Optical pulse-drive for the AC Josephson voltage standard realized

Optical pulse-drive for the AC Josephson voltage standard realizedThe pulse-driven Josephson Voltage Standard was successfully operated with optical pulses in a “proof-of-principle” experiment. Fast photodiodes (28 GBit/s) were mounted by flip-chip technology to special carrier chips for operation at 4.2 K

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 Nb_xSi_1-x as barriers.

To increase the output voltage, more JAWS circuits will be operated in parallel in future (today there are up to 8 in parallel). To reduce at the same the high costs of the complete JAWS system, the number of room temperature HF-channels of the pulse-pattern-generator should be decreased too. Both is possible by implementation of high-performance photodiodes directly at 4.2 K. Within a close cooperation in the framework of the EMPIR projects Q-WAVE and QuADC between JV (Norway), the university of Southeast-Norway USN and NPL (Great Britain) a flip-chip technology was established for the connection of these photodiodes onto special fabricated superconducting carrier chips (made at PTB clean room). The pulses will be transferred from a laser-setup via optical fiber directly to the photodiodes at 4.2 K. From here, the generated electrical pulses will be further guided to the JAWS chip.

First measurements were performed to evaluate the basic features of this optical setup. Furthermore, first sine waveforms were successfully synthesized with high spectral purity at the maximal possible clock-frequency of 15 GHz. Because for this “proof-of-principle” experiment only one photodiode was used, the synthesized waveforms are unipolar with a DC-shift (see time-domain in figure1). For a JAWS chip with 3000 Josephson junctions stable laser bias margins of 1 mA were achieved.

Figure 1: Frequency spectrum and time domain of an unipolar sine waveform synthesized with optical pulses (with cold photodiodes) with a signal frequency of 1875 Hz and a signal amplitude of 6.6 mV RMS (18.6 mV PP, resp.). A JAWS array with 3000 Josephson junctions was operated at a clock frequency of 15 GHz.