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IEEE Transactions on Applied Superconductivity

Precision Comparison of Sine Waveforms With Pulse-Driven Josephson Arrays

Oliver Kieler, Ralf Behr, Detlef Schleußner, L. Palafox, J. Kohlmann

Published Jan 3, 2013• 33 citations
Publisher

Abstract

The equivalence of two sinusoidal waveforms synthesized by pulse-driven Josephson arrays using two Josephson arbitrary waveform synthesizers (JAWS) has been investigated by a direct, high-precision comparison. The arrays are based on double-stacked SNS-type Josephson junctions (superconductor-normal conductor-superconductor) with NbxSi1-xas barrier material. Two arrays, consisting of 4000 junctions each, are arranged on one silicon chip and are connected in opposite voltage direction directly at the chip at 4.2 K. Careful phase tuning between the two JAWS systems allowed the minimization of the residual ac voltage, which was characterized by a fast digitizer and a lock-in amplifier. For an output voltage of 8.8 mV, a difference voltage of 0.14 nV with 0.25 nV uncertainty was measured, i.e., (1.6 ±2.9) × 10-8in relative units. This result demonstrates the excellent agreement of both voltages and therefore the quantization of the JAWS itself.

DOI
10.1109/tasc.2013.2237817

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The equivalence of two sinusoidal waveforms synthesized by pulse-driven Josephson arrays using two Josephson arbitrary waveform synthesizers (JAWS) has been investigated by a direct, high-precision comparison. The arrays are based on double-stacked SNS-type Josephson junctions (superconductor-normal conductor-superconductor) with NbxSi1-xas barrier material. Two arrays, consisting of 4000 junctions each, are arranged on one silicon chip and are connected in opposite voltage direction directly at the chip at 4.2 K. Careful phase tuning between the two JAWS systems allowed the minimization of the residual ac voltage, which was characterized by a fast digitizer and a lock-in amplifier. For an output voltage of 8.8 mV, a difference voltage of 0.14 nV with 0.25 nV uncertainty was measured, i.e., (1.6 ±2.9) × 10-8in relative units. This result demonstrates the excellent agreement of both voltages and therefore the quantization of the JAWS itself.
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.
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.
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.
Superconductor-normal metal-superconductor(SNS) Josephson junction series arrays are developed and investigated for applications in the Josephson arbitrary waveform synthesizer (JAWS). The series arrays contain up to about 10 000 junctions arranged in a meanderlike design. AC voltages and arbitrary waveforms are synthesized by operation of the Josephson junctions by short current pulses. Higher harmonics are at least 110 dB below the fundamental. To verify the generated waveforms, a spectrum analyzer that has initially been calibrated using a binary-divided programmable Josephson voltage standard is used.
We developed a fabrication process to establish large arrays of up to 5-stacked Josephson junctions for the Josephson Arbitrary Waveform Synthesizer (JAWS). SNS-type Josephson junctions with NbxSi1-xbarriers are used for this application. By modifying our standard window process, e.g. to add a CMP (chemical mechanical polishing) and an ALD (atomic layer deposition) step, the yield of this process was increased. An output voltage of 1 V RMS could be achieved by using 4 JAWS arrays in series with a total number of 60,000 5-stacked junctions. The investigation of the current-voltage characteristics (IVC) showed an interesting and potentially useful feature. The required pulse amplitude per junction for getting the maximum Shapiro step width reduces for higher junction stacks. We found that the pulse power linearly reduces with the square of the junction number in the array. The origin of this effect might be a strong neighbour-neighbour interaction of the junctions, which are closely arranged within the stacked configuration. This might generate a strong self-synchronization of the junctions.