Stacked Nanoscale Josephson Junction Arrays for High-Performance Voltage Standards

Alexander M. Klushin, S. Schornstein, H. Kohlstedt +3 more

Stacked Josephson junction arrays located at the rf current antinodes of a superconducting microstripline resonator were investigated. The circuits were designed to obtain highly accurate voltages at cm-waveband microwave driving and low rf-power level. Nb-Al/AlO/sub x/-Nb Josephson junctions with tunneling area of 60 /spl mu/m/spl times/300 /spl mu/m and critical current density up to 7 A/cm/sup 2/ were used. We compared the average voltages of the rf induced maximal current steps in arrays with up to three junctions per stack. Quantized current steps up to 300 mV were observed under microwave irradiation at frequency of 19.20 GHz in circuits with 384 three-junction stacks. The dynamics of stacked tunnel junctions in the presence of rf-currents will be discussed with respect to application as voltage standards.

Atsumi Wada, Kazuhiro Yamaki, Akinobu Irie

Victor K. Kornev, Igor I. Soloviev, N. V. Klenov +1 more

Oliver Kieler, Ruediger Wendisch, Rolf-Werner Gerdau +3 more

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.

D. Georgakopoulos, Ilya F. Budovsky, Samuel P. Benz

Abstract Josephson voltage standards (JVSs) provide a primary realization of the volt, the unit of electromotive force. They generate direct current (dc) voltages up to 10 V and show agreement better than 1 nV V−1 at 10 V. For JVSs based on Josephson junction arrays (JJAs) that are driven by sinusoidal radiofrequency (RF) power, commonly referred to as continuous wave-driven JJAs (CWD JJAs), the minimum voltage that can be generated is limited to the voltage across one Josephson junction (JJ) for practical devices. To achieve this resolution, they may require a perfect JJA chip. JVSs based on a pulse-driven (PD) JJA require high performance electronics (i.e. high bandwidth, low distortion and jitter, pulse shaping filters and large memory) to achieve their minimum and maximum voltage. We have combined two CWD JJAs and two PD JJAs driven by two microwave inputs to one chip to generate quasi-continuous dc voltages up to the sum of the full-scale voltages of both JJAs that are robust to the imperfections of the JJs and have relaxed requirements on the RF electronics driving the JJA, compared to the existing CWD JVSs and PD JVSs, respectively. By use of the JJA chip at the National Measurement Institute Australia, we demonstrate its feasibility to generate voltages up to 1 V. Preliminary evaluation of the system shows that the voltage uncertainty can be 11 nV (k = 2) or better and the theoretical resolution is better than 1 nV from 0 V to 1 V. The main requirement is that all the JJs must have quantum locking ranges with respect to the power and frequency of the RF bias and for the PD JJAs to have a constant voltage over a range of dc bias current. Although this development is not a replacement for existing state-of-the-art JVSs, we anticipate that it will be an alternative fit-for-purpose solution for metrological applications under non-ideal operating conditions or when the components of the state-of-the-art solutions are not available.

S. Pagano, Sandro Barbanera, Vincenzo Boffa +6 more