Josephson Amplifier for Qubit Readout
Baleegh Abdo, Flavius Schackert, Michael Hatridge, Chad Rigetti and, Michel Devoret

TL;DR
This paper presents a Josephson amplifier designed for quantum-state qubit readout, demonstrating high gain, tunability, and good agreement with theoretical predictions, advancing microwave quantum measurement technology.
Contribution
It introduces a Josephson amplifier with specific design features and characterizes its performance, showing alignment with theoretical models and potential for improvements.
Findings
Maximum gain of about 20 dB achieved
Bandwidth of 9 MHz demonstrated
Center-frequency tunability of 60 MHz achieved
Abstract
We report on measurements of a Josephson amplifier (J-amp) suitable for quantum-state qubit readout in the microwave domain. It consists of two microstrip resonators which intersect at a Josephson ring modulator. A maximum gain of about 20 dB, a bandwidth of 9 MHz, and a center-frequency tunability of about 60 MHz with gain in excess of 10 dB have been attained for idler and signal of frequencies 6.4 GHz and 8.1 GHz, in accordance with theory. Maximum input power measurements of the J-amp show a relatively good agreement with theoretical prediction. We discuss how the amplifier characteristics can be improved.
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