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Journal of Physics Conference Series

Development of Josephson junction series arrays for synthesis of AC voltages and arbitrary waveforms

J. Kohlmann, F Müller, Ralf Behr, Daniel Hagedorn, Oliver Kieler, L. Palafox, J. Niemeyer

Published Jun 1, 2006• 11 citations
Publisher

Abstract

Josephson junctions operated by short pulses make the synthesis of AC and arbitrary waveforms possible. Series arrays of intrinsically shunted SINIS and SNS Josephson junctions were developed and characterized for applications in this Josephson arbitrary waveform synthesizer (S: Superconductor, I: Insulator, N: Normal conductor). First measurements under pulse irradiation are presented.

DOI
10.1088/1742-6596/43/1/339

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Josephson junctions operated by short pulses make the synthesis of AC and arbitrary waveforms possible. Series arrays of intrinsically shunted SINIS and SNS Josephson junctions were developed and characterized for applications in this Josephson arbitrary waveform synthesizer (S: Superconductor, I: Insulator, N: Normal conductor). First measurements under pulse irradiation are presented.
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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.
Sub-μm\mu{\rm m}μm SNS Josephson Junction Arrays for the Josephson Arbitrary Waveform Synthesizer
The experimental set-up of a Josephson arbitrary waveform synthesizer (JAWS) has been further optimized to generate AC voltages with fundamental accuracy. Josephson junction series arrays for the JAWS, consisting of sub-mum SNS type junctions (superconductor-normal conductor-superdonductor), are fabricated using a technology which combines electron-beam lithography and chemical-mechanical polishing. Bipolar pulses are realized by the superposition of unipolar pulses from a pulse pattern generator and a continuous sinusoidal microwave provided by a signal generator. A second-order delta-sigma modulator is optimized to generate a high-quality pulse code.
Arrays of Josephson junctions have been used successfully for many years as the primary standard of voltage. A recent development has been the fabrication of arrays, based on non-hysteretic Josephson junctions, which allows the voltage output to be changed almost instantaneously. When these arrays are divided into binary segments a ‘digital-to-analogue converter’ with quantum accuracy is created. The properties of two types of array, a SINIS array, using intrinsic shunting of the Josephson junctions, and a SIS array with external, on-chip shunting have been investigated. The accuracy of the generated voltages and AC properties of the arrays when driven with a purpose-built, programmable current bias source are discussed. It is anticipated that a Josephson standard having waveforms with calculable rms values of AC voltage will soon be available.