Demonstrating magnetic field robustness and reducing temporal T1 noise in transmon qubits through magnetic field engineering

TL;DR

This study shows that applying controlled magnetic fields during cooldown can significantly reduce temporal T1 noise in transmon qubits, enhancing their stability without compromising average coherence times.

Contribution

It demonstrates that magnetic field engineering can suppress T1 fluctuations in superconducting qubits, challenging the traditional view of magnetic fields as purely harmful.

Findings

Magnetic fields up to 600 mG do not degrade average T1 or qubit frequency.

Applied and trapped magnetic flux reduce T1 fluctuations by over twofold.

Magnetic field stabilization is linked to impurity polarization and quasiparticle management.

Abstract

The coherence of superconducting transmon qubits is often disrupted by fluctuations in the energy relaxation time (T1), limiting their performance for quantum computing. While background magnetic fields can be harmful to superconducting devices, we demonstrate that both trapped magnetic flux and externally applied static magnetic fields can suppress temporal fluctuations in T1 without significantly degrading its average value or qubit frequency. Using a three-axis Helmholtz coil system, we applied calibrated magnetic fields perpendicular to the qubit plane during cooldown and operation. Remarkably, transmon qubits based on tantalum-capped niobium (Nb/Ta) capacitive pads and aluminum-based Josephson junctions (JJs) maintained T1 lifetimes near 300 {\mu}s even when cooled in fields as high as 600 mG. Both trapped flux up to 600 mG and applied fields up to 400 mG reduced T1 fluctuations by more than a factor of two, while higher field strengths caused rapid coherence degradation. We attribute this stabilization to the polarization of paramagnetic impurities, the role of trapped flux as a sink for non-equilibrium quasiparticles (QPs), and partial saturation of fluctuating two-level systems (TLSs). These findings challenge the conventional view that magnetic fields are inherently detrimental and introduce a strategy for mitigating noise in superconducting qubits, offering a practical path toward more stable and scalable quantum systems.