Circuits for Josephson Voltage Standards
Research
Overdamped Josephson junctions can be realized by different types of junctions. For binary-divided series arrays we use SINIS junctions consisting of a multilayer of Superconductor / Insulator / Normal metal / Insulator / Superconductor. We fabricate these arrays in the clean room center of PTB using thin film technology. We use Niobium as a superconductor, 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 SINIS junctions we successfully realized first 10-V arrays consisting of about 70 000 SINIS 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 10 GHz can be generated, corresponding to an output voltage of 20 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 output voltage of 100 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. The SNS junctions are well suitable for operation at 10 GHz.
