Stability of superconducting resonators: motional narrowing and the role of Landau-Zener driving of two-level defects

TL;DR

This study investigates the frequency stability of superconducting resonators, revealing motional narrowing effects and the influence of Landau-Zener transitions on two-level defects, which impact quantum device coherence.

Contribution

It demonstrates how drive power affects defect dynamics and introduces a new understanding of noise mitigation in superconducting resonators.

Findings

Resonators exhibit similar telegraph noise fluctuations.

Motional narrowing occurs with increased drive power.

Defect switching rates follow Landau-Zener transition power-law dependence.

Abstract

Frequency instability of superconducting resonators and qubits leads to dephasing and time-varying energy-loss and hinders quantum-processor tune-up. Its main source is dielectric noise originating in surface oxides. Thorough noise studies are needed in order to develop a comprehensive understanding and mitigation strategy of these fluctuations. Here we use a frequency-locked loop to track the resonant-frequency jitter of three different resonator types---one niobium-nitride superinductor, one aluminium coplanar waveguide, and one aluminium cavity---and we observe strikingly similar random-telegraph-signal fluctuations. At low microwave drive power, the resonators exhibit multiple, unstable frequency positions, which for increasing power coalesce into one frequency due to motional narrowing caused by sympathetic driving of individual two-level-system defects by the resonator. In all three devices we probe a dominant fluctuator, finding that its amplitude saturates with increasing drive power, but its characteristic switching rate follows the power-law dependence of quasiclassical Landau-Zener transitions.