Pulse-driven SNS arrays for a quantum-based synthesizer for arbitrary waveforms

A quantum-based synthesizer for generation of AC voltages of arbitrary waveforms (Josephson Arbitrary Waveform Synthesizer, JAWS) is based on SNS Josephson junctions due to their high critical current density of about 100 kA/cm². SNS junctions allow the integration of a large number of sub-µm junctions in short compact series arrays (lumped arrays), which have a typical length of about 1.5 mm allowing an operation at 10 GHz. In spite of the geometric limitations, these lumped arrays are a simple solution for a broad-band operation for the JAWS. The goal of the ongoing development is to further increase the output voltage of the synthesized AC waveforms.
The integration density of these pulse-driven arrays was significantly increased by arranging the Josephson junctions within a meandered lumped array (cf. Fig. 1). The whole meander forms the inner conductor of a co-planar waveguide, which is dimensioned to an impedance of 50 Ohm. Up to 10,752 junctions were arranged in meanders consisting of up to 21 parallel lines within an area of only 62 µm x 1.4 mm.

Fig. 1: Design and scanning electron microscope picture of a meander-like SNS array consisting of 10,752 Josephson junctions arranged into 21 parallel meander lines. The junction size amounts to 0.7 µm x 0.7 µm each.

These arrays were fabricated in the clean room center at PTB using a process specially optimized for Josephson junctions of very small size (sub-µm junctions). Ebeam lithography and chemical-mechanical polishing (CMP) are key technologies in this fabrication process. In this way large series arrays can be fabricated with high yield and low parameter spread. Fig. 2 shows a scheme of the overall process.

Fig. 2: Scheme of the SNS process for series arrays of sub-µm Josephson junctions.

To work as a JAWS the arrays are driven by a sequence of short current pulses delivered by an pulse pattern generator (PPG). The pulse pattern contains the sigma delta code of the waveform to be synthesized. Bipolar waveforms are generated by superimposing a continuous microwave (about 10 GHz) on the unipolar pulses of the PPG (cf. Fig. 3).

Fig. 3: Block diagram of the set-up for bipolar pulse operation of the JAWS.

Sinusoidal AC voltages with frequencies of about 2.5 kHz and peak voltages of up to 214 mV (rms voltage: 153 mV) were synthesized using these new arrays. The synthesized AC voltage is characterized by its frequency spectrum (cf. Fig. 4). Higher harmonics are significantly suppressed and nearly undistinguishable from the noise floor. Due to the high noise floor, the harmonic distortion is only -66 dBc. This value is too high and needs to be improved. The reasons for the high noise level are not well understood so far.

Fig. 4: Frequency spectrum of a 2,5 KHz sine wave with a maximum peak voltage of 214 mV generated by a 21 meander-line array consisting of 10,752 Josephson junctions. Higher harmonics are strongly suppressed and nearly undistinguishable from the noise floor.