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IEEE Transactions on Applied Superconductivity

Optimization and Validation of the Design of Superconductor-Insulator-Superconductor Mixer-Based Amplifier Circuits

Yoshinori Uzawa, Wenlei Shan, Akira KAWAKAMI, Yosuke Murayama, Sho Masui, Takafumi Kojima, Kazumasa Makise, Takatomi KUMAGAI

Published Dec 23, 2025• 0 citations
Publisher

Abstract

We propose an optimization method for a superconductor-insulator-superconductor (SIS) mixer-based amplifier circuit that employs SIS up- and down-converters. Such a novel amplifier configuration could enable superior low-noise performance for future broadband amplifier systems, such as those for large arrays of superconducting sensors or qubits. Optimization was achieved by identifying a solution in which the mutually dependent impedances of the SIS mixers converge under given conditions. A circuit design using this method predicted an amplifier gain of approximately 20 dB and a noise temperature of approximately 2 K at 4 K operation when practical current-voltage characteristics of Nb SIS junctions are used in the simulation. To verify the validity of this method, an amplifier circuit was prepared using waveguide Nb/AlOx/Nb mixer modules. By varying the phase delay between the two SIS mixers, changes in gain were observed, resulting from different convergence conditions of their mutually dependent impedances, which closely matched the predictions made using this method. These results are promising for the development of an integrated circuit including SIS amplifiers and superconducting detector arrays, with bandwidths of 10 GHz or above. This may also enable novel nonreciprocal microwave circuits such as gyrators, isolators, and circulators.

DOI
10.1109/tasc.2025.3646187

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A circuit that produces broadband impedance matching with low-current-density junctions at submillimeter wavelengths for tuning superconductor–insulator–superconductor (SIS) mixers is described; it consists of half-wavelength distributed SIS junctions connected alternately by half-wavelength microstrip lines. It is made of Nb and was designed for two practical source impedances using numerical calculation based on the Mattis–Bardeen theory of the anomalous skin effect at 660 GHz, assuming SIS junctions with a current density of 5 kA/cm 2 . An SIS mixer tuned using this circuit had a large fractional bandwidth of more than 18%, below -10 dB matching, even though the assumed ω C J R N product was about 16. Investigation of the mixer performance using Tucker's quantum theory of mixing showed that it has excellent broadband and low-noise characteristics compared with conventional mixers consisting of half- or full-wavelength distributed elements.
This thesis describes the development, modeling, and optimization of Nb superconducting microstrip tuning circuits for Superconductor-Insulator-Superconductor mixers. The ultimate goal is to extend the quantum limited noise performance of Nb/AlOx/Nb SIS mixers to above the Nb gap frequency (680 GHz). The mixers are intended to be used in the Atacama Large Millimeter Array (ALMA) band 9 frequency range (600-720 GHz). The behavior of Nb microstrip tuning circuits especially above the gap frequency is modeled using the complex conductivity given by the Mattis-Bardeen theory of the anomalous skin effect [72]. A computer code utilizing two-port network theory is utilized to predict and optimize the RF coupling efficiency and bandwidth of the tuning circuits and embedding impedances. The effect of material and geometrical variations on the coupling has been evaluated. The installment of the devices in the waveguide has been found to be a crucial factor explaining the observed scattering in measurement results for nominally identical devices. Despite parameter variations, modeling of the RF coupling efficiency proved to be successful within the limitations set by scattering of the measured results. Using this model, the tuning circuit geometry has been optimized for bandwidth and maximum coupling efficiency. During the work several SIS devices have been fabricated and Fourier transform spectroscopy measurements have been made for RF coupling evaluation. Finally, using simulations, it has been found that application of Nb/AlN/Nb junctions would increase the bandwidth to an extent which should dramatically decrease the sensitivity to parameter variations.
We present the design and preliminary experimental results of superconductor-insulator-superconductor (SIS) mixers for Atacama large millimeter array (ALMA) band 10 (780-950GHz) receivers. To achieve wide frequency coverage as well as low noise temperature, we integrated high current density (10 kA/cm2) and small-sized (one micrometer in diameter) Nb/AlOx/Nb SIS junctions with a tuning microstrip line using NbTiN ground plane and Al wiring layers. High quality NbTiN films with Tc reaching 14.5 K and Al films with residual resistivity ratio as large as 15 have been achieved. To minimize the waveguide transmission loss and facilitate the fabrication of mixer block, a full height waveguide was adopted. The waveguide probe was optimized to achieve low input impedance in the frequency range of 780-950 GHz while adopting a backshort cavity of zero depth. Preliminary RF testing with a Nb/Al microstrip tuning circuit demonstrated a DSB receiver noise temperature about 400 K at 832.5 GHz, which promises the accommodation of receiver performance in ALMA specification with low loss NbTiN/Al microstrip lines.