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Conference on Precision Electromagnetic Measurements

Temperature Controller for the Josephson Arbitrary Waveform Synthesizer

Xiaosong Chen, Kunli Zhou, Yang Shi, Qina Han, Bingkun Chen, Jinxin Xu, Jianting Zhao, Yan Rong Yang, Jifeng Qu

Published Jul 8, 2024• 0 citations
Publisher

Abstract

This paper presents a cryocooled Josephson arbitrary waveform synthesizer (JAWS) developed at the National Institute of Metrology (NIM), China. A 2-volt Josephson junction chip is mounted on the second stage cold head of a cryostat. We develop a temperature controller to measure the output voltage of the temperature sensor, and control the temperature of the second-stage cold head. The controller is composed of a current source of high stability, relays, a preamplifier, a low pass filter and a 24-bit analog to digital converter, and a microcontroller unit. Preliminary results show that the cooling power at 4.2 K is about 279 mW and the temperature fluctuation is within 40 mK.

DOI
10.1109/cpem61406.2024.10731961

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This paper presents a cryocooled Josephson arbitrary waveform synthesizer (JAWS) developed at the National Institute of Metrology (NIM), China. A 2-volt Josephson junction chip is mounted on the second stage cold head of a cryostat. We develop a temperature controller to measure the output voltage of the temperature sensor, and control the temperature of the second-stage cold head. The controller is composed of a current source of high stability, relays, a preamplifier, a low pass filter and a 24-bit analog to digital converter, and a microcontroller unit. Preliminary results show that the cooling power at 4.2 K is about 279 mW and the temperature fluctuation is within 40 mK.
This paper describes the current status of developing Josephson arbitrary waveform synthesizer (JAWS) chips in NIM (National Institute of Metrology, China). The chip has one data input channel and two Josephson junction arrays, each array consisted by 4000 double-stack Nb/NbxSi1–x/Nb junctions. The two arrays were driven with a single pulse-generator channel by using one on-chip Wilkinson power divider and inside/outside DC blocks. Root-mean-square (rms) output magnitude of 300 mV was generated with this JAWS chip.
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.
We created a Josephson Arbitrary Waveform Synthesizer (JAWS) with a root-mean-square (rms) output magnitude of 2 V. This system is composed of two 1 V chips operating on a cryocooler. By controlling the relative phase of the two chips' output voltage, we add the voltages in quadrature and create rms output voltages between (1+1) V=2 V and (1-1) V=0 V. We use this control to compare the two halves of the system as a function of the relative phase, and measure a difference of 8 parts in 108at a nominal rms amplitude of 1 V and 1 kHz.
This paper describes the status of developing Josephson arbitrary waveform synthesizer (JAWS) chips at NIM (National Institute of Metrology, China). To obtain high junction integration density and fewer data input channels, the chip employs an on-chip Wilkinson power divider and inside/outside dc blocks, enabling both arrays to be driven by a single pulse-generator channel. In addition, the tapered coplanar waveguide structure is used to ensure the microwave uniformity of the long-junction array. Each array consisted of 4000 double-stack Nb/NbxSi1−x/Nb junctions, and 16,000 junctions are integrated in the chip in total. The JAWS chip demonstrates good performance, capable of synthesizing a 300 mV root mean square (rms) voltage with exceptionally low harmonic distortion. Dc and ac voltage-current characteristics measurements indicate that the junctions are with a critical current of 2.5 mA, and a normal-state resistance of 4.5 mΩ per junction. Contact aligners are manually operated to fabricate the chips, and process errors in the fabrication are estimated in this paper.
We demonstrate Josephson arbitrary waveform synthesizers (JAWS) with increased operating temperature range for temperatures below 4 K. These JAWS synthesizers were fabricated with externally-shunted Nb/a\boldsymbol{a}a-Si/Nb junctions whose critical current exhibits improved temperature stability compared to the self-shunted Nb/Nb0.15\boldsymbol{ _{0.15} }0.15​Si0.85\boldsymbol{ _{0.85} }0.85​/Nb junctions typically used. Vertical stud resistors made of 230 nm of PdAu were developed to provide the milliohm shunt resistance required for junction overdamping while maintaining a small footprint suitable for high-density series arrays embedded in a coplanar waveguide. We evaluated the performance of these resistors from 3.8 K down to 20 mK. We designed, fabricated and tested a JAWS circuit with 4650 externally shunted Nb/a\boldsymbol{a}a-Si/Nb JJs with a critical current density (Jc\boldsymbol{J}_{\mathbf{c}}Jc​) of 0.12 mA/μ\boldsymbol{\mu }μm2\boldsymbol{^{2}}2and critical current (Ic\boldsymbol{I}_{\mathbf{c}}Ic​) of 3 mA. This circuit was designed to be mounted to the 3 K stage of a dilution refrigerator and used to control and calibrate a qubit mounted at the 10 mK stage. To increase the circuit density of the JAWS circuits we made arrays of two-junction vertical stacks. Current-voltage (I-V) curves of this JAWS circuit with stacked junctions under microwave excitation show Shapiro steps with quantum-locking ranges similar to those of JAWS circuits used for qubit control.
We have recently created a 4 V rms cryocooled JAWS (Josephson Arbitrary Waveform Synthesizer) using 204,960 nearly identical Josephson junctions (JJs) that are embedded in coplanar-wave guides. The JJs are pulse-biased at repetition rates up to 16×109pulses per second to create quantum-accurate, calculable AC waveforms at frequencies from DC to greater than 1 MHz. This system has metrological applications including in precision ac voltage calibrations, comparisons of arbitrary impedances, and ac power measurements.