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Conference on Precision Electromagnetic Measurements

Progress on the Opto-Electronic Bias for Pulse-Driven Josephson Voltage Standards at PTB

Marco Kraus, Oliver Kieler, Ralf Behr, L. Palafox

Published Jul 8, 2024• 1 citation
Publisher

Abstract

After demonstrating bipolar waveforms in 2020, PTB’s optical pulse transmission drive has been expanded for two independent Josephson junction arrays. Passive optical power splitters divide the two independent optical channels at room temperature. Balanced photodiodes modules in liquid helium then generate ternary electrical pulses to drive the nearby Josephson junction arrays via coplanar waveguides less than 10 mm long. The outputs of the Josephson junction arrays are connected in series and we successfully synthesized bipolar sinewaves at 1 kHz and 35 mV rms amplitude with higher harmonics suppressed by more than 96 dBc.

DOI
10.1109/cpem61406.2024.10646125

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After demonstrating bipolar waveforms in 2020, PTB’s optical pulse transmission drive has been expanded for two independent Josephson junction arrays. Passive optical power splitters divide the two independent optical channels at room temperature. Balanced photodiodes modules in liquid helium then generate ternary electrical pulses to drive the nearby Josephson junction arrays via coplanar waveguides less than 10 mm long. The outputs of the Josephson junction arrays are connected in series and we successfully synthesized bipolar sinewaves at 1 kHz and 35 mV rms amplitude with higher harmonics suppressed by more than 96 dBc.
Abstract We have successfully operated two Josephson junction arrays in parallel with photonically generated current pulses to achieve higher output voltages with our optical pulse drive setup. Passive optical power splitters divide each optical channel at room temperature in order to illuminate multiple cryogenic photodiodes at 4 K , located near the arrays. The balanced photodiode configurations bias the Josephson junctions with ternary current pulses to generate a 1 kHz bipolar sinusoidal voltage with a nominal root mean square amplitude of 85 mV . We determined a spurious free dynamic range and total harmonic distortion of 110.6 dBc and − 105.0 dB from the spectrum of the synthesized signal. Applying an additional dither current to each array individually yielded a quantum locking range of 0.4 mA and 0.6 mA . Furthermore, the quantum locked operation was confirmed to remain stable when the optical output power of the laser diodes and the DC bias of the Mach–Zehnder modulators were varied. A stability measurement of the output amplitude over more than half an hour confirmed sufficiently wide operating margins, further demonstrating a reliable optical Josephson arbitrary waveform synthesizer setup suitable for generating quantum-based waveform with high spectral purity traceable to fundamental constants. To our knowledge, this is the highest amplitude generated with optically driven Josephson junctions so far and the first successful proof of principle for parallelizing the optical pulse drive to achieve higher output voltages in the future.
Pulse-driven Josephson voltage standards are used to generate quantum-based, spectrally pure AC voltage signals. The current pulses that drive the Josephson arrays can be transmitted optically and recent progress has demonstrated the operation of fast photodiodes at 4 K. In order to generate bipolar waveforms, two photodiodes are required to generate positive and negative pulses close to the Josephson array. We describe our optoelectronic setup and will present first results on a ternary pulse drive at the conference.
The report describes the most recent PTB developments and applications of large Josephson arrays for the generation of very precise sine waves and arbitrary waveforms. By means of large70 GHz sine-wave-driven binary arrays, AC voltages with frequencies up to 1 kHz and amplitudes of more than 10 V were generated with an uncertainty of better than 1μV/V. For the fabrication of this array type with up to 70000 junctions in series connection, an SNS junction technology with NbxSix-1barriers is now completely installed and permanently in use. With pulse-driven arrays of Josephson junctions, arbitrary waveforms were generated with a level of higher harmonics smaller than -120 dBc for frequencies from 150 Hz to 100 kHz at rms voltages less than 100 mV. These results were achieved with extended arrays of up to 4795 very small SNS junctions with Hf0.5Ti0.5barriers. Also the NbSi-technology technology has successfully been applied for the fabrication of pulse-driven Josephson arrays. As it seems to be difficult to considerably increase the wave amplitude by enhancing the junction number of the arrays, experiments were performed to combine a binary array system with a pulse-driven system with the goal to make use of the high amplitude of stepwise generated waves without losing the low noise and precision of quantum based rf waveforms generated by pulse trains at a clock frequency of 15 GHz.
An array of Josephson junctions (JJs) was driven with photonically generated current pulses to synthesize a high-fidelity 1 kHz bipolar voltage waveform with a quantum-based amplitude that can be directly related to fundamental constants. A photodiode capable of producing high average photocurrent was used to generate large-amplitude current pulses that were ac-coupled to a JJ array. The resulting bipolar current pulses have enabled the first demonstration of quantum-based bipolar waveform synthesis with an optical drive. We measured the quantum locking range with respect to several operating parameters, including 1.2 mA with respect to a dc bias current applied to the array, confirming the robust synthesis of bipolar waveforms.
The establishment at PTB of an AC Josephson voltage standard (Josephson Arbitrary Waveform Synthesizer- JAWS) based on pulse-driven Josephson arrays is focused on achieving an output voltage of at least 1 Vrms which is required for many metrology applications. In this paper, we approached this goal by increasing the number of active junctions in two ways. Firstly, we fabricated arrays containing triple-stacked SNS-type Josephson junctions with NbxSi1-xas barrier material. We obtained current operation margins of about 0.2 mA with arrays of up to 9000 Josephson junctions at an rms voltage of 355 mV. Secondly, we used a new 8-channel ternary pulse pattern generator (PPG) to bias up to 8 arrays connected in series. An output voltage of VRMS= 1006 mV(VPP= 2.845 V) was achieved by using 8 arrays (arranged on 4 separate chips) with 63000 junctions in total. Higher harmonics are suppressed by at least -116 dBc. The fabrication process and the experimental setup will be described, as well as experimental results that are leading towards our 1 V goal.
We developed an optical pulse-drive for the operation of the Josephson Arbitrary Waveform Synthesizer (JAWS) using a fast photodiode (PD) operated at 4 K, close to the JAWS chip. The optical pulses are transmitted to the PD by an easily removable optical fiber attached to it. A bare-lensed PD is mounted by flip-chip technique to a custom-made silicon-carrier chip. This carrier chip is equipped with coplanar waveguides to transmit the electrical pulses from the PD to the JAWS chip mounted on a separate printed circtuit board (PCB). The main components of this optical setup are a laser source, a high-speed Mach-Zehnder modulator, and the modulator driver. The waveform pattern is supplied by a commercial pulse pattern generator providing up to 15 GHz electrical return-to-zero (RTZ)-pulses. Unipolar sinusoidal waveforms were synthesized. Using a JAWS array with 3000 junctions, an effective output voltage of 6.6 mV root mean square (RMS) at the maximum available clock-frequency of 15 GHz was achieved. Higher harmonics were suppressed by more than 90 dBc at laser-bias operation margins of more than 1 mA.