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IEEE Transactions on Applied Superconductivity

Cryogenic Analog Microwave Variable Attenuator Using rf-SQUIDs for Quantum Systems

Navjot K. Khaira, Tejinder Singh, Raafat R. Mansour

Published Feb 14, 2023• 3 citations
Publisher

Abstract

We present the design, fabrication and testing of a low-temperature superconducting (LTS) analog microwave variable attenuator. The two-port circuit includes two 90ohybrid couplers and a phase shifter to realize a variable attenuation at the output port. The wideband analog phase shifter is implemented using LTS microstrip transmission line (TL) coupled to an array of radio frequency superconducting quantum interference devices (rf-SQUIDs). The phase shift is achieved by passing a dc current through a TL adjacent to the SQUID-loaded microstrip line which controls the flux through the rf-SQUIDs. The proposed variable attenuator shows more than 30 dB of precise analog flat attenuation tuning range over 8 GHz to 12 GHz with a dc control current ranging from 0 mA to 4.5 mA. The device exhibits a wideband operation with variable attenuation seen not only in theX-band but also inKu-band. The device is fabricated using a multilayer niobium-based superconducting process SFQ5ee by MIT Lincoln Laboratory, and has a compact footprint of 1.3 mm2. To the best of our knowledge, this is the first implementation of a variable attenuator using distributed rf-SQUIDs. This device has implications in a variety of applications such as cryogenic receivers and qubit circuit measurements.

