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IEEE Transactions on Applied Superconductivity

Tunable Impedance Matching for Josephson Junction Reflection Amplifier

Visa Vesterinen, Juha Hassel, Heikki Seppä

Published Nov 14, 2012• 3 citations
Publisher

Abstract

It has been recently demonstrated that a single Josephson junction with a properly engineered impedance environment can be used as a reflection amplifier approaching the quantum limit at microwave frequencies. In this article the authors describe an improved microwave setup for such a device. Voltage-tunable capacitors enable precise impedance matching across a wide range of operating points. Also, a branch-line coupler instead of a microwave circulator is used for the separation of the input and the output signals to make the system more compact and affordable. Furthermore, a custom-designed bias tee is introduced. The authors present characterization results of individual components as well as of the complete system. In particular, noise spectroscopy is used to study amplifier stability as a function of input matching.

DOI
10.1109/tasc.2012.2227653

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It has been recently demonstrated that a single Josephson junction with a properly engineered impedance environment can be used as a reflection amplifier approaching the quantum limit at microwave frequencies. In this article the authors describe an improved microwave setup for such a device. Voltage-tunable capacitors enable precise impedance matching across a wide range of operating points. Also, a branch-line coupler instead of a microwave circulator is used for the separation of the input and the output signals to make the system more compact and affordable. Furthermore, a custom-designed bias tee is introduced. The authors present characterization results of individual components as well as of the complete system. In particular, noise spectroscopy is used to study amplifier stability as a function of input matching.
An impedance matched parametric amplifier (IMPA) with Josephson junctions is fabricated and characterized. A hybrid structure containing coplanar and strip structures is implemented to realize an impedance taper line and a plate capacitor in an LC nonlinear resonator based on Josephson junctions. The upper plate of the capacitor is isolated with SiN x without grounding as well as the strips. Such easily-prepared designs greatly reduce the requirements for lithography alignment and precision, which makes the fabrication process more reliable. The experimental results show that in such IMPA a gain higher than 25 dB with a bandwidth of about 100 MHz can be obtained. This broadband amplifier operates close to the quantum limit. By adjusting the working point, a higher bandwidth of about 400 MHz can be obtained with a gain of about 17 dB.
We report the efficient coupling of a 50 Ω microwave circuit to a high impedance conductor. We use an impedance transformer consisting of a λ/4 co-planar resonator whose inner conductor contains an array of superconducting quantum interference devices (SQUIDs), providing it with a tunable lineic inductance L∼80 μ0, resulting in a characteristic impedance ZC∼1 kΩ. The impedance matching efficiency is characterized by measuring the shot noise power emitted by a dc biased tunnel junction connected to the resonator. We demonstrate matching to impedances in the 15 to 35 kΩ range with bandwidths above 100 MHz around a resonant frequency tunable between 4 and 6 GHz.
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Josephson junction parametric amplifiers are playing a crucial role in the readout chain in superconducting quantum information experiments. However, their integration with current 3D cavity implementations poses the problem of transitioning between waveguide, coax cables, and planar circuits. Moreover, Josephson amplifiers require auxiliary microwave components, like directional couplers and/or hybrids, that are sources of spurious losses and impedance mismatches that limit measurement efficiency and amplifier tunability. We have developed a wireless architecture for these parametric amplifiers that eliminates superfluous microwave components and interconnects. This greatly simplifies their assembly and integration into experiments. We present an experimental realization of such a device operating in the 9–11 GHz band with about 100 MHz of amplitude gain-bandwidth product, on par with devices mounted in conventional sample holders. The simpler impedance environment presented to the amplifier also results in increased amplifier tunability.
We present a fishbone-like lumped-element artificial transmission line to overcome impedance mismatch in a reflection-type Josephson parametric amplifier between a nonlinear resonator and an external transmission line. Using this easily prepared architecture, we design and fabricate a broadband Josephson parametric amplifier, which has gain in an excess of 20 dB with a bandwidth of hundreds of MHz. Furthermore, by varying the working point of the device, the operating frequency of amplification can be tuned in a wide frequency range of 1 GHz while the amplifier operates in the mode of either three-wave mixing or four-wave mixing. Such a parametric amplifier is suitable for engineering applications of superconducting circuit quantum electrodynamics.
In this article, we present an experimental study of microwave resonators made out of Josephson junctions. The junctions are embedded in a transmission line geometry so that they increase the inductance per length for the line. By comparing two devices with different input/output coupling strengths, we show that the coupling capacitors, however, add a significant amount to the total capacitance of the resonator. This makes the resonators with high coupling capacitance to act rather as lumped element resonators with inductance from the junctions and capacitance from the end sections. Based on a circuit analysis, we show that the input and output couplings of the resonator are limited to a maximum value of ωrZ0/4Zr, where ωr is the resonance frequency and Z0 and Zr are the characteristic impedances of the input/output lines and the resonator, respectively.