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Experimental Study on the Influence of Noise on the Accurate Measurement of Critical Current of Josephson Junction in Un-Shielded Environment

Zhou Jian, Juan Wang, Yewen Yao, Yongliang Wang, Man Wang, Wei Peng

Published Jan 1, 2023• 0 citations
Publisher
DOI
10.2139/ssrn.4395475

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The critical current (Ic) of a Josephson junction (JJ) is a significant physical parameter that plays a crucial role in the design of subsequent devices and circuits, as well as in the optimization of manufacturing processes. To ensure accurate measurements, testing should ideally be conducted in a magnetically shielded room. However, due to laboratory constraints, tests are often performed in unshielded environments, where they are inevitably influenced by various environmental noises. This introduces significant uncertainty into the measured Ic values, thereby impacting the design of subsequent superconducting circuits. This study examines the impact of conductive, radiated, and flux noise on the repeatability of Ic measurements in an unshielded environment and establishes an empirical formula for predicting Ic values in shielded environments based on unshielded data. The deviation between the empirically predicted Ic and the experimentally measured value is less than 1%, verifying the effectiveness of the proposed formula. This approach offers a relatively accurate and cost-effective method for testing the critical current (Ic) of Josephson junctions (JJs) under unshielded conditions.

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The measurement of critical currentIc, as an important parameter of un-shunted Josephson junctions (JJ), is critical for designing superconductor analog devices or digital circuits. However, precise measurement ofIcfrom current–voltage (I–V) characteristic curves often faces difficulties, such as the disturbance of environmental magnetic and electromagnetic waves (EM) noise, which significantly decreases theIcvalue from ideal critical currentIc0and increases the uncertainty ofIcmeasured value (>10 %). In this article, a test procedure is proposed for measuring theI–Vcurve of un-shunted JJ under a magnetic shielding environment, effectively avoiding deficiencies of conventional methods such as poor uncertainty and lowerIcmeasurement value. The uncertainty ofIcis investigated based on different superconducting JJs, and the results show that the combined uncertainty ofIcis less than 2% (the current stepμA). Also, a formula expressing the relationship ofIcwith the radius of JJ is proposed, which may provide a good way of predictingIcfor practical experimental measurements. The effectiveness of the procedure and the formula are both verified by experiments based on the Nb/Al-AlOx/Nb samples. The method is helpful for round-robin standard measurement onIcof un-shunted JJ and shortens the investigation process feedback time of the RSFQ integrated circuit.

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For Josephson bicrystal YBaCuO junctions 3-1.5 $\mu$m in size, the response to an external signal was measured. Receivers with similar $I_{c}R_{N}$ values at helium temperatures and different RNvalues were considered. The amplitudes of the Shapiro steps were measured for various external signal powers. The dependence of the output voltage noise on structure temperature was measured. It is shown that fluctuations of the critical current have the greatest influence on the output noise of the receiver. The volt-watt sensitivity and noise-equivalent power were evaluated for a 165 GHz external signal. Broadband classical detection of THz radiation by a YBaCuO Josephson junction (JJ) was studied based on resistively and capacitively shunted Josephson Junction model. Optimal temperature operating ranges for the use of each structure were shown.

IEEE All-Russian Microwave Conference (RMC)•Nov 23, 2022•2 citations
The critical current (Ic) of a Josephson junction (JJ) is a significant physical parameter that plays a crucial role in the design of subsequent devices and circuits, as well as in the optimization of manufacturing processes. To ensure accurate measurements, testing should ideally be conducted in a magnetically shielded room. However, due to laboratory constraints, tests are often performed in unshielded environments, where they are inevitably influenced by various environmental noises. This introduces significant uncertainty into the measured Ic values, thereby impacting the design of subsequent superconducting circuits. This study examines the impact of conductive, radiated, and flux noise on the repeatability of Ic measurements in an unshielded environment and establishes an empirical formula for predicting Ic values in shielded environments based on unshielded data. The deviation between the empirically predicted Ic and the experimentally measured value is less than 1%, verifying the effectiveness of the proposed formula. This approach offers a relatively accurate and cost-effective method for testing the critical current (Ic) of Josephson junctions (JJs) under unshielded conditions.
The measurement of critical currentIc, as an important parameter of un-shunted Josephson junctions (JJ), is critical for designing superconductor analog devices or digital circuits. However, precise measurement ofIcfrom current–voltage (I–V) characteristic curves often faces difficulties, such as the disturbance of environmental magnetic and electromagnetic waves (EM) noise, which significantly decreases theIcvalue from ideal critical currentIc0and increases the uncertainty ofIcmeasured value (>10 %). In this article, a test procedure is proposed for measuring theI–Vcurve of un-shunted JJ under a magnetic shielding environment, effectively avoiding deficiencies of conventional methods such as poor uncertainty and lowerIcmeasurement value. The uncertainty ofIcis investigated based on different superconducting JJs, and the results show that the combined uncertainty ofIcis less than 2% (the current stepμA). Also, a formula expressing the relationship ofIcwith the radius of JJ is proposed, which may provide a good way of predictingIcfor practical experimental measurements. The effectiveness of the procedure and the formula are both verified by experiments based on the Nb/Al-AlOx/Nb samples. The method is helpful for round-robin standard measurement onIcof un-shunted JJ and shortens the investigation process feedback time of the RSFQ integrated circuit.
For Josephson bicrystal YBaCuO junctions 3-1.5 μ\muμm in size, the response to an external signal was measured. Receivers with similar IcRNI_{c}R_{N}Ic​RN​ values at helium temperatures and different RNvalues were considered. The amplitudes of the Shapiro steps were measured for various external signal powers. The dependence of the output voltage noise on structure temperature was measured. It is shown that fluctuations of the critical current have the greatest influence on the output noise of the receiver. The volt-watt sensitivity and noise-equivalent power were evaluated for a 165 GHz external signal. Broadband classical detection of THz radiation by a YBaCuO Josephson junction (JJ) was studied based on resistively and capacitively shunted Josephson Junction model. Optimal temperature operating ranges for the use of each structure were shown.