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IEEE Transactions on Applied Superconductivity

Challenges of Josephson Junction Arrays for ac Voltage Generation by Microwave Pulse Power Modulation

Alexander S. Katkov, Alexander M. Klushin, G. P. Telitchenko, Ralf Behr, J. Niemeyer

Published Jun 1, 2005• 4 citations
Publisher

Abstract

Voltage pulses with a determined amplitude and width might be the basis for high-precision ac voltage measurements in the future. The pulse width modulation technique makes possible the generation of arbitrary waveforms. In this paper, arrays of SINIS Josephson junctions were investigated which produce a 1 V voltage step under irradiation at a frequency of the order of 70 GHz. This step has an amplitude of 1 mA whereof a region of 0.3 mA overlaps with the critical current. This allows the generation of a 1 V voltage pulse by switching the microwave power at constant dc bias current. The results of this investigation will be presented and also the array parameters which are required for obtaining a relative uncertainty of 0.1 ppm in ac voltage measurements.

DOI
10.1109/tasc.2005.849833

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Voltage pulses with a determined amplitude and width might be the basis for high-precision ac voltage measurements in the future. The pulse width modulation technique makes possible the generation of arbitrary waveforms. In this paper, arrays of SINIS Josephson junctions were investigated which produce a 1 V voltage step under irradiation at a frequency of the order of 70 GHz. This step has an amplitude of 1 mA whereof a region of 0.3 mA overlaps with the critical current. This allows the generation of a 1 V voltage pulse by switching the microwave power at constant dc bias current. The results of this investigation will be presented and also the array parameters which are required for obtaining a relative uncertainty of 0.1 ppm in ac voltage measurements.
The DC voltage across a Josephson junction is exactly determined by the number of single flux quanta Φ0passing the junction per second. By locking this flux train to an external sinusoidal microwave with the frequency f or a pulse train with the pulse rate f in such a way that one flux quantum is transferred per microwave period or per pulse respectively, the junction DC voltage becomes V=Φ0f in the simplest case. The precision of V is only determined by the uncertainty of the frequency control. As the output voltage is relatively small (V=140μV for f=70GHz), large series arrays of n Josephson junctions must be used to increase the voltage to practical levels V=nΦ0f. With a sinusoidal 70 GHz microwave drive and series arrays of up to 14000 SIS tunnel junctions modern DC voltage standards were developed in the past which may reach a reproducibility of a few parts in 10-10. These standards are commercially available and form the basis of the legal voltage metrology of most of the national laboratories worldwide. Because the reference voltages of these standards are widely overlapping it is difficult to rapidly adjust the desired voltage. It was therefore impossible to use these quantum voltmeters for the precise analysis of AC voltages. For this purpose the arrays had to be modified in many respects. The direct way to generate arbitrary Josephson waveforms V(t) is to rapidly alter the frequency f(t) with respect to the desired waveform V(t)=nf(t)Φ0. It requires a precise flux transfer control over a very wide frequency range.
A 2 V Shapiro step with a current height of about 0.3 mA was successfully generated at 4.2 K with a single array including 65 536 Nb/TiN/Nb Josephson junctions under microwave irradiation at 16 GHz. The critical current density was 89 kA cm − 2 , and the junction size was 3 µm. The n = 1 step height decreased exponentially as the number of junctions N increased when the number of junctions was less than 30 000, a result that agreed well with the dissipated junction model. On the other hand, the step height was nearly constant when the number of junctions was more than about 30 000, a figure much larger than that theoretically calculated from the model. Using an array including such a large number of Josephson junctions represents a promising way for ac waveform synthesis to increase output voltage to about 1 V rms .
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.
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.