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Conference on Precision Electromagnetic Measurements

An AC Josephson source for Johnson noise thermometry

Samuel P. Benz, John M. Martinis, Paul D. Dresselhaus, Sae Woo Nam

Published Jun 25, 2003• 3 citations
Publisher

Abstract

We have adapted the Josephson arbitrary waveform synthesizer to create a quantized voltage noise source suitable for calibrating the cross-correlation electronics of a Johnson noise thermometer system. The requirements of long term stability and low voltage amplitude allow dramatic simplification of the bias electronics compared to previous circuits. We describe the bias technique and the superconducting integrated circuit used to generate the pseudo-noise waveforms.

DOI
10.1109/cpem.2002.1034907

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We have adapted the Josephson arbitrary waveform synthesizer to create a quantized voltage noise source suitable for calibrating the cross-correlation electronics of a Johnson noise thermometer system. The requirements of long term stability and low voltage amplitude allow dramatic simplification of the bias electronics compared to previous circuits. We describe the bias technique and the superconducting integrated circuit used to generate the pseudo-noise waveforms.
A Johnson noise thermometer (JNT) determines the thermodynamic temperature through cross-correlation measurement of the Johnson noise in a sense resistor at an unknown temperature. In the quantum voltage calibrated JNT system, a superconductive quantum voltage noise source (QVNS) is required to produce artificial pseudo-random noise to calibrate the gain of the cross-correlation electronics. In this paper, we present the design, fabrication, and characterization of the QVNS chip. Compared with our previous design, a new straightforward mirror symmetric layout is implemented. For this layout, the coplanar waveguides (CPWs) have the same lengths and transmission parameters. Equal pulse magnitudes are delivered to each Josephson junction array under the same output settings of the bipolar pattern generator. The modified QVNS chip is thereby enhanced because the quantum locking range is enlarged. A copper foil shielding package is used to eliminate crosstalk in this design. The first spectral comparison of two Josephson junction (JJ) arrays of with and without shielding is presented in this paper. The comparative results demonstrate that the shielding is effective. The abovementioned improvements enable us to synthesize both single and multitone waveforms with good spectral results, such that the chip satisfies the requirement of a QVNS-based JNT system for temperature measurements.
We have developed a quantum voltage noise source (QVNS) based on pulse-driven Josephson arrays and optimized its waveform synthesis for use with Johnson noise thermometry (JNT). The QVNS synthesizes multitone waveforms with equal amplitude harmonic tones and random relative phases in order to characterize the amplitude-frequency response of the analog and digital electronics of the JNT system. We describe the QVNS circuit design and operation, including the lumped-array Josephson-junction circuit and the unipolar bias technique. In particular, we describe the primary design considerations that determine the voltage accuracy of the harmonic tones.
The Johnson noise thermometer is used to precisely measure Boltzmann constant by comparing the thermal noise caused by charge movement and the quantized voltage reference noise synthesized by the quantum voltage noise source (QVNS). The QVNS signal is synthesized based on quantized voltage pulses produced by two channels of superconducting Josephson junction arrays, which are designed for cross-correlation electronics. The Nb/NbxSi1-x/Nb Josephson junction is used as a core device of QVNS chip in this work for its non-hysteretic current-voltage (I-V) characteristics and conveniently adjustable barrier parameters.In this paper, we present the design consideration, fabrication process, and measurement results of the QVNS chip. The QVNS chip contains two Josephson junction arrays, each consists of four 6 μm×12 μm junctions and is embedded in a 50 Ω coplanar waveguide transmission line. The random noise in signals from the two driven channels is eliminated by cross-correlation, and then an accurate quantum noise is obtained. Test chips with different areas of Josephson junctions are also designed on the same mask, aiming at estimating the variation range of Ic. The typical fabrication process for voltage standard chips in our laboratory is used for preparing the QVNS chip.The sample is measured at 4.2 K. The DC I-V curve shows that the critical current Ic is 6.1 mA. The I-V characteristics of the junctions under 5 GHz microwave radiation are measured. For a series array of four junctions, a 41.44 μV one-stage Shapiro step is observed. Calculation shows that the error between the measurement and theoretical value of 41.36 μV is about 1.9‰, which means that the QVNS chip performs well under microwave radiation and can be used for synthesizing the AC quantum voltage reference noise.A single-frequency 100 kHz sinusoidal waveform is synthesized by the QVNS chip under pulse driven signal. A spectrum of the synthesized sinusoidal waveform shows a single peak, which means that the digital pulse signal is perfectly filtered by Josephson junction arrays and the synthesized signals possess quantum accuracy. The results indicate that our chip has good dynamic response and works well in synthesizing a single-frequency AC quantum voltage signal. This work can provide core devices for the noise thermometry system and support the precise measurement of Boltzmann constant as well as redefinition of Kelvin in future. As a next step, the design and package will be further improved, and the probe module will be optimized to reduce the measurement uncertainty.
A quantum-accurate waveform with an rms output amplitude of 1 V has been synthesized for the first time. This fourfold increase in voltage over previous systems was achieved through developments and improvements in bias electronics, pulse-bias techniques, Josephson junction array circuit fabrication, and packaging. A recently described ac-coupled bipolar pulse-bias technique was used to bias a superconducting integrated circuit with 25 600 junctions, which are equally divided into four series-connected arrays, into the second quantum state. We describe these advancements and present the measured 1 V spectra for 2 Hz and 10 Hz sine waves that remained quantized over a 0.4 mA current range. We also demonstrate a 2 kHz sine wave produced with another bias technique that requires no compensation current and remains quantized at an rms voltage of 128 mV over a 1 mA current range. Increasing the clock frequency to 19 GHz also allowed us to achieve a maximum rms output voltage for a single array of 330 mV.
We have been developing a Johnson noise thermometer using an integrated quantum voltage noise source (IQVNS) as a reference signal source. IQVNS is a superconducting integrated circuit that generates pseudo random signals. The power spectral density of output signal of IQVNS can be described with quantum accuracy by fundamental physical constants e and h, the clock frequency of the circuit, and a numerical coefficient. In the previous version of IQVNS the numerical coefficient was fixed by the circuit design. However, to cope with the measurement of thermodynamic temperature in a wide temperature range, it is desirable to make the output of IQVNS variable. In this study, we improved part of the design of the device to be able to change the power spectral density of the output signal and confirmed by measurement that the output signal can be variable as designed.