays of SNS Josephson junctions are investigated for this application at the Physikalisch-Technische Bundesanstalt (PTB). The series arrays consisting of Nb/HfTi/Nb SNS junctions are embedded in the center line of a coplanar waveguide transmission line. To increase the ac output voltage, the Josephson junction series array is designed as a meander-like structure. A broadband response of the arrays is required for the complex pulse operation of the JAWS. The arrays are investigated by dc measurements and for test purposes under continuous microwave irradiation at different frequencies. Flat Shapiro steps were measured for meander-like series arrays with 21 parallel lines containing up to 10 752 junctions. A sine wave with a frequency of 2.5 kHz and a peak voltage of 214 mV Peak was synthesized under pulse mode operation.