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-x barriers 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.
To make the experimental setup of the Josephson arbitrary waveform synthesizer less complex by reducing the number of RF cables between room and cryogenic temperature, we developed on-chip RF power dividers. By integration of these components, we can eventually increase the number of Josephson junctions operated by one single pulse-pattern generator channel, and thus reduce the costs of the setup. At Physikalisch-Technische Bundesanstalt, we designed, fabricated, and investigated the performance of two different RF power dividers types: the serial-parallel and the Wilkinson power divider. Spectrally pure sinusoidal waveforms were successfully synthesized with both types of power dividers. With the Wilkinson power divider, we obtained 17.55 mV (rms) at a clock frequency of 15 GHz combined with a test array of 1000 Josephson junctions. As for the serial-parallel power divider combined with a test array of 2000 Josephson junctions, we synthesized rms output voltages of 19.0 mV. Index Terms-AC Josephson voltage standards, pulse-driven Josephson series arrays, serial-parallel power divider, SNS Josephson Junctions (JJs), wilkinson power divider. I. INTRODUCTION T HE Josephson arbitrary waveform synthesizer (JAWS) is based on a series array of nonhysteretic SNS Josephson junctions (JJs) (S: superconducting, N: normal metal) driven by a high-speed digital sequence of short current pulses from a commercial pulse-pattern generator (PPG), in which the corresponding waveform is encoded by ΣΔ analog-to-digital conversion. These pulse-driven series arrays operated at 4 K enable spectrally pure ac voltages to be synthesized in a wide frequency range from dc up to MHz [1]-[3]. The short current pulses (pulse repetition frequency f p) transfer flux quanta Ф 0 = h/2e (h is Planck's constant and e the elementary charge) through each junction, which result in a quantized ac voltage of high spectral purity with no drift and low noise. According to the Josephson equation, the output ac voltage and its frequency are in general Manuscript
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