研究目的
To study the effect of phase shifters on the performance of Josephson junction array devices, specifically in canceling microwave reflection and achieving consistent Shapiro step margins across branches.
研究成果
The study demonstrates that inserting phase shifters in Josephson junction array circuits can cause asymmetry and power imbalances, leading to inconsistent Shapiro step widths across branches. Removing the phase shifters results in identical ac I-V curves and step margins for all branches, suggesting that phase shifters may not be necessary in such circuits. Further research is required to validate these findings for longer junction arrays.
研究不足
The length of the Josephson junction array is small (about 0.13 cm), and limitations in fabrication process and measurement conditions prevented obtaining good curves at the design frequency of 20 GHz for the phase shifters. Future work is needed for longer arrays and improved fabrication consistency.
1：Experimental Design and Method Selection:
The study compares two circuit scenarios—one with 90° phase shifters and one without—both incorporating a 2-stage Wilkinson power divider to split input microwave power into four branches. The phase shifter is based on a one-stage low-pass filter with an LC-network in π model. Advanced Design System (ADS) is used for simulation and optimization of circuit elements.
2：Sample Selection and Data Sources:
Josephson junction array circuits are designed and fabricated using typical micro-processing technology for Nb/NbxSi1-x/Nb junctions. Each branch consists of 128 2-stacked junctions (total 256 junctions) with a size of 6×10 μm2 and a terminal resistance of 50 Ω.
3：List of Experimental Equipment and Materials:
Equipment includes a microwave source, DC blocks, Wilkinson power dividers, phase shifters (designed with capacitance of 151 fF and inductance of 402 pH), coplanar waveguide transmission lines with 50 Ω characteristic impedance, and measurement setups in liquid helium at 4.2 K. Materials involve superconducting Nb/NbxSi1-x/Nb junctions.
4：2 K. Materials involve superconducting Nb/NbxSi1-x/Nb junctions. Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: Chips are fabricated and measured at 4.2 K. DC I-V characteristics are measured first to determine critical current (Ic = 2.6 mA) and characteristic voltage (Vc = 27.42 μV). Then, ac I-V characteristics are measured for each branch under both circuit scenarios, with microwave power at 19 dBm and frequency at 10.6 GHz.
5：2 K. DC I-V characteristics are measured first to determine critical current (Ic = 6 mA) and characteristic voltage (Vc = 42 μV). Then, ac I-V characteristics are measured for each branch under both circuit scenarios, with microwave power at 19 dBm and frequency at 6 GHz. Data Analysis Methods:
5. Data Analysis Methods: S-parameters of the phase shifter are simulated using ADS. Ac I-V curves are compared between branches to assess Shapiro step widths and power imbalances.
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Advanced Design System
ADS
Keysight Technologies
Used for simulation and optimization of circuit elements, including S-parameters of the phase shifter.
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Wilkinson Power Divider
2-stage
Not specified
Splits input microwave power into four parts for the Josephson junction arrays.
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Phase Shifter
One-stage low-pass filter with LC-network in π model
Not specified
Designed to provide a 90° phase shift to cancel microwave reflection in branches.
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Coplanar Waveguide
CPW transmission line
Not specified
Transmits microwave signals with embedded Josephson junctions in the central signal line.
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Josephson Junction
Nb/NbxSi1-x/Nb, 2-stacked
Not specified
Forms the array for quantum voltage standard applications, exhibiting Shapiro steps under microwave irradiation.
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Termination Resistance
50 Ω resistor
Not specified
Terminates the transmission lines to match impedance and reduce reflections.
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Microwave Source
Not specified
Not specified
Provides microwave power for exciting the Josephson junctions.
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DC Block
Not specified
Not specified
Blocks DC signals while allowing microwave signals to pass in the circuit.
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