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“Proof-of-principle” experiment: JAWS chip and “PPG-on-chip”


A 3-chip hybrid-combination was used for operation of a JAWS chip together with dummy chips of a “PPG-on-chip” and nTron-amplifier.



For the operation of the pulse-driven AC Josephson voltage standard – also called Josephson arbitrary waveform synthesizer (JAWS) – a fast pulse pattern generator (PPG) is needed. This PPG delivers bipolar pulses with a data rate of up to 30 Gbit/s. The 8-channel version of this PPG is rather expensive. Due to the fact that the output voltage of the JAWS should be increased in the near future even more channels might become necessary. An alternative device could be a good solution for this problem.

According to the fast development of RSFQ-electronics in the last year (integration density and yield), there might be a realistic chance to use an RSFQ-shift register as “PPG-on-chip”. Within a cooperation with the CIS institute (Erfurt), the IPHT and Supracon AG (both Jena), and the MIT (Cambridge, USA) the feasibility of this idea will be investigated.

For a “proof-of-principle” experiment two superconducting dummy chips of a “PPG-on-chip” and nTron pulse-amplifier were designed at CIS and fabricated at PTB. By flip-chip technology the nTron was connected to the shift-register chip at CIS. This hybrid chip was finally connected to a JAWS chip and investigated at PTB.

The pulses of a room-temperature PPG were transferred through this hybrid chip to the JAWS chip to investigate the properties of the JAWS chip in this configuration. First measurements showed wide Shapiro steps (see figure), but only up to 2 GHz clock frequency. Furthermore, sinusoidal waveforms were synthesized. Pure spectra were achieved up to 1 GHz clock frequency and a sigma-delta code amplitude of 30%, which corresponds to a signal output voltage of 5 mV peak-peak. Additional measurements performed at MIT showed the principle functionality of the nTron chip successfully. The results will be presented at the ASC 2018 conference.
As a next step active prototypes of the “PPG-on-chip” and nTron chip together with a JAWS chip will be investigated and optimized, possibly in an externally funded cooperation project.

Current-voltage characteristic of a JAWS chip

Figure 1: Current-voltage characteristic of a JAWS chip with 4000 Josephson junctions versus pulse amplitude for irradiation of pulses with a frequency of 2 GHz. The pulses were transmitted through a dummy “PPG-on-chip” and nTron amplifier-chip.