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中国物理学会期刊

超导约瑟夫森结物理参数的实验推算

Experimentally estimating of physical parameters of the fabricated superconducting Josephson junctions

CSTR: 32037.14.aps.70.20210393
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  • 超导约瑟夫森结是实现超导量子计算和微波单光子探测的核心器件, 其物理参数很难直接测定. 与之前常用的测量结微波激励效应估计方法不同, 本文通过实验测量低频电流驱动下的约瑟夫森结I-V曲线及其跳变电流统计分布, 并与基于标准电阻电容分路结模型数值模拟进行比对, 推算出了约瑟夫森结的临界电流I_\rm c、电容C、电阻R及阻尼参数\beta_\rm c等物理参数. 结果表明, 所推算的参数值与基于微观理论推导所得到的Ambgaokar-Baratoff公式基本符合, 可供约瑟夫森结的器件参数按需设计和制备工艺的参数设置等参考.

     

    Superconducting Josephson junctions are the key devices for superconducting quantum computation and microwave single photon detection. It is important to fabricate the Josephson junctions with designable parameters. Different from the typical methods to calibrate the parameters of the Josephson junctions,, e.g., by using the microwave drivings and measuring the ratio of hysteresis current to critical one, in this paper we achieve the calibrations with the low frequency current biases. First, we measure the I-V characteristic curves of the fabricated Al/AlOx/Al junctions. Second, we measure the statistical distributions of the jump currents of the Josephson junction samples driven by the low frequency (@71.3 Hz) biased currents at an extremely low temperature of 50 mK. These two sets of experimental data are utilized to estimate the typical parameters of the Josephson junction, i.e., junction capacitance, critical current, and the damping coefficient, which are difficult to be directly measured in the usual experiments. The critical current and capacitance of the Josephson junction are estimated by fitting the statistical distribution of the measured jump currents with the relevant theoretical model of the "particle" escape from the potential driven by the thermal excitations and quantum tunnelings. With the calibrated critical current of the junction, the relation between I/I_\rmc and \rmd\varphi/\rmd\tau,\,\tau=\omega_\rmct (with \omega_\rmc being the plasmon frequency) is obtained from the measured I\text-V curve. Using the standard resistively capacitance shunted junction model to fit such a relation, the damping coefficient of the junction can be estimated. With the estimated critical current, capacitance, and damping coefficient, the resistance R_n of the junction at the working temperature is calibrated consequently. It is shown that our estimated results are in good agreement with that predicted by the famous Ambgaokar-Baratoff formula. Physically, the method demonstrated here possesses two advantages. First, it is relatively insensitive to the noise during the measurement of the junction's I-V characteristic curve, compared with the usual method to calibrate damping coefficient by measuring the ratio of hysteresis current to critical current. Second, only the low frequency driving is required to measure the jump current of the junction for estimating the damping coefficient. The microwave driving is not necessary. Hopefully, the present work is useful for the on-demand designs of the Josephson junctions for various applications.

     

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