# High-gain and large-bandwidth Josephson parametric amplifier influenced by Fabry-Pérot interference

**Authors:** Shingo Kono, Jesper Ilves, Arjan F. van Loo, Yoshiki Sunada, C. W. Sandbo Chang, Yutaka Takeda, Kenshi Yuki, Takeaki Miyamura, Kohei Matsuura, Kazuki Koshino, Yasunobu Nakamura  
**arXiv:** [2604.13881](https://arxiv.org/abs/2604.13881)  
**Date:** 2026-04-15  
**Summary page:** https://www.summarizepaper.com/fr/paper/2604.13881v1/

## Abstract

Quantum-limited parametric amplifiers are essential components for many quantum technologies operating in the microwave domain. Achieving both high gain and broad bandwidth, however, remains challenging due to trade-offs between gain and bandwidth, pump efficiency, and dynamic range. Moreover, high-gain broadband amplifiers become increasingly sensitive to their external electromagnetic environment, which can distort their gain spectra and hinder reliable operation. Here, we present an accurate theoretical model and a systematic design methodology for a flux-driven, lumped-element Josephson parametric amplifier based on a SQUID array. Our device achieves near-quantum-limited, phase-preserving amplification with a net gain of 20 (maximally 44) dB and a 3-dB bandwidth of $\sim$50 ($\lesssim$0.2) MHz. We further show that the gain spectra exhibit pronounced sensitivity to weak reflections in the input-output waveguide caused by impedance mismatches in the microwave environment. By incorporating Fabry-Pérot-type interference into a quantum input-output model, we analytically reproduce these complex spectral features and identify how they depend on the physical parameters of the environment. More generally, our results provide a practical framework for separating the intrinsic dynamics of parametric amplifiers from environmental effects. This approach enables reliable characterization and optimization of amplifier performance while providing a systematic strategy for diagnosing microwave reflections and engineering environmental interference to shape amplifier gain spectra, thereby offering a pathway toward robust, reproducible, and truly quantum-limited microwave amplification.