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High-precision pulse-driven AC Josephson voltage standard up to 1 V


By the triple stacking of Josephson junctions and by the series connection of eight circuits with a total of 63 000 junctions, the output voltage of a pulsedriven AC Josephson voltage standard could be increased to an effective value of 1 V. Such voltage standards allow spectrally pure, arbitrary waveforms to be generated and are, therefore, also referred to as Josephson Arbitrary Waveform Synthesizers (JAWS).

Scanning electron microscope images of a JAWS circuit with individual details of the circuit, and a photo of the JAWS chip, contacted on a special chip carrier.

An essential component of a JAWS are series arrays composed of SNS Josephson junctions which are manufactured in PTB’s Clean Room Center on the basis of NbxSi1-x barriers in a complex multilayer thin-film process. The extremely stable and reproducible deposition conditions allow single layers of adapted and well-defined thickness to be manufactured which allowed Josephson junctions – which are stacked three times (i.e. four Nb- and three NbxSi1-x-layers) – to be successfully processed at PTB for the first time with a high fabrication yield. On a chip (10 mm × 10 mm), two circuits with up to 9000 Josephson junctions each can be integrated. By combining eight circuits on four chips, a total of 63 000 Josephson junctions could be connected in series. The generation of sine waves with an output voltage of 1 V and an outstanding spectral purity (signal-to-noise ratio better than 120 dB) is achieved. In a (worldwide unique) precision comparison with an AC quantum voltmeter, an excellent agreement of (3.5 ± 11.7) nV/V could be demonstrated at a frequency of 250 Hz. For the future, effective voltages of 1 V open up a great number of new application possibilities for the JAWS in the field of electrical AC voltage metrology.