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Pulse-driven AC Josephson voltage standard with an output voltage of 2.25 VRMS

28.11.2017

To increase the output voltage of the pulse-driven Josephson voltage standard, 16 arrays, implemented on 8 chips, were connected in series. This configuration leads to the operation of 162,000 Josephson junctions in total, resulting in an output voltage of 2.25 VRMS (6.36 VPP).

 

 

The pulse-driven AC Josephson voltage standard enables the generation of arbitrary and spectrally pure waveforms, and it is referred to as Josephson arbitrary waveform synthesizer (JAWS). Essential components of the JAWS are series arrays of SNS Josephson junctions (SNS: superconductor – normal conductor – superconductor). The circuits are fabricated in the PTB clean-room facility in a modified window-type process. Hereby, niobium serves as superconductor and NbxSi1-x as barriers.

The extremely stable and reproducible deposition process provides appropriate and well-defined thicknesses of the individual layers. As a result, large series arrays of stacked Josephson junctions can be fabricated successfully, which significantly increases the integration density of the JAWS.

By operating of up to 8 chips in series, with up to 24,000 Josephson junctions per chip (2 arrays per chip), 162,000 junctions are operated simultaneously, achieving a total output voltage of 2.25 VRMS, (6.36 VPP, resp.). To operate these series arrays, two high-quality 8-channel pulse pattern generators and two 8-channel cryogenic sample rods have been used.

The figure illustrates the frequency spectrum and the time domain of a sinusoidal waveform with a frequency of 250 Hz. Higher harmonics were suppressed to an excellent level of 127 dBc. However, minor distortion peaks are still present in the background noise, and are likely caused by a high-frequency crosstalk. Thus, we aim to reduce such interferences by optimizing the experimental setup, including an RF shield that is directly integrated on-chip.

 

Frequency spectrum and time domain of a sine waveform synthesized with a frequency

Figure 1: Frequency spectrum and time domain of a sine waveform synthesized with a frequency of 250 Hz and a signal amplitude of 2.25 VRMS (6.36 VPP, resp.). Here, 16 JAWS arrays (integrated onto 8 chips) were connected in series, i.e. and 162,000 Josephson junctions were operated simultaneously.