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IEEE Transactions on Applied Superconductivity

Fabrication and Evaluation of Waveguide Josephson Array Oscillators at Millimeter-Wave Lengths for SIS Mixer-Based Amplifiers

Yosuke Murayama, Akira Kawakami, Wenlei Shan, Shohei Ezaki, Akihira Miyachi, Sho Masui, Takafumi Kojima, Yoshinori Uzawa

Published Dec 12, 2024• 3 citations
Publisher

Abstract

In this study, a 150 GHz waveguide Josephson array oscillator was designed, fabricated, and evaluated as a local oscillator (LO) source for superconductor-insulator-superconductor (SIS) mixer-based amplifiers. These amplifiers aim to reduce the power consumption of microwave amplifiers used in large-scale multi-pixel heterodyne receivers for radio astronomy and superconducting quantum computers. We fabricated a Josephson array oscillator of 11 Nb/Al–AlOx/Nb Josephson junctions with Cu shunt resistors, each of which is placed on the center of a half-wavelength microstrip line resonator. When the LO signal from the oscillator was introduced into the balanced SIS mixers via a waveguide, the oscillator successfully pumped four SIS mixers simultaneously, delivering a total estimated LO power of 0.22 µW at 146.7 GHz. The SIS mixers operated without a millimeter-wave isolator at an LO noise temperature of 4–6 K. These results indicate the Josephson array oscillator's potential as an LO source for SIS mixer-based amplifiers.

DOI
10.1109/tasc.2024.3515971

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In this study, a 150 GHz waveguide Josephson array oscillator was designed, fabricated, and evaluated as a local oscillator (LO) source for superconductor-insulator-superconductor (SIS) mixer-based amplifiers. These amplifiers aim to reduce the power consumption of microwave amplifiers used in large-scale multi-pixel heterodyne receivers for radio astronomy and superconducting quantum computers. We fabricated a Josephson array oscillator of 11 Nb/Al–AlOx/Nb Josephson junctions with Cu shunt resistors, each of which is placed on the center of a half-wavelength microstrip line resonator. When the LO signal from the oscillator was introduced into the balanced SIS mixers via a waveguide, the oscillator successfully pumped four SIS mixers simultaneously, delivering a total estimated LO power of 0.22 µW at 146.7 GHz. The SIS mixers operated without a millimeter-wave isolator at an LO noise temperature of 4–6 K. These results indicate the Josephson array oscillator's potential as an LO source for SIS mixer-based amplifiers.
We propose a low-noise and low-power-consumption microwave amplifier using superconductor-insulator-superconductor (SIS) mixers as quasi-particle frequency up- and down-converters pumped by a Josephson array oscillator in the millimeter-wave band. A proof-of-concept study of an amplifier with two W-band waveguide mixers using Nb/Al-AlOx/Nb junctions in a cascade connection demonstrated an average gain of 6 dB and a noise temperature of 12 K from nearly DC to 5 GHz. We also designed and fabricated a Josephson array oscillator at 100 GHz, which consisted of 31 overdamped Josephson junctions placed at each half-wavelength in an Nb microstrip line. The estimated output power of the oscillator was 0.2 μW at 100 GHz, which can be increased by increasing the number of the oscillator junctions. These results suggest that the proposed SIS mixer-based amplifier is suitable for on-chip integrated circuits to be implemented in large-scale multipixel SIS receivers and quantum computers.
A waveguide Josephson array oscillator operating at a frequency of 100 GHz was designed and fabricated to evaluate its pumping capability as a local oscillator (LO) for our proposed SIS mixer-based amplifier, which incorporates SIS up- and down-converters. The LO power requirement was estimated by simulation to be several nanowatts when the SIS up-converter is optimized for noise temperature and conversion gain under the condition that the input port at the microwave frequencies is connected to a practical 50-W system. The fabricated waveguide Josephson array oscillator module has 31 Nb Josephson junctions placed at the center of a half-wavelength resonator of an Nb microstripline. The output power from the waveguide oscillator module was evaluated by connecting it to the waveguide SIS detector module in a 4-K cryostat. Photon-assisted tunneling steps were clearly observed, from which the detected power was estimated to be approximately 17 nW at 106 GHz. These results suggest that the Josephson array oscillator is capable of being used in the SIS mixer-based amplifier as the LO.
This paper introduces a potential low-power-consumption and low-noise microwave amplifier operated at cryogenic temperature, essential for large-scale multi-pixel heterodyne receivers and fault-tolerant quantum computers. The amplifier employs two millimeter-wave superconductor-insulator-superconductor (SIS) mixers as amplifying elements and a millimeter-wave Josephson array oscillator as the local oscillator source. A proof-of-concept experiment utilizing waveguide SIS mixer modules demonstrated an average gain of approximately 7.5 dB and a noise temperature of around 10 K at microwave frequencies. Additionally, a waveguide Josephson array oscillator module, developed to validate the design, exhibited an output power of roughly 52 nW, estimated from the response of a waveguide SIS detector connected to the oscillator module. These findings indicate the feasibility of realizing a monolithically integrated amplifier chip incorporating SIS mixers and a Josephson array oscillator.
All-refractory material superconducting millimeter-wave oscillators have been designed and investigated experimentally with different superconductor-insulator-superconductor (SIS) mixers integrated on the same chip. Tested structures include a flux-flow oscillator (FFO) based on a long Josephson junction, a coupling section, and an SIS detector with tuned out junction capacitance. Coupling sections were designed as multistep microstrip quarter-wave impedance transformers. All junctions have been fabricated on the basis of a high-quality trilayer Nb-AlO/sub x/-Nb process. Microwave oscillations in the frequency range 75-500 GHz have been detected experimentally. The level of delivered power was estimated from the pumped I-V curve of the strongly coupled single junction detector. Coupled power levels higher than 0.1 mu W at 256 GHz were achieved. A spectral linewidth of the FFO of less than 1 MHz has been estimated experimentally. The first attempt to create an integrated receiver based on an FFO and an SIS array mixer integrated on the same chip was made in the 2-mm wavelength band.>
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlOx/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (Jc) of 10kA/cm2. In this paper, we describe their design, fabrication and preliminary experimental results.
Abstract We developed a low-noise, compact, balanced superconductor–insulator–superconductor (SIS) mixer, operating in the 787–950 GHz radio frequency range. A waveguide mixer block was designed to integrate all the key components, such as a radio frequency (RF) 90° hybrid coupler, two identical SIS mixer chips, bias-tees, and an intermediate frequency power-combiner. The RF waveguide 90° hybrid coupler consists of branch lines with wide slots optimized by numerical simulation, for ease of fabrication. The balanced mixer was installed into a cartridge type receiver, originally developed for the Atacama Large Millimeter/submillimeter Array Band 10 (787–950 GHz). The receiver demonstrated double sideband noise temperatures of approximately 200 K for most of the band, without any correction for loss in front of the receiver. The local oscillator noise rejection ratio was estimated to be more than 15 dB within the measured frequency range.