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IEEE International Superconductive Electronics Conference (ISEC)

Optical pulse-drive and on-chip power splitter for the pulse-driven AC Josephson Voltage Standard

Oliver Kieler, Hao Tian, Rolf-Werner Gerdau, Ruediger Wendisch, J. Kohlmann, Per Alfred Øhlckers, Eivind Bardalen, Muhammad Nadeem Akram, Ralf Behr, L. Palafox, Jane Ireland, Jonathan M. J. Williams, Bjørnar Karlsen, Helge Malmbekk

Published Jul 1, 2019• 0 citations
Publisher

Abstract

The pulse-driven Josephson Voltage Standard, also called Josephson Arbitrary Waveform Synthesizer (JAWS) is already well established for different applications in AC voltage metrology. To further increase the output voltage towards 10 V and to reduce the complexity of the JAWS systems we investigated two different approaches, which finally can be combined. One approach is to integrate an optimized on-chip power splitter to reduce the number of high-frequency (HF) channels from room temperature down to 4 K. A pulse pattern generator with less HF outputs will directly reduce the complexity and costs of a JAWS system. The second approach is to use an optical pulse-drive implementing cold photodiodes close to the JAWS chip. The use of optical fiber will have two main advantages: the optical fibers will reduce the high frequency noise and will enable an easy splitting into parallel optical channels. We will present first results with both approaches.

DOI
10.1109/isec46533.2019.8990917

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IEEE Transactions on Applied Superconductivity•Nov 10, 2014•97 citations
The pulse-driven Josephson Voltage Standard, also called Josephson Arbitrary Waveform Synthesizer (JAWS) is already well established for different applications in AC voltage metrology. To further increase the output voltage towards 10 V and to reduce the complexity of the JAWS systems we investigated two different approaches, which finally can be combined. One approach is to integrate an optimized on-chip power splitter to reduce the number of high-frequency (HF) channels from room temperature down to 4 K. A pulse pattern generator with less HF outputs will directly reduce the complexity and costs of a JAWS system. The second approach is to use an optical pulse-drive implementing cold photodiodes close to the JAWS chip. The use of optical fiber will have two main advantages: the optical fibers will reduce the high frequency noise and will enable an easy splitting into parallel optical channels. We will present first results with both approaches.
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.
This paper presents the basic principle of the pulse-driven ac Josephson voltage standard, also called “Josephson arbitrary waveform synthesizer” (JAWS). We will mainly focus on activities and developments at the Physikalisch-Technische Bundesanstalt (PTB). The JAWS is suitable to generate high precision quantized waveforms, which can be arbitrarily programmed in the frequency and time domain with extremely high spectral purity (typical signal-to-noise ratios SNR of 120–140 dBc). A brief overview of the fabrication technology for highly integrated JAWS circuits with more than 50 000 junctions per chip will be described. This includes the stacked junction technology with up to 5-stacked junctions. The experimental setup and the key-features of JAWS will be shown too. New approaches designed to further increase the output voltage, to simplify the experimental setup, and to extend the range of applications are the on-chip power dividers, the optical pulse drive, and the RF-JAWS. A selected application of the JAWS is described to demonstrate the usability of sophisticated JAWS waveforms for the complex characterization of electrical devices. The PTB JAWS circuits already have a significant impact on the quantum ac voltage metrology community. JAWS chips made at PTB have been distributed to more than 10 national metrology and other research institutes within the framework of international research projects.
The root-mean-square (rms) output voltage of the NIST Josephson arbitrary waveform synthesizer (JAWS) has been doubled from 1 V to a record 2 V by combining two new 1 V chips on a cryocooler. This higher voltage will improve calibrations of ac thermal voltage converters and precision voltage measurements that require state-of-the-art quantum accuracy, stability, and signal-to-noise ratio. We achieved this increase in output voltage by using four on-chip Wilkinson dividers and eight inner-outer dc blocks, which enable biasing of eight Josephson junction (JJ) arrays with high-speed inputs from only four high-speed pulse generator channels. This approach halves the number of pulse generator channels required in future JAWS systems. We also implemented on-chip superconducting interconnects between JJ arrays, which reduces systematic errors and enables a new modular chip package. Finally, we demonstrate a new technique for measuring and visualizing the operating current range that reduces the measurement time by almost two orders of magnitude and reveals the relationship between distortion in the output spectrum and output pulse sequence errors.
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.
The output voltage of Josephson arbitrary waveform synthesizers (JAWS) is limited by the number of Josephson junctions (JJs) that can be driven by a single pulse-generator channel. Here, we double the number of JJs driven by one generator channel to 51 200 JJs by distributing the pulse bias between four JJ arrays by use of two layers of Wilkinson dividers. We use this single bias to generate a voltage at 1 kHz with an rms magnitude of 1 V. This voltage is quantum-accurate over an operating current range of 1.4 mA. For comparison, the operating current range of a recent design that uses a single layer of Wilkinson dividers is twice as large, but requires two pulse-bias channels to generate 1 V. We also show that we can recover this performance by incorporating in the pulse generator a finite-impulse response (FIR) filter that acts as an equalizer. The FIR filter creates a custom transfer function that compensates for the bandwidth-limited transfer function of the Wilkinson dividers. Optimizing the FIR filter parameters increases the operating current range from 1.4 to 2.7 mA. This ability to drive additional JJ arrays with a single pulse-generator channel will enable future JAWS chips and systems to achieve significantly larger output voltages. This will increase the voltage range for JAWS calibrations of ac thermal converters and improve precision voltage measurements that require quantum-accurate, stable, and distortion-free waveforms with a large signal-to-noise ratio.
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.