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Josephson Technology

Working Group 2.43


Josephson arrays

The AC voltage circuits contain overdamped Josephson junctions, which can be realized by different types of junctions. For binary-divided series arrays we now use SNS junctions, after we used SINIS junctions for several years. The junctions consist of a trilayer of Superconductor / Normal metal / Superconductor or a multilayer of Superconductor / Insulator / Normal metal / Insulator / Superconductor, respectively. We fabricate these arrays in the clean room center of PTB using thin film technology. We use Niobium for the superconductive layers, Niobium-Silicon or Aluminum as a normal metal and Aluminumoxid fabricated by thermal oxidation of Aluminum as an insulator, respectively. The characteristic properties of the junctions make their operation at microwave frequencies around 70 GHz possible. Besides 1 V arrays containing 8192 junctions we successfully fabricate  10 V arrays consisting of about 70 000 junctions.

While the binary-divided arrays are operated at a fixed microwave fequency, Josephson series arrays can besides be operated by a train of short current pulses. The sequence of pulses at any time determines the output voltage of these pulse-driven Josephson voltage standards. Changes of the sequence of pulses make possible the generation of arbitrary waveforms with this JAWS (Josephson Arbitrary Waveform Synthesizer). Using commercial pulse pattern generators, pulse repetition rates up to about 15 GHz can be generated, corresponding to an output voltage of 30 µV for a single Josephson junction. Josephson junction series arrays are again required in order to increase the output voltage (for instance, 5000 Josephson junctions for an peak voltage of 150 mV). The Josephson junction series arrays of the JAWS are typically made of SNS junctions, which consist of a multilayer of Superconductor / Normal metal / Superconductor. These SNS junctions are well suitable for operation at 15 GHz.  The junctions now contain Niobium-Silicon as the normal metal, after we also used SINIS junctions or SNS junctions with a barrier made of Hafnium-Titanium for many years. Using Niobium-Silicon junctions, we are also able to fabricate stacked junctions, which increases the integration density.