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IEEE Transactions on Applied Superconductivity

On the Synthesis of Stepwise Quantum Waves Using a SNIS Programmable Josephson Array in a Cryocooler

Bruno Trinchera, V. Lacquaniti, A. Sosso, Matteo Fretto, Paolo Durandetto, E. Monticone

Published Dec 7, 2016• 13 citations
Publisher

Abstract

This paper presents recent advances in the application of binary-divided 1 V array, consisting of 8192 intrinsically shunted SNIS overdamped Josephson junctions (JJs), for the synthesis of stepwise waves with quantum accuracy. The maximum output voltage is ensured by opportunely driving the subsections of the SNIS array by means of three states bias-current setpoints to the Shapiro steps n = 0, ±1 or n = ±2, respectively. Reconfigurable digital modular electronics has been designed to bias individually each of the 13 subsections of the SNIS array. A two-stage closed cycle refrigerator equipped with LF and RF electrical lines is employed for cooling-down the SNIS array for temperatures ranging from 3.6 K to above 7 K. Stepwise sine waves with rms amplitude ranging from 1 to 2 V using the first (n = 1) and second (n = 2) Shapiro steps, different temperatures and bias-current setpoints have been synthesized up to the kHz range. The synthesized waves have been recorded and analyzed by a high-precision differential sampling system. We report the results of the first characterizations carried out with the new multibit current source and an improved version of the sample holder designed to optimize the heat dissipation of the SNIS array for operation in cryocooler setups.

DOI
10.1109/tasc.2016.2636569

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This paper presents recent advances in the application of binary-divided 1 V array, consisting of 8192 intrinsically shunted SNIS overdamped Josephson junctions (JJs), for the synthesis of stepwise waves with quantum accuracy. The maximum output voltage is ensured by opportunely driving the subsections of the SNIS array by means of three states bias-current setpoints to the Shapiro steps n = 0, ±1 or n = ±2, respectively. Reconfigurable digital modular electronics has been designed to bias individually each of the 13 subsections of the SNIS array. A two-stage closed cycle refrigerator equipped with LF and RF electrical lines is employed for cooling-down the SNIS array for temperatures ranging from 3.6 K to above 7 K. Stepwise sine waves with rms amplitude ranging from 1 to 2 V using the first (n = 1) and second (n = 2) Shapiro steps, different temperatures and bias-current setpoints have been synthesized up to the kHz range. The synthesized waves have been recorded and analyzed by a high-precision differential sampling system. We report the results of the first characterizations carried out with the new multibit current source and an improved version of the sample holder designed to optimize the heat dissipation of the SNIS array for operation in cryocooler setups.
An alternating programmable Josephson voltage standard (AC-PJVS) configured as a multibit digital-to-analog converter has been developed to explore the staircase-approximated waveforms synthesis at frequencies up to the acoustic range. The AC-PJVS setup is based on state-of-the-art modular electronics able to bias individually the subsections of a binary-divided array consisting of 8192 intrinsically shunted SNIS junctions. High order Shapiro steps have been employed for waveform synthesis having amplitude beyond 1 V by finely trimming the SNIS temperature using a two stage cryocooler. Alongside the experiments on the synthesis, the experimental setup was improved for direct comparisons between quantum staircase waves and conventional sine waves based on the use of a room temperature-operating phase sensitive detector and on a differential precision sampling system. Both methods maximize the accuracy obtained from the AC-PJVS synthesizer, allowing, in principle, its use for primary LF AC voltage metrology.
Quantum Digital-to-Analog Converters (DACs) based on programmable Josephson array (PJVS) represent the most advanced devices currently employed in ac voltage calibrations. Pulse-driven Josephson technology, compared to PJVS, provides cleaner and faster voltage signals and is the main topic of frontier developments. Although the extension of PJVS frequency range appears to be arduous, some enhancement are still practicable. In this work we demonstrate the possibility of synthesizing stepwise quantum voltage waveforms with half of the bias lines and fewer Josephson junctions by exploiting both first and second Shapiro steps, with no losses in terms of performances.
Binary-divided 1 V arrays were fabricated for programmable Josephson voltage standards in close cooperation between INRIM and PTB. The arrays, consisting of 8192 overdamped Nb/Al-AlO(x)Nb SNIS Josephson junctions, were successfully operated at microwave frequencies around 70 GHz. While the characteristic voltage of the junctions ranged from 0.25 to 0.6 mV, the current margins were larger than 1 mA for the Shapiro step n = 1. In addition, flat Shapiro steps of higher order (n = 3 and 4) were generated at increased microwave power levels due to the large characteristic voltage, demonstrating the small parameter spread of the SNIS junctions. Operation at temperatures up to 6-7 K has also been verified, measuring quantized voltages up to 1.25 V on the n = 1 step.
A quantum-accurate waveform with an rms output amplitude of 1 V has been synthesized for the first time. This fourfold increase in voltage over previous systems was achieved through developments and improvements in bias electronics, pulse-bias techniques, Josephson junction array circuit fabrication, and packaging. A recently described ac-coupled bipolar pulse-bias technique was used to bias a superconducting integrated circuit with 25 600 junctions, which are equally divided into four series-connected arrays, into the second quantum state. We describe these advancements and present the measured 1 V spectra for 2 Hz and 10 Hz sine waves that remained quantized over a 0.4 mA current range. We also demonstrate a 2 kHz sine wave produced with another bias technique that requires no compensation current and remains quantized at an rms voltage of 128 mV over a 1 mA current range. Increasing the clock frequency to 19 GHz also allowed us to achieve a maximum rms output voltage for a single array of 330 mV.
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.
A synthesizer of precision ac waveforms based on a binary superconductor insulator-normal metal-insulator-superconductor (SINIS) Josephson junction array has been developed. The array segments are individually biased using a high-speed bias source which gives a switching time of approximately 250 ns between voltage steps. The accuracy limitations of the system due to limited risetime and bandwidth are discussed. Comparisons between the synthesizer and a planar multijunction thermal converter (PMJTC) indicate that it is possible to synthesize sine waves with known rms value having an accuracy in the range of 1 part in 10/sup 7/.