In situ frequency tuning of superconducting resonators via nonlinear kinetic inductance

Superconducting microresonators have diverse applications, including microwave kinetic inductance detectors, microwave superconducting quantum interference device multiplexers, and superconducting qubits. Arrays of such devices are typically addressed using microwave frequency combs with probe tones matched to individual device resonances. However, resonator frequency collisions caused by wafer non-uniformity and fabrication variations significantly limit usable device yields. In this Letter, we present a technique that mitigates these frequency collisions without the need for ex post facto processing. By leveraging nonlinear kinetic inductance and persistent current in a superconducting loop, we achieve in situ tuning of individual resonator frequencies within an array during device cooldown, effectively resolving frequency collisions in a way that is both highly flexible and reversible. We successfully demonstrate this technique by tuning a small array of four resonators to both identical frequencies and a uniformly spaced frequency comb. This in situ resonator tuning approach provides a universal solution for improving yield and multiplexing density in large resonator arrays, addressing a critical need for scaling up superconducting detector and qubit systems.