# 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/en/paper/2604.13881v1/

## Key points

- High-performance quantum-limited parametric amplifiers are crucial in the realm of quantum technologies operating in the microwave domain.
- Achieving high gain and broad bandwidth in these amplifiers is challenging due to trade-offs such as gain versus bandwidth, pump efficiency, and dynamic range.
- A novel theoretical model and design methodology for a flux-driven Josephson parametric amplifier based on a SQUID array was introduced by Shingo Kono and colleagues.
- The innovative device achieved near-quantum-limited, phase-preserving amplification with a net gain of 20 dB (maximally 44 dB) and a 3-dB bandwidth of approximately 50 MHz (less than or equal to 0.2 MHz).
- The amplifier's gain spectra were found to be sensitive to weak reflections in the input-output waveguide caused by impedance mismatches within the microwave environment.
- By incorporating Fabry-Pérot-type interference into a quantum input-output model, researchers were able to reproduce complex spectral features analytically and determine their dependence on environmental parameters.
- This study provides a practical framework for distinguishing between intrinsic amplifier dynamics and environmental effects, enabling reliable characterization and optimization of amplifier performance.

## Summary

In the realm of quantum technologies operating in the microwave domain, high-performance quantum-limited parametric amplifiers play a crucial role. Achieving both high gain and broad bandwidth in these amplifiers presents a significant challenge due to various trade-offs such as gain versus bandwidth, pump efficiency, and dynamic range. In a recent study by Shingo Kono and colleagues, a novel theoretical model and design methodology were introduced for a flux-driven Josephson parametric amplifier based on a SQUID array. This innovative device was able to achieve near-quantum-limited, phase-preserving amplification with an impressive net gain of 20 dB (maximally 44 dB) and a substantial 3-dB bandwidth of approximately 50 MHz (less than or equal to 0.2 MHz). One notable finding from this research was the pronounced sensitivity of the amplifier's gain spectra to weak reflections in the input-output waveguide caused by impedance mismatches within the microwave environment. By incorporating Fabry-Pérot-type interference into a quantum input-output model, the researchers were able to analytically reproduce these complex spectral features and determine their dependence on environmental parameters. This study provides a practical framework for distinguishing between intrinsic amplifier dynamics and environmental effects. This approach enables reliable characterization and optimization of amplifier performance while offering a systematic strategy for diagnosing microwave reflections and manipulating environmental interference to shape amplifier gain spectra. Ultimately, these findings pave the way towards robust, reproducible, and truly quantum-limited microwave amplification in future quantum technologies.