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Josephson Technology

Working Group 2.43

Research:

JAWS - AC waveforms of highest quality:
highly stable and quantum accurate

We investigate Josephson junction series arrays operated by short current pulses, in order to generate AC voltages of arbitrary waveforms. The series arrays for this Josephson Arbitrary Waveform Synthesizer (JAWS) consist of SNS junctions (a trilayer of Superconductor / Normal metal / Superconductor). We use Niobium for the superconductive layers and amorphous Silicon with an Niobium content of about 20 % prepared in a co-sputtering process for the barrier. A single Josephson series array contains up to 9,000 Josephson junctions. Two series arrays are placed on a single Silicon chip of 1 cm edge size and can be operated simultaneously. We fabricate the series arrays in Opens internal link in current windowthin film technology in the clean room centre of PTB. The electron beam lithography being an important tool enables a high quality and yield of the circuits.

The experimental JAWS measurement set-up is based on a pulse pattern generator (PPG), which delivers ternary pulses (-1 / 0 / +1) with a maximum pulse repetition frequency of 15 GHz to the Josephson arrays. The chips are operated in liquid Helium at a temperature of 4.2 K (about -269°C). Using two arrays of 9,000 junctions each on a chip, we were able to generate waveforms in the frequency range from about 10 Hz to 1 MHz with rms (root-mean-square) voltages up to 355 mV (peak-to-peak voltage 1 V). Higher harmonics are typically suppressed by 110 dBc to 120 dBc. Figure 1 shows a sine wave with a rms voltage of 50 mV (peak-to-peak voltage 141.4 mV) and a frequency of 58.6 Hz as an example.

The JAWS enables us to synthesise arbitrary waveforms in the frequency and in the time domain, which is of great interest for different applications. Two examples evidently illustrate the large potential of the JAWS: we realised the acronym “JAWS” using 132 harmonics as an example for the frequency domain, and we generated a heart-like signal in the time domain.

In close cooperation with Opens internal link in current windowworking group 2.63, we successfully synthesised waveforms with a root-mean-square (RMS) voltage of 1 V (peak-peak voltage 2.845 V) by series operation of 8 Josephson arrays with 63,000 junctions altogether in August 2014. We now use the 1 V system for different applications.

Figure 1: Frequency spectrum (top) and time domain (bottom) of a synthesized sine wave with a frequency of 58.6 Hz and an rms voltage of 50 mV (peak-to-peak voltage 141.4 mV). Higher harmonics are suppressed below the noise level. The signal-to-noise ratio is 119 dBc.

 

 

Figure 2: Frequency spectrum of a synthesised multi-tone waveform containing 132 higher harmonics to show the acronym “JAWS”.

 

 

Figure 3: Time domain of a synthesised wave form, which corresponds to the signal of a human heart.