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International Kharkiv Symposium on Physics and Engineering of Microwaves, Millimeter and Submillimeter Waves

PTB's status of AC voltage metrolgy by microwave- and pulsetrain-driven josephson junction arrays

J. Niemeyer, Oliver Kieler, F. Muller, J. Kohlmann, Ralf Behr, L. Palafox, Detlef Schleußner, M. Florian Beug, J. Schurr

Published Jun 1, 2013• 0 citations
Publisher

Abstract

The report describes the most recent PTB developments and applications of large Josephson arrays for the generation of very precise sine waves and arbitrary waveforms. By means of large70 GHz sine-wave-driven binary arrays, AC voltages with frequencies up to 1 kHz and amplitudes of more than 10 V were generated with an uncertainty of better than 1μV/V. For the fabrication of this array type with up to 70000 junctions in series connection, an SNS junction technology with NbxSix-1barriers is now completely installed and permanently in use. With pulse-driven arrays of Josephson junctions, arbitrary waveforms were generated with a level of higher harmonics smaller than -120 dBc for frequencies from 150 Hz to 100 kHz at rms voltages less than 100 mV. These results were achieved with extended arrays of up to 4795 very small SNS junctions with Hf0.5Ti0.5barriers. Also the NbSi-technology technology has successfully been applied for the fabrication of pulse-driven Josephson arrays. As it seems to be difficult to considerably increase the wave amplitude by enhancing the junction number of the arrays, experiments were performed to combine a binary array system with a pulse-driven system with the goal to make use of the high amplitude of stepwise generated waves without losing the low noise and precision of quantum based rf waveforms generated by pulse trains at a clock frequency of 15 GHz.

DOI
10.1109/msmw.2013.6622163

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The report describes the most recent PTB developments and applications of large Josephson arrays for the generation of very precise sine waves and arbitrary waveforms. By means of large70 GHz sine-wave-driven binary arrays, AC voltages with frequencies up to 1 kHz and amplitudes of more than 10 V were generated with an uncertainty of better than 1μV/V. For the fabrication of this array type with up to 70000 junctions in series connection, an SNS junction technology with NbxSix-1barriers is now completely installed and permanently in use. With pulse-driven arrays of Josephson junctions, arbitrary waveforms were generated with a level of higher harmonics smaller than -120 dBc for frequencies from 150 Hz to 100 kHz at rms voltages less than 100 mV. These results were achieved with extended arrays of up to 4795 very small SNS junctions with Hf0.5Ti0.5barriers. Also the NbSi-technology technology has successfully been applied for the fabrication of pulse-driven Josephson arrays. As it seems to be difficult to considerably increase the wave amplitude by enhancing the junction number of the arrays, experiments were performed to combine a binary array system with a pulse-driven system with the goal to make use of the high amplitude of stepwise generated waves without losing the low noise and precision of quantum based rf waveforms generated by pulse trains at a clock frequency of 15 GHz.
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.
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.
Series arrays of overdamped Josephson junctions compose the basis of AC Josephson voltage standards. Presently, an advanced technology is based on Josephson junctions consisting of an NbxSi1-xbarrier. Using junction stacks containing five Josephson junctions, series arrays of 15,000 junctions have been successfully fabricated at PTB for the pulse-driven Josephson Arbitrary Waveform Synthesizer (JAWS). The operation of eight arrays simultaneously enable spectrally pure waveforms with output voltages above 1 V to be synthesized.
The paper is about development of the lumped Josephson junction arrays (JJAs) for the generation of accurate quantum-based ac voltages. By using semiconductor pattern generators, it was possible to synthesize unipolar and bipolar waveforms with amplitudes of almost 10 mV and frequencies of up to 122 kHz. Higher harmonics and other unwanted noise signals were suppressed by -90 dBc. Because of the unique layout of our JJA, it was possible to connect the circuit directly to the ground, thereby eliminating common-mode signals
The Josephson arbitrary waveform synthesizer (JAWS) allows AC voltages to be generated with fundamental accuracy. Lumped arrays of superconductor/normal conductor/superconductor (SNS) Josephson junctions were developed and investigated for this application. Combining electron-beam lithography and chemical-mechanical polishing, series arrays of sub-micrometer SNS junctions consisting of Nb/HfTi/Nb were fabricated. The junction size varied from 0.04 mum2to 4 mum2in series arrays up to 2560 junctions in different microwave designs. Stable Shapiro steps are generated in a frequency range from 5 GHz to 20 GHz using continuous microwave irradiation. Using a simple pulse pattern at different pulse repetition frequencies, the arrays show current-voltage characteristics with flat Shapiro steps. Sinusoidal AC voltages were synthesized using a commercial pulse pattern generator. Peak-to-zero voltages of some mV for frequencies below 1 kHz were achieved. Higher harmonics were measured to be more than 80 dB below the fundamental tone. The behavior of the arrays was investigated in more detail by measuring the amplitude-frequency spectra.