用于精密测量玻尔兹曼常数的量子电压噪声源芯片研制

量子噪声温度计系统可通过比较导体中电子运动的热噪声和量子电压参考噪声精密测量玻尔兹曼常数,其中量子电压噪声源所合成的量子电压参考噪声由一组超导约瑟夫森结阵产生.本文详细介绍了基于Nb/Nb_xSi_(1-x)/Nb约瑟夫森结的量子电压噪声源芯片的设计、制备及测试;采用脉冲驱动模式,合成了具有量子精度的100 kHz交流量子电压信号.结果表明:本文所研制的噪声温度计核心芯片已具备了合成交流电压的功能,可为后续玻尔兹曼常数精密定值、重新定义及复现热力学温度研究提供核心器件.

Development of quantum voltage noise source chip for precision measurement of Boltzmann constant

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/Nb_xSi_1-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 I_c. 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 I_c 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.