DOI
10.1109/tasc.2023.3244768

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We present the design, fabrication and testing of a low-temperature superconducting (LTS) analog microwave variable attenuator. The two-port circuit includes two 90ohybrid couplers and a phase shifter to realize a variable attenuation at the output port. The wideband analog phase shifter is implemented using LTS microstrip transmission line (TL) coupled to an array of radio frequency superconducting quantum interference devices (rf-SQUIDs). The phase shift is achieved by passing a dc current through a TL adjacent to the SQUID-loaded microstrip line which controls the flux through the rf-SQUIDs. The proposed variable attenuator shows more than 30 dB of precise analog flat attenuation tuning range over 8 GHz to 12 GHz with a dc control current ranging from 0 mA to 4.5 mA. The device exhibits a wideband operation with variable attenuation seen not only in theX-band but also inKu-band. The device is fabricated using a multilayer niobium-based superconducting process SFQ5ee by MIT Lincoln Laboratory, and has a compact footprint of 1.3 mm2. To the best of our knowledge, this is the first implementation of a variable attenuator using distributed rf-SQUIDs. This device has implications in a variety of applications such as cryogenic receivers and qubit circuit measurements.
This article presents a low-temperature superconductor (LTS)-based microwave phase shifter. The device utilizes a superconducting hybrid coupler monolithically integrated with two tunable reflective loads. An array of radio frequency superconducting quantum interference devices (rf-SQUIDs) is used to achieve inductive tuning in reflective loads, resulting in a broadband true time delay phase shift. The rf-SQUIDs are current controlled using an on-chip dc bias circuitry. The proof-of-concept fabricated device provides excellent RF performance in theX{X}X- and Ku -band with a phase shift of 49° at 10 GHz and 67° at 15 GHz. The insertion loss is < 1.5 dB for the entire range from 8 to 18 GHz. The methods to further improve the obtained phase shift are discussed. The overall device has a chip footprint of 0.6 mm2 and is fabricated using MIT-Lincoln Lab (MIT-LL) SFQ5ee eight-layer Niobium (Nb)-based superconducting fabrication process.
This letter presents a low-temperature superconductor (LTS)-basedX{X}X- andK{K}K-band analog ultra-wideband phase shifters realized by coupling an array of radio-frequency superconducting quantum interference devices (rf-SQUIDs) to a microwave transmission line (TL). The proposed device is realized using MIT-LL SFQ5ee eight-layer niobium-based process. The device is designed by distributing 2256 rf-SQUIDs coupled throughout the line exhibiting experimental analog current-controlled phase shift of 130° and insertion loss variation of 0.46 ± 0.23 dB inX{X}X-band atfc{f} _{c}fc​= 10 GHz. In theK{K}K-band, a phase shift of 328° and insertion loss of 2.5 ± 1 dB inK{K}K-band atfc{f} _{c}fc​= 22.5 GHz is observed. Theoretical analysis of the phase shifter is discussed.
A novel design of a radio-frequency (RF) power limiter for protecting sensitive superconductor receiver components from high-power microwave signals is presented. In this approach, two low-Tc{T} _{c}Tc​superconductor microstrip lines, one of which is coupled with an array of RF superconducting quantum interference devices (rf-SQUIDs), are combined with a pair of hybrid couplers to provide the power limiting operation at very low RF power levels. The key concept is built on the fact that a microstrip line coupled to an array of rf-SQUIDs behaves as a power-dependent phase shifter and its phase can be controlled by the input RF power. This phenomenon is exploited to achieve the desired RF power-limiting operation. The device is fabricated using an eight-layer niobium-based superconducting process node SFQ5ee by MIT Lincoln Laboratory (MIT-LL). A monolithically integrated wideband hybrid coupler is used in this design as it dictates the bandwidth of the overall device. The operating frequency of the power limiter is 10 GHz with a 2-GHz bandwidth. The performance of the device is measured for input RF power levels ranging from −30 to +10 dBm, as well as arising intermodulation products are experimentally investigated. The output power increases linearly with the input power up to −15 dBm, and for higher power levels, the device offers an increasing attenuation to limit the output RF power starting above −15 dBm.
The authors report on a novel microwave phase shifter featuring rapid electronic adjustment, continuous phase control true time delay operation, high device fault tolerance, and very broadband operation. By coupling a large number of superconducting quantum interference devices (SQUIDs) to a superconducting microstrip transmission line, a variable magnetic medium in which the wave velocity is controlled electronically is created. The authors have measured 60 degrees phase shift at 10 GHz, and wideband operation from 5 to 15 GHz for an 8-cm-long Nb transmission line coupled to 1600 SQUIDs, each containing a single Nb/AlO/sub x//Nb tunnel junction. The observed phase shift corresponds to a change in wave velocity of about 1 part in 60.>
We implement a broadly tunable phase shifter for microwaves based on superconducting quantum interference devices (SQUIDs) and study it both experimentally and theoretically. At different frequencies, a unit transmission coefficient, |S21| = 1, can be theoretically achieved along a curve where the phase shift is controllable by the magnetic flux. The fabricated device consists of three equidistant SQUIDs interrupting a transmission line. We model each SQUID embedded at different positions along the transmission line with two parameters, capacitance and inductance, the values of which we extract from the experiments. In our experiments, the tunability of the phase shift varies from 0.07 × π to 0.14 × π radians along the full-transmission curve with the input frequency ranging from 6.00 GHz to 6.28 GHz. The reported measurements are in good agreement with simulations, which is promising for future design work of phase shifters for different applications.
Abstract Here we consider a tunable superconducting cavity that can be used either as a tunable coupler to a qubit inside the cavity or as a tunable low noise, low temperature, RF filter. Our design consists of an array of radio-frequency superconducting quantum interference devices (rf SQUIDs) inside a superconducting cavity. This forms a tunable metamaterial structure which couples to the cavity through its magnetic plasma frequency. By tuning the resonant frequency of the metamaterial through an applied magnetic flux, one can tune the cavity mode profile. This allows us to detune the cavity initially centered at 5.593 GHz by over 200 MHz. The maximum quality factor approaches that of the empty cavity, which is 4.5 × 106. The metamaterial electromagnetic response is controlled via a low-frequency or dc magnetic flux bias, and we present a control line architecture that is capable of applying sufficient magnetic flux bias with minimal parasitic coupling. Together this design allows for an in-situ tunable cavity which enables low-temperature quantum control applications.