Granular Aluminum as a Nonlinear Medium for Superconducting Parametric Amplifiers

Quantum-limited parametric amplifiers based on superconducting circuits are essential for the readout of microwave quantum systems. They are typically realized by exploiting the nonlinearity of Josephson junctions (JJs), which can be modulated through the application of strong pump tones. Despite the remarkable progress achieved over the past decade, JJ-based amplifiers still suffer from two major limitations that hinder their applicability in next-generation quantum technologies: sensitivity to magnetic fields beyond the millitesla range and non-negligible higher-order nonlinearities. Disordered superconductors have recently emerged as an alternative nonlinear medium capable of overcoming these limitations. Among them, granular aluminum (grAl) offers a particularly rich and versatile source of nonlinearity, well suited for the implementation of parametric devices. In this work, we leverage the intrinsic properties of grAl to develop a practical standing-wave parametric amplifier, called grAlPA, with a design resembling a superconducting Bose–Hubbard dimer. We demonstrate the magnetic-field resilience of the grAlPA, achieving near quantum-limited performance for in-plane magnetic fields up to 1 T. Furthermore, by exploiting the interplay between the intrinsically diluted higher-order nonlinearities and the low self-capacitance of grAl, we realize an amplifier exhibiting unconventional gain–bandwidth scaling up to gain levels of 25 dB. These results positions grAl as a compelling nonlinear platform for the next-generation of parametric amplifiers and open new opportunities for future quantum technologies.