Enhancement of Superconducting Qubit Coherence via Structured Low-Frequency Toroidal Magnetic Fields

This work demonstrates a novel approach to enhancing superconducting qubit coherence through controlled low-frequency electromagnetic environments. A toroidal coil geometry is used to generate spatially coherent magnetic fields at cryogenic temperatures, enabling precise control of the qubit environment without modifying device materials or fabrication processes. A Ta/Si λ/4 CPW resonator is placed at the center of a 3D-printed toroidal coil, and Bayesian optimization is employed to tune the applied magnetic field parameters. The optimization converges to a narrow frequency band centered at 7.831 Hz, resulting in a significant increase in qubit relaxation time from 1.02 ms to 4.7 ms (4.6× improvement), along with a substantial enhancement in internal quality factor. Control experiments confirm that the observed effect is not attributable to thermal, mechanical, or broadband electromagnetic artifacts. The results suggest that structured low-frequency electromagnetic environments can act as an additional control parameter for suppressing decoherence, introducing environmental engineering as a complementary axis alongside materials and circuit design